KR20250088810A - A variable optical transmission device comprising an electrophoretic medium having a combination of light-reflecting and light-absorbing pigment particles - Google Patents

Abstract

A variable optical transmittance device is disclosed, comprising two optically transparent electrode layers and a microcell layer having a plurality of microcells. Each of the plurality of microcells comprises a first type of electrically charged pigment particles, a second type of electrically charged pigment particles, a charge control agent, and a non-polar liquid. Upon application of an electric field, the amount of light passing through the device can be modulated.

Description

A variable optical transmission device comprising an electrophoretic medium having a combination of light-reflecting and light-absorbing pigment particles

This application claims priority to U.S. Provisional Patent Application No. 63/436,124, filed December 30, 2022, which is incorporated by reference in its entirety along with all other patents and patent applications disclosed herein.

The present invention relates to a variable light transmission device. Specifically, the present invention relates to a microcell electro-optical device comprising an electrophoretic medium comprising a first type of electrically charged pigment particles, a second type of electrically charged pigment particles, and a charge control agent. The electrophoretic medium is capable of switching between optical states using an electric field. The first type of electrically charged pigment particles comprise a light-reflecting pigment, while the second type of electrically charged pigment particles comprise a light-absorbing pigment. The present invention also relates to a method of driving the device for switching between optical states. The variable light transmission device can modulate the amount of light and other electromagnetic radiation passing therethrough. It can be used in mirrors, windows, sunroofs, and similar items. For example, the present invention can be applied to windows that can modulate infrared radiation to control the temperature inside buildings and vehicles. Examples of electrophoretic media that can be incorporated into various embodiments of the present invention include, for example, U.S. Patent Nos. 7,116,466, 7,327,511, 8,576,476, 10,319,314, 10,809,590, 10,067,398, 10,067,398, and 11,143,930, and U.S. Patent Application Publication Nos. 2014/0055841, 2017/0351155, 2017/0235206, 2011/0199671, 2020/0355979, 2020/0272017, 2021/0096439, and U.S. Patent Application No. Includes electrophoretic media described in U.S. Pat. No. 17/935,386, the contents of which are incorporated herein by reference in their entirety.

Particle-based electrophoretic displays, in which multiple electrically charged pigment particles move through a suspended fluid under the influence of an electric field, have been the subject of intensive research and development for many years. Such displays can have the properties of good brightness and contrast, wide viewing angle, state bistability, and low power consumption compared to liquid crystal displays.

The terms "bistable" and "bistability" are used herein in their customary sense in the art to refer to a display comprising a display element having first and second display states having at least one optical characteristic that differs, such that after any given element is actuated, by an addressing pulse of finite duration, it will assume its first or second display state, and after the addressing pulse is terminated, that state will persist for at least a number of times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. Published U.S. Patent Application No. 2002/0180687 discloses that certain particle-based electrophoretic displays capable of grayscale are stable not only in their extreme black and white states but also in their intermediate gray states, as are some other types of electro-optical displays. This type of display is more appropriately referred to as "multi-stable" rather than bistable, but for convenience the term "bistable" may be used herein to encompass both bistable and multi-stable displays.

As mentioned above, the electrophoretic medium requires the presence of a suspending fluid. In most prior art electrophoretic media, this suspending fluid is a liquid, but the electrophoretic medium can be created using a gas suspending fluid. Such gas-based electrophoretic media appear to be susceptible to the same types of problems as liquid-based electrophoretic media due to particle settling when the media is used in an orientation that permits such settling, for example in signs where the media is positioned in a vertical plane. In fact, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of the gas suspending fluid compared to the liquid case allows for faster settling of the electrically charged pigment particles.

Numerous patents and applications assigned to or in the name of MIT (Massachusetts Institute of Technology), E Ink Corporation, E Ink California, LLC and related companies describe various techniques used in encapsulation and microcell electrophoresis and other electro-optical media. Encapsulated electrophoretic media comprise a number of small capsules, each capsule itself comprising an internal phase containing electrophoretically mobile particles in a liquid medium and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form a coherent layer positioned between two electrodes. In microcell electrophoretic displays, the electrically charged pigment particles and liquid are not encapsulated within the microcapsules, but instead are held within a plurality of cavities formed within a carrier medium, typically a polymeric film. The techniques described in these patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814.

(b) Capsules, binders and encapsulation processes; see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719.

(c) Microcell structures, wall materials and methods of forming microcells; see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906.

(d) Methods of filling and sealing microcells; see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088.

(e) Films and subassemblies comprising electro-optical materials; see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564.

(f) Backplanes, adhesive layers, and other auxiliary layers and methods used in displays; see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624.

(g) Color formation and color control; see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564.

(h) Methods of driving displays; see, for example, U.S. Patent Nos. 7,012,600 and 7,453,445.

(i) Applications of displays; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348.

(j) non-electrophoretic displays, such as those described in U.S. Patent No. 6,241,921 and U.S. Patent Application Publication No. 2015/0277160; and applications of encapsulation and microcell technology other than displays; see, e.g., U.S. Patent Application Publication Nos. 2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in the encapsulated electrophoretic medium can be replaced by a continuous phase, thus creating a so-called polymer-dispersed electrophoretic display wherein the electrophoretic medium comprises a plurality of discrete droplets of a non-polar liquid and a continuous phase of a polymeric material, and that the discrete droplets of the electrophoretic medium within such polymer-dispersed electrophoretic display can be considered capsules or microcapsules even though there is no discrete encapsulating membrane associated with each individual droplet; see, for example, the aforementioned 2002/0131147. Accordingly, for the purposes of the present application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the electrically charged pigment particles and suspension are not encapsulated within microcapsules, but instead are held within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281 and Published U.S. Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.

Although electrophoretic media are often opaque (e.g., because the particles in many electrophoretic media substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be configured to operate in a so-called "shutter mode", where one display state is substantially opaque and one is light transmissive; see, e.g., U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Electrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can also operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optical displays can also operate in a shutter mode.

Encapsulated or microcell electrophoretic displays typically do not suffer from the clustering and precipitation failure modes of traditional electrophoretic devices, and offer additional advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. The use of the word "printing" is intended to encompass all forms of printing and coating, including but not limited to, pre-metered coating, such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating, such as knife-over-roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition; and other similar techniques. Thus, the resulting display can be flexible. Additionally, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

One potentially important market for electrophoretic media is windows with variable optical transmittance. As energy performance of buildings and vehicles becomes increasingly important, electrophoretic media can be used as coatings on windows, allowing the proportion of incident radiation transmitted through the window to be electronically controlled by changing the optical state of the electrophoretic media. Effective implementation of such "variable transmission" ("VT") technology in buildings is expected to provide: (1) reduced unwanted heating effects during hot weather, thereby reducing the amount of energy required for cooling, the size of air conditioning systems, and peak power demand; (2) increased use of natural daylight, thereby reducing energy used for lighting and peak power demand; and (3) increased occupant comfort by increasing both thermal and visual comfort. Even greater benefits are expected in automobiles, where the ratio of glass surface to enclosure is considerably greater than in conventional buildings. Specifically, effective implementation of VT technology in automobiles is expected to provide, in addition to the aforementioned benefits, (1) increased driving safety, (2) reduced glare, (3) improved mirror performance (by using electro-optical coatings on the mirrors), and (4) increased ability to use head-up displays. Other potential applications of VT technology include privacy glass and glare guards in electronic devices.

The present technology provides an example of a device comprising an electrophoretic medium between electrode layers, which can achieve a closed optical state (opaque state) and an open optical state (transparent state) and can be switched between these states by the application of an electric field across the electrophoretic medium. However, conventional electrophoretic devices using conventional structures and waveforms require long switching times, making the devices less desirable. The inventors of the present invention surprisingly found that a specific device having a microcell layer and a specific waveform can achieve efficient switching between the open optical state and the closed optical state.

In one aspect, the present invention provides a variable optical transmission device comprising two optically transparent electrode layers and a microcell layer having a plurality of microcells. Each of the plurality of microcells comprises a first type of electrically charged pigment particles and a second type of electrically charged pigment particles, a charge control agent, and a non-polar liquid. Upon application of an electric field, the amount of light passing through the device can be modulated.

A variable optical transmittance device of the present invention comprises a first optically transparent electrode layer, a second optically transparent electrode layer, and a microcell layer. The microcell layer comprises a plurality of microcells and a sealing layer. The microcell layer is disposed between the first optically transparent electrode layer and the second optically transparent electrode layer. Each microcell comprises an electrophoretic medium, the electrophoretic medium comprising a first type of electrically charged pigment particles, a second type of electrically charged pigment particles, a charge control agent, and a non-polar liquid. The charge control agent has an amount in the electrophoretic medium expressed as a weight percent of the charge control agent based on the weight of the electrophoretic medium. Each microcell of the plurality of microcells has a microcell aperture. The sealing layer spans the microcell apertures of the plurality of microcells. Each microcell of the plurality of microcells comprises a microcell bottom layer, a protruding structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, and the microcell bottom inner surface includes an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The protrusion structure has a protrusion base, a protrusion surface, a protrusion apex, and a protrusion height. The protrusion apex is a point or a set of points of the protrusion structure, wherein the point or set of points has a shorter distance from the microcell opening than all other points of the protrusion structure. The protrusion height is the distance between the protrusion base and the protrusion apex. The protrusion surface is a surface of the protrusion structure that does not include the protrusion apex, which contacts the electrophoretic medium. The microcell wall has a microcell inner wall surface and a microcell wall top surface. The microcell inner wall surface is a surface of the microcell wall of the microcell that contacts the electrophoretic medium. The microcell wall top surface is a surface of the microcell wall of the microcell that contacts the sealing layer. The channel has a channel height, and the channel height is 50% of the protrusion height. The unexposed microcell bottom inner surface contacts the protrusion base. The channel is a volume between the exposed microcell bottom inner surface, the protruding surface, and the microcell inner wall surface. The charge control agent of the electrophoretic medium of the variable optical transmittance device can be from 1 weight percent to 8 weight percent based on the weight of the electrophoretic medium. The molecular structure of the charge control agent can include a quaternary ammonium functional group and a nonpolar tail. The nonpolar liquid of the electrophoretic medium can include a material selected from the group consisting of an aliphatic hydrocarbon, an alicyclic hydrocarbon, an aromatic hydrocarbon, a halogenated aliphatic hydrocarbon, a polydimethylsiloxane, or a mixture thereof.

The first type of electrically charged pigment particles can be light reflective and the second type of electrically charged pigment particles can be light absorbing. The first type of electrically charged pigment particles can be white. The second type of electrically charged pigment particles can be black. The first type of electrically charged pigment particles can have the same charge polarity as the second type of charged pigment particles. The first type of electrically charged pigment particles and the second type of electrically charged pigment particles can be positive. When both the first type of electrically charged pigment particles and the second type of electrically charged pigment particles are positively charged, the zeta potential of the first type of electrically charged pigment particles can be lower than the zeta potential of the second type of electrically charged pigment particles.

The first type of electrically charged pigment particles and the second type of electrically charged pigment particles can be negatively charged. When both the first type of electrically charged pigment particles and the second type of electrically charged pigment particles are negatively charged, the zeta potential of the first type of electrically charged pigment particles can be higher than the zeta potential of the second type of electrically charged pigment particles. The higher zeta potential of the first type of electrically charged pigment particles means that the zeta potential of the first type of electrically charged pigment particles is less negative than the zeta potential of the second type of electrically charged pigment particles. That is, for example, when the zeta potential of the second type of electrically charged pigment particles is -15 eV, the zeta potential of the first type of electrically charged pigment particles can be -10 eV.

The first type of electrically charged pigment particles can have an opposite charge polarity to the second type of charged pigment particles. The first type of electrically charged pigment particles can be negatively charged and the second type of electrically charged pigment particles can be positively charged. Alternatively, the first type of electrically charged pigment particles can be positively charged and the second type of electrically charged pigment particles can be negatively charged. The first type of electrically charged pigment particles can have an average particle size that is greater than an average particle size of the second type of charged pigment particles. The average particle size corresponds to an average diameter of a largest dimension of the electrically charged pigment particles.

The protruding structures are: (a) a cone, (b) a cone on a cylinder - the cylinder has a base and the base of the cylinder is the protruding base of the protruding structure - , (c) a tetrahedron, (d) a tetrahedron on a triangular prism - the triangular prism has a triangular base and the triangular base is the protruding base of the protruding structure - , (e) a triangular prism - the triangular prism has a square base and the square base is the protruding base of the protruding structure - , (f) a square pyramid with a square base - the square base is the protruding base of the protruding structure - , (g) a square pyramid on a square rectangular parallelepiped - the rectangular cuboid has a square base and the square base is the protruding base of the protruding structure - , (h) a square pyramid on a rectangular parallelepiped - the rectangular parallelepiped has a right parallelogram base and the right parallelogram base is the protruding base of the protruding structure - , (i) a pentagonal pyramid - the pentagonal pyramid is - , (j) a pentagonal pyramid on a pentagonal prism - the pentagonal prism has a pentagonal base and the pentagonal base is the protruding base of the protruding structure - , (k) a hexagonal pyramid - the hexagonal pyramid has a hexagonal base and the hexagonal base is the protruding base of the protruding structure - , (l) a hexagonal pyramid on a hexagonal prism - the hexagonal prism has a hexagonal base and the hexagonal base is the protruding base of the protruding structure - , (m) a polygonal pyramid - the polygonal pyramid has a polygonal base and the polygonal base is the protruding base of the protruding structure - , (n) a polygonal pyramid on a polygonal prism - the polygonal pyramid has a polygonal base and the polygonal base is the protruding base of the protruding structure - . The protruding structure may be a cone, and the inclination of the cone may be from 5 degrees to 20 degrees or from 5 degrees to 10 degrees. The protruding structure may be a cone on a cylinder. The cylinder may have a base, and the base of the cylinder is a protruding base of the protruding structure, and the inclination of the cone may be 10 degrees or less, and the inclination of the cone may be 5 degrees to 20 degrees. The protruding structure may be a geometric solid of a pyramid having a base with n sides, wherein the base with n sides is a protruding base of the protruding structure, and n is an integer from 7 to 12, (m) a pyramid having a base with n sides, on a prism having a base with n sides, wherein the base of the prism having n sides is a protruding base of the protruding structure, and n is 7 to 12.

The variable optical transmission device can be switched to an open optical state by application of a first electric field between the first optically transparent electrode layer and the second optically transparent electrode layer via a first waveform.

Application of a first waveform between the first optically transparent electrode layer and the second optically transparent electrode layer can cause movement of the first electrically charged pigment particles toward the channel, resulting in switching of the variable optical transmission device to an open optical state. The second type of electrically charged pigment particles in the open optical state can be positioned within the channel.

Application of the second waveform can cause movement of the first type of electrically charged pigment particles toward the first optically transmissive electrode layer at a velocity having a lateral component, resulting in a closed optical state. The second waveform can comprise at least one positive voltage and at least one negative voltage, wherein the second waveform has a net positive or net negative impulse. The closed optical state has lower optical transmission than the open optical state. The second waveform can comprise an AC waveform, wherein the AC waveform has a duty cycle of 5% to 45%. Alternatively, the second waveform can comprise a DC offset waveform formed by superimposing a DC voltage component and the AC waveform. The second waveform can be DC-imbalanced.

In one example, the second waveform can comprise an AC waveform, the AC waveform having a frequency, and the AC waveform having a duty cycle of from 5% to 45%. The AC waveform can have a duty cycle of greater than 50%, greater than 55%, greater than 60%, or greater than 65%. The AC waveform can have a duty cycle of from 55% to 95%, from 58% to 90%, from 60% to 88%, from 65% to 85%, or from 70% to 80%. The AC waveform can have a duty cycle of less than 50%, less than 45%, less than 40%, or less than 35%. The AC waveform can have a duty cycle of from 5% to 45%, from 8% to 40%, from 10% to 38%, from 10% to 40%, from 15% to 35%, or from 20% to 30%. The AC waveform can be a square waveform, a sinusoidal waveform, a triangular waveform, or a sawtooth waveform. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge controlling agent in the electrophoretic medium, expressed as weight percent of the charge controlling agent based on the weight of the electrophoretic medium, can be from 400 Hz to 2000 Hz.

The AC waveform can be a square waveform having two or more cycles. In this case, the positive voltage and the negative voltage of the AC waveform have the same amplitude; the amplitude of the AC waveform can be from 10 V to 200 V; the frequency of the AC waveform can be from 0.1 Hz to 6000 Hz or from 100 Hz to 3000 Hz; the amplitude of the AC waveform can be from 10 V to 200 V or from 20 V to 180 V, and the frequency of the AC waveform can be from 0.1 Hz to 6000 Hz or from 100 Hz to 3000 Hz. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge control agent in the electrophoretic medium, expressed as weight percent of the charge control agent based on the weight of the electrophoretic medium, can be from 400 Hz to 2000 Hz.

In another example, the second waveform can comprise a waveform formed by superimposing a DC voltage component and an AC waveform, the AC waveform having a frequency and an amplitude. The frequency of the AC waveform can be from 0.1 Hz to 6000 Hz, from 100 Hz to 3000 Hz, or from 400 Hz to 2000 Hz. The amplitude of the AC waveform can be from 10 V to 200 V or from 20 V to 180 V. The DC voltage component has an amplitude of from 0.1 V to 500 V. A ratio of the frequency of the AC waveform, expressed in Hz, to a content of the charge controlling agent in the electrophoretic medium, expressed as a weight percent of the charge controlling agent based on the weight of the electrophoretic medium, can be from 400 Hz to 2000 Hz. The second waveform can comprise an AC waveform having a DC offset. The AC waveform can be selected from the group consisting of square waveform, sine waveform, triangle waveform and sawtooth waveform.

The electrophoretic medium can include a first type of electrically charged pigment particles and a second type of electrically charged pigment particles. The first type of electrically charged pigment particles can be light reflective. The second type of electrically charged pigment particles can be light absorbing. The first type of electrically charged pigment particles can be white. The second type of electrically charged pigment particles can be black. The first type of electrically charged pigment particles can have the same polarity as the second type of electrically charged pigment particles. The first type of electrically charged pigment particles can have an opposite polarity as the second type of electrically charged pigment particles.

The top surface of the microcell wall may have a light-blocking layer, and the light-blocking layer is disposed between the top surface of the microcell and the sealing layer. The light-blocking layer may include a light-absorbing pigment. The light-blocking layer may include a black pigment.

In another aspect, the present invention provides a variable optical transmission device. The variable optical transmission device comprises a first optically transparent electrode layer, a second optically transparent electrode layer, and a microcell layer. The microcell layer comprises a plurality of microcells and a sealing layer. The microcell layer is disposed between the first optically transparent electrode layer and the second optically transparent electrode layer. Each microcell comprises an electrophoretic medium, wherein the electrophoretic medium comprises electrically charged pigment particles, a charge control agent, and a non-polar liquid. Each microcell of the plurality of microcells has a microcell aperture. A sealing layer spans the microcell apertures of the plurality of microcells. Each microcell of the plurality of microcells comprises a microcell bottom layer, a protruding structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, wherein the microcell bottom inner surface comprises an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The protruding structure has a protruding base, a protruding surface, a protruding apex, and a protruding height. A protrusion apex is a point or set of points of a protrusion structure, wherein the point or set of points has a shorter distance from the microcell opening than all other points of the protrusion structure. A protrusion height is a distance between a protrusion base and a protrusion apex. A protrusion surface is a surface of a protrusion structure that does not include a protrusion apex, which contacts an electrophoretic medium. A microcell wall has a microcell inner wall surface and a microcell wall top surface. The microcell inner wall surface is a surface of a microcell wall of a microcell that contacts an electrophoretic medium. The microcell wall top surface is a surface of a microcell wall of a microcell that contacts a sealing layer. A channel has a channel height, which is 50% of the protrusion height. An unexposed microcell bottom inner surface contacts the protrusion base. A channel is a volume between the exposed microcell bottom inner surface, the protrusion surface, and the microcell inner wall surface.

Application of a first electric field between the first light-transmitting electrode layer and the second light-transmitting electrode layer via the first waveform causes movement of the electrically charged pigment particles toward the channel, resulting in switching of the variable light-transmitting device to an open optical state. The electrically charged pigment particles in the open optical state are located inside the channel.

Application of a second electric field between the first optically transparent electrode layer and the second optically transparent electrode layer via the second waveform causes movement of the electrically charged pigment particles toward the first optically transparent electrode layer at a velocity having a lateral component, resulting in a closed optical state. The second waveform can be a DC imbalance. The second waveform can include at least one positive voltage and at least one negative voltage, and the second waveform has a pure positive or pure negative impulse. The closed optical state has lower optical transmission than the open optical state.

The second waveform can include an AC waveform, the AC waveform having a frequency, and the AC waveform having a duty cycle of 5% to 45%. The AC waveform can be a square waveform, a sinusoidal waveform, a triangle waveform, or a sawtooth waveform.

The AC waveform can be a square waveform having two or more cycles, and the positive voltage and the negative voltage of the AC waveform have the same amplitude. The amplitude of the square AC waveform can be from 10 V to 200 V, and the frequency of the square AC waveform can be from 0.1 Hz to 6000 Hz or from 100 Hz to 3000 Hz. The amplitude of the square AC waveform can be from 10 V to 200 V or from 20 V to 180 V, and the frequency of the square AC waveform can be from 0.1 Hz to 6000 Hz or from 100 Hz to 3000 Hz. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge controlling agent in the electrophoretic medium, expressed as weight percent of the charge controlling agent based on the weight of the electrophoretic medium, can be from 400 Hz to 2000 Hz.

Alternatively, the second waveform can comprise a waveform formed by superimposing a DC voltage component and an AC waveform, the AC waveform having an amplitude and a frequency, and the DC voltage component having an amplitude. The frequency of the AC waveform can be from 0.1 Hz to 6000 Hz, from 100 Hz to 3000 Hz, or from 400 Hz to 2000 Hz, and the amplitude of the AC waveform can be from 10 V to 200 V or from 20 V to 180 V. The amplitude of the DC voltage component can be from 0.1 V to 500 V. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge controlling agent in the electrophoretic medium, expressed as a weight percent of the charge controlling agent based on the weight of the electrophoretic medium, can be from 400 Hz to 2000 Hz. The second waveform can comprise an AC waveform having a DC offset. The AC waveform can be selected from the group consisting of square waveform, sine waveform, triangle waveform and sawtooth waveform.

Figure 1 is an example of a cylindrical particle in a liquid under the influence of an applied electric field and the resulting force on the particle.
FIGS. 2A, 2B, 2C and 2D illustrate side views of examples of a portion of a variable optical transmission device of the present invention.
Figure 3 illustrates a side view of a microcell in an open optical state and a side view of a microcell in a closed optical state. The electrophoretic medium comprises one type of electrically charged pigment particles.
FIG. 4 is an example of a first embodiment of the present invention; this example is a DC unbalanced waveform that can be applied to a variable optical transmission device to achieve a closed state; the waveform includes an AC waveform having a duty cycle greater than 50%.
FIG. 5 is an example of a second embodiment of the present invention; this example is a DC unbalanced waveform that can be applied to a variable optical transmission device to achieve a closed state; the waveform is a superposition of a DC voltage component and an AC waveform.
FIG. 6 illustrates the force exerted by electrically charged pigment particles on the surface of a conical protrusion of a variable light transmission device of the present invention.
FIG. 7 illustrates a portion of a variable light transmission device having an electrophoretic medium including a first type of electrically charged pigment particles and a second type of electrically charged pigment particles, wherein the first type of electrically charged pigment particles have the same polarity as the second type of electrically charged pigment particles.
FIG. 8 illustrates a portion of a variable light transmission device having an electrophoretic medium including a first type of electrically charged pigment particles and a second type of electrically charged pigment particles, wherein the first type of electrically charged pigment particles have an opposite polarity to the second type of electrically charged pigment particles.
Figure 9 provides a graph of light reflection, transmission and absorption versus layer thickness of a layer comprising light absorbing pigment particles.
Figure 10 provides a graph of light reflection, transmission and absorption versus layer thickness of a layer comprising light reflective pigment particles.
Figure 11 illustrates a graph providing the effect of layer thickness on reflection, transmission and absorption of a closed optical state layer comprising a combination of light reflective pigment particles and light absorbing pigment particles.
Figure 12 illustrates a portion of a variable optical transmission device including a light blocking layer on the inner surface of the exposed microcell bottom. The electrophoretic medium includes one type of electrically charged pigment particles.
Figure 13 illustrates a portion of a variable optical transmission device including a light blocking layer on the upper surface of the microcell wall. The electrophoretic medium includes one type of electrically charged pigment particles.
Figure 14 illustrates a plan view of a microcell of the variable penetration device used in the example.
Figure 15 illustrates a cross-sectional view of a microcell of the variable penetration device used in the example.
Figure 16 provides micrographs of the open and closed optical states of the variable optical transmission device of Example 1, where the optical states result from different waveforms.
Figure 17 provides micrographs of the open and closed optical states of the variable optical transmittance device of Example 2, wherein the electrophoretic medium of the device comprises different concentrations of charge control agent.
FIG. 18 is a micrograph of a microcell array of the variable optical transmittance device of Example 5, wherein the light-blocking composition comprises black pigment particles distributed throughout the entire microcell (closed optical state).
FIG. 19 is a micrograph of a microcell array of a variable optical transmittance device from Example 5, wherein black pigment particles of the light-blocking composition are moved into the channels of the microcells (closed optical state).
FIG. 20 is a micrograph of a closed optical state of a microcell array of a variable optical transmittance device of Example 6, wherein white electrically charged pigment particles of the light-blocking composition are distributed throughout the entire microcell (closed optical state).
FIG. 21 is a micrograph of the open optical state of the microcell array of the variable optical transmittance device of Example 6, where white electrically charged pigment particles of the light blocking composition are moved into the channels (open optical state).
FIG. 22 is a micrograph of the open optical state of the microcell array of the variable optical transmittance device of Example 7, wherein the electrophoretic medium of the device comprises white and black pigment particles.

The distance of a point from a plane is the shortest perpendicular distance from that point to the plane. The shortest distance from a point to a plane is the length of a perpendicular line parallel to the normal vector falling from a given point to a given plane.

The distance between two planes in three-dimensional space is the shortest distance between the planes. It is the shortest distance between any point on one plane and any point on the other plane.

The slope of a cone is defined as the angle formed by (a) the vertex A on the circumference of the base of the cone, (b) the first arm, which is the line connecting point A (the vertex) and the center of the base of the cone, and (c) the second arm, which is the line connecting point A (the vertex) and the vertex of the cone.

The term "electrically charged pigment particles" may refer to charged pigment particles that do not have any polymerizable material on the surface of the pigment particle. The term "electrically charged pigment particles" may also refer to pigment particles that have polymerizable material on the surface of the pigment particle.

"Microcell inner wall surface" is the surface of the microcell wall that contacts the electrophoretic medium of the microcell.

The "microcell wall top surface" is the surface of the microcell wall that is in contact with the sealing layer of the microcell. If there is a light-blocking layer on the microcell wall top surface, the light-blocking layer is disposed between the microcell wall top surface and the sealing layer.

The term "DC-balanced waveform" or "DC-balanced drive waveform" applied to a pixel is a drive waveform which, when integrated over the application period of the entire waveform, results in a drive voltage applied to the pixel that is substantially zero. DC balance may be achieved by balancing each stage of the waveform, i.e., the initial positive voltage will be chosen such that when integrated over the subsequent negative voltage, the result will be zero or substantially zero. When the waveform is not DC-balanced, it is referred to as a "DC-imbalanced waveform" or "DC-imbalanced drive waveform." The drive waveform applied to the pixel may have at least one additional pulse of a DC-imbalanced portion and an opposite impulse to ensure that the entire waveform applied to the pixel is DC-balanced. This additional pulse may be applied prior to the DC-imbalanced portion of the waveform (a pre-pulse). Common examples of DC unbalance waveforms include (a) square or sinusoidal AC waveforms having a duty cycle less than (or greater than) 50%, and (b) square or sinusoidal AC waveforms having a DC offset.

The term "impulse" is the integral of voltage with respect to time. That is, for a waveform pulse having voltage V applied over time t, the impulse is V×t. The impulse can be positive if the polarity of the voltage V is positive, or negative if the polarity of the voltage V is negative.

The term “pure positive impulse” of the waveform means that during application of the waveform, the negatively electrically charged pigment particles will be attracted to and move towards the first light-transmitting electrode layer.

The term "lateral component of velocity" in relation to the movement of electrically charged pigment particles in the microcell of the variable optical transmittance device of the present invention is the velocity in the horizontal direction. For the purpose of this definition, the velocity of the electrically charged particles is a vector resulting from the vectorial addition of the horizontal velocity (Vh) and the vertical velocity (Vv), and for the movement of the electrically charged pigment particles within the electrophoretic microcell, the vertical direction is assumed to be from the first optically transparent electrode layer to the second optically transparent electrode layer or from the second optically transparent electrode layer to the first optically transparent electrode layer. In the same system, the horizontal direction of the movement of the electrically charged pigment particles within the electrophoretic microcell is from one side of the microcell wall to the other side of the microcell wall, which direction is parallel to the first optically transparent electrode layer. Therefore, the phrase "the velocity of the electrically charged pigment particles has a lateral component" means that the magnitude of the velocity in the horizontal direction is greater than zero.

The phenomenon of induced-charge-electro-osmosis (ICEO) can be used to laterally move polarizable particles, such as pigment particles, present in an electrophoretic medium. That is, the polarizable particles can move parallel to the electrode layers that separate the electrophoretic medium. In the presence of an electric field, the particle can experience a force caused by the polarization of the particle (or by the polarization of an adsorbed conductive coating on the particle surface or an electric double layer around the particle). This force can cause a disturbance in the flow of mobile charges, such as ions or charged micelles, in the electrophoretic medium, as illustrated in FIG. 1 for a cylindrical particle (101) surrounded by a liquid of the electrophoretic medium in an applied electric field. This figure is reproduced by the paper of Bazant and Squires, J. Fluid Mech., 2004, 509, 217-252.

A perfectly symmetric spherical particle will experience no net force, but less symmetrical particles will experience a force whose component is perpendicular to the direction of the applied electric field. The cooperative flow generated by clusters of particles each subjected to these forces can cause "swirl" in an electrophoretic medium containing a large number of particles. According to the theory presented in the paper by Bazant and Squires, the maximum velocity of this swirling for a particular particle is will be roughly given by equation (1).

Equation (1)

In equation (1), is the electric field strength, is the dielectric constant of the solvent, is the viscosity of the electrophoretic fluid, is the applied sinusoidal AC frequency, is the time scale for forming a screening charge layer by the movement of charges contained in the solvent around the charge. Time scale is given by equation (2).

Equation (2)

In equation (2), is the Debye screening length, is the particle radius, is the diffusion constant of charge carriers in the fluid.

According to equation (1), as the frequency increases, As the value of increases, the maximum velocity of the induced charge flow decreases. Also, for values significantly greater than 1, For the value of , the maximum swirling speed is is proportional to the square of the ratio. The induced charge flow occurs in the same direction regardless of the polarity of the applied electric field, and therefore can be driven by an alternating electric field.

When the electrophoretic medium is contained within microcells, as is preferred in electrophoretic displays, the geometry of the induced flow is influenced by the geometry of the particular microcells employed. For example, in the simplest case of two parallel electrodes, it has been shown that, with appropriate electric field strength and AC frequency, the flow can assume a roll structure with periodic spacing corresponding to the width of the gap between the electrodes.

The inventors of the present invention have fabricated a switchable device using a complex microcell structure formed by an embossing method. In one example, the embossed structure includes a conical protrusion at the bottom of each microcell. FIGS. 2A, 2B, and 2C illustrate examples of a variable light transmission device according to the present invention, wherein the protrusion structure of the variable light transmission device is a cylindrical cone. As illustrated in FIGS. 2A, 2B, and 2C, the cone of the protrusion structure can direct the electrophoretic flow of particles into the channel. When an electric field applied across the electrophoretic medium has an appropriate polarity with respect to the polarity of the electrically charged pigment particles, the electrically charged pigment particles will migrate toward the channel. For example, when the electrically charged pigment particles are positively charged and have a negative polarity toward the second optically transparent electrode as a result of an applied voltage across the optically transparent electrode, the electrically charged pigment particles will migrate toward the channel. The same movement will occur when the electrically charged pigment particles are negatively charged and positively polarized to the second optically transparent electrode as a result of an applied voltage across the optically transparent electrode. Figures 2a, 2b and 2c illustrate cross-sections (not to scale) of a portion of a tunable optically transparent device showing only one microcell of the plurality of microcells of the device. All three Figures 2a, 2b and 2c are identical with respect to the device structure illustrated, but different portions of the device are indicated in each of the drawings.

Portions of the variable light transmitting device (200) of FIGS. 2A, 2B and 2C include a microcell layer including a plurality of microcells and a sealing layer. Although only a single microcell is shown in FIGS. 2A, 2B and 2C, an entire variable light transmitting device including a plurality of microcells can be envisioned. The variable light transmitting device can include a first light transmitting substrate (201), a first light transmitting electrode layer (202), a microcell layer (203) including a plurality of microcells (204) and a sealing layer (206), a second light transmitting electrode layer (207) and a second light transmitting substrate (208). Each microcell of the plurality of microcells (204) includes an electrophoretic medium (209) including electrically charged pigment particles, a charge control agent and a non-polar liquid. Components of the electrophoretic medium (electrically charged pigment particles, charge control agent, non-polar liquid) are not shown in FIGS. 2A, 2B and 2C. Each microcell of the plurality of microcells (204) has a microcell opening (205), and a sealing layer (206) spans the microcell openings (205) of the plurality of microcells (204). Each microcell of the plurality of microcells (204) includes a microcell bottom layer (210), a protruding structure (217), a microcell wall (212) and a channel (215). The microcell bottom layer (210) has a microcell bottom inner surface (211), and the microcell bottom inner surface (211) includes an exposed microcell bottom inner surface (211a) and an unexposed microcell bottom inner surface (211b). The unexposed microcell bottom surface (211b) contacts the protruding base (218). The exposed microcell bottom inner surface (211a) is highlighted in Fig. 2a by being represented by a thicker line.

In this example, the protrusion structure (217) is a cone on a cylinder. The protrusion structure (217) has a protrusion base (218), a protrusion surface (221), a protrusion apex (219), and a protrusion height (220). The protrusion apex (219) is a point or set of points of the protrusion structure (217) that has a shorter distance from the microcell aperture (205) than all other points of the protrusion structure (217). In the examples of the variable light transmitting devices of FIGS. 2A, 2B, and 2C, the protrusion apex (219) is the apex of the cone of the protrusion structure. The protrusion height (220) is the distance between the protrusion base (218) and the protrusion apex (219). If the protrusion structure (217) has more than one point, such as a protrusion apex (219) that includes a planar surface, the protrusion height (220) is the distance between the planar surface and the protrusion base (218) of the protrusion structure (217). A microcell layer comprising a plurality of microcells (204) having protruding structures (217) can be fabricated by embossing a thermoplastic or thermosetting precursor layer using a pre-patterned mold, and then releasing the mold. The precursor layer can be cured by radiation, cooling, solvent evaporation, or other means during or after the embossing step.

The microcell wall (212) has a microcell inner wall surface (213) and a microcell wall top surface (214). The microcell inner wall surface (213) is in contact with the electrophoretic medium (209). The microcell wall top surface (214) is a surface of the microcell wall (212) of the microcell that is in contact with the sealing layer (206). The microcell inner wall surface (213) is highlighted in FIG. 2b by being represented by a thicker line.

The channel (215) is a volume between the exposed microcell bottom inner surface (211a), the microcell inner wall surface (213), and the protruding surface (221). The channel (215) is the volume location where most of the electrically charged particles exist in the open optical state of the device. The channel (215) has a channel height (216) which is 50% of the protruding height (220). Thus, the channel height, together with the exposed microcell bottom inner surface (211a), the microcell inner wall surface (213), and the protruding surface (221), further defines the channel. The protruding surface (221) is highlighted in FIG. 2c by being represented by a thicker line.

FIG. 2d illustrates an example of a variable light transmission device according to the present invention, wherein the protruding structure of the variable light transmission device is a cylindrical cone. The variable light transmission device of FIG. 2d is similar to that illustrated by FIGS. 2a, 2b, and 2c, but shows a larger portion of the device including four microcells. The variable light transmission device (200) includes a first light transmitting substrate (201), a first light transmitting electrode layer (202), a microcell layer (203) including a plurality of microcells (204) and a sealing layer (206), a second light transmitting electrode layer (207), and a second light transmitting substrate (208). Each microcell of the plurality of microcells includes an electrophoretic medium including electrically charged pigment particles (222), a charge control agent, and a non-polar liquid. Each microcell of the plurality of microcells (204) has a microcell opening, and a sealing layer (206) spans the microcell openings of the plurality of microcells. Each microcell of the plurality of microcells includes a microcell bottom layer (210), a protruding structure (217), a microcell wall (212), and a channel (215). The variable optical transmission device illustrated in FIG. 2d is in a closed optical state.

When a first electric field is applied between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207) via the first waveform, if the polarity of the electrically charged pigment particles (222) and the voltage polarity of the second light-transmitting electrode layer are opposite to each other, movement of the electrically charged pigment particles (222) toward the channel is caused. If the polarity of the electrically charged pigment particles (222) and the voltage polarity of the second light-transmitting electrode layer are opposite to each other, the electrically charged pigment particles (222) will be attracted by the second light-transmitting electrode, and the variable light-transmitting device will be switched to an open optical state, and the open optical state has a higher light transmittance than the closed optical state. The open optical state is illustrated in FIG. 3A, where the electrically charged pigment particles (222) are represented by black filled circles. In this example, the electrophoretic medium comprises one type of electrically charged pigment particles (222).

Application of a second electric field between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207) via the second waveform causes the electrically charged pigment particles (222) to move toward the first light-transmitting electrode layer (202) with a velocity. This leads to a closed optical state as illustrated in FIG. 3b. The velocity has a lateral component. If there were no lateral component of the velocity, the closed optical state would not occur because the electrically charged pigment particles (222) would move from the open channel toward the first light-transmitting electrode layer (202), but these electrically charged pigment particles (222) would occupy a region near the perimeter of the microcell near the sealing layer (206). That is, the electrically charged pigment particles (222) would not spread over the entire surface of the first light-transmitting electrode layer (202). Therefore, the closed optical state will not be formed effectively because the closed optical state will have relatively high light transmittance.

Above, it is shown that during the movement of the electrically charged pigment particles towards the second light-transmitting electrode layer, it is somewhat easier to switch from a closed optical state to an open optical state, since the inclination of the protrusion structure (e.g., the cone in FIGS. 3a and 3b) will impart a lateral component to their velocity when they hit the protrusion surface of the protrusion structure.

It is possible to shape the electric field within the variable optical transmission device by making the electrical conductivities of the electrophoretic medium and the cone substantially different from each other. For example, if the cone is much less conductive than the electrophoretic medium, the electric field lines will tend to direct the electrically charged pigment particles into the channel. However, even in this case, it may still be necessary to provide a more substantial horizontal force component to redistribute the electrically charged pigment particles from the channel into the overall microcell volume. Furthermore, in current state of the art, the resistivity of the cone material and the electrophoretic medium are each about 10 ¹⁰ When the electric field lines are approximately equal to each other, it is easier to assemble and operate the device, in which case the electric field lines will be oriented approximately vertically through the microcell. Therefore, it would be desirable to use a waveform that imparts lateral motion to the electrically charged pigment particles.

The variable optical transmission device can be switched to the open optical state by applying a first electric field between the first optically transparent electrode layer and the second optically transparent electrode layer via a first waveform to cause movement of the first type of electrically charged pigment particles toward the channel, whereby switching of the variable optical transmission device to an open optical state occurs, wherein the first type of electrically charged pigment particles in the open optical state are located within the channel. The variable optical transmission device can be switched to the closed optical state by applying a second electric field between the first optically transparent electrode layer and the second optically transparent electrode layer via a second waveform to cause movement of the first type of electrically charged pigment particles toward the first optically transparent electrode layer at a velocity having a lateral component, whereby switching to the closed optical state occurs, wherein the second waveform comprises a series of at least two positive and negative pulses having pure positive or pure negative impulses, wherein the closed optical state has lower optical transmission than the open optical state.

The second waveform can be a DC unbalance. The second waveform can include at least one positive voltage and at least one negative voltage, and the second waveform has a pure positive or pure negative impulse. The selection of a pure positive or pure negative impulse depends on the polarity of the electrically charged pigment particles to be moved to a location in the electrophoretic medium near the sealing layer. Specifically, if the closed state involves the movement of the first type of electrically charged pigment particles, which are negatively charged, a pure positive impulse is required to move these particles from the channel toward the first optically transparent electrode layer. In other words, such movement should be such that the net result of the applied voltage is the attraction of the negatively charged particles by the positive voltage of the first optically transparent electrode layer toward the second optically transparent electrode layer. Conversely, if the closed state involves movement of the first type of electrically charged pigment particles that are positively charged, a pure negative impulse is required to move the electrically charged pigment particles from the channel near the second light-transmitting electrode layer (207) toward the first light-transmitting electrode layer.

A second electric field applied between the two optically transparent electrode layers via a second waveform achieves a closed optical state.

In the first embodiment, the second waveform comprises an AC waveform having a duty cycle different from 50%. An example of the second waveform of the first embodiment is illustrated in FIG. 4.

The AC waveform can have a positive or negative DC bias. The DC bias can be achieved by controlling the duty cycle of the waveform. The duty cycle for the positive DC biased waveform is greater than 50%. The duty cycle for the positive DC biased waveform can be greater than 55%, greater than 60%, or greater than 65%. The duty cycle for the positive DC biased waveform can be between 55% and 95%, between 58% and 90%, between 60% and 88%, between 65% and 85%, or between 70% and 80%. Similarly, the duty cycle for the negative DC biased waveform is less than 50%. The duty cycle for the negative DC biased waveform can be less than 45%, less than 40%, or less than 35%. The duty cycle for the negative DC biased waveform can be from 5% to 45%, from 8% to 40%, from 10% to 38%, from 15% to 35%, or from 20% to 30%.

The waveform illustrated in the example of FIG. 4 comprises an AC square waveform having two or more cycles. Each cycle can include a first pulse of amplitude V1 applied for a time period t1 and a second pulse of amplitude V2 applied for a time period t2, where V1 is positive and V2 is negative, and where t1 is greater than t2. When the amplitude of V1 is equal to the amplitude of V2 (|V1| = |V2|), a DC bias is achieved by the difference in the time periods. In the case of the example of FIG. 4, a positive DC bias exists because the positive voltage (V1) is applied for a time period t1 that is longer than the time period t2 of the negative voltage (V2). The positive DC bias means that when the electrically charged pigment particles of the tunable optical transmittance device are negatively charged, the electrically charged pigment particles will migrate toward the first optically transmittable electrode layer of the device. The duty cycle of the waveform can be calculated by equation (3).

Equation (3)

In the waveform example of Fig. 4, the amplitude of V1 can be equal to the amplitude V2 (|V1| = |V2|), but in general, the amplitudes V1 and V2 can be different from each other.

The example drive waveform of FIG. 4 is a DC unbalanced waveform. However, one or more additional pulses may be included in the waveform of FIG. 4 of the opposite impulse, which may cause the overall waveform applied to the pixel to be DC balanced. This additional pulse (or additional pulses) may be applied prior to the DC unbalanced waveform (a pre-pulse). Also, the example waveform of FIG. 4 is a square AC waveform. Other examples of AC waveforms that may be used include sinusoidal waveforms, triangle waveforms, and sawtooth waveforms.

The AC waveform of the first embodiment can have an amplitude of 10 V to 200 V and a frequency of 0.1 Hz to 6000 Hz. The AC waveform can have an amplitude of 15 V to 180 V, 20 V to 160 V, 25 V to 150 V, or 30 V to 140 V. The AC waveform can have a frequency of 0.5 Hz to 5000 Hz, 1 Hz to 4000 Hz, 5 Hz to 3000 Hz, 10 Hz to 2000 Hz, 15 Hz to 1000 Hz, 20 Hz to 800 Hz, or 25 Hz to 600 Hz. The ratio of the frequency of the AC waveform to the weight percent content of the charge control agent based on the weight of the electrophoretic medium can be from 400 Hz to 2000 Hz.

In a second embodiment, the second waveform may include a waveform formed by superimposing a DC voltage component and an AC waveform. An example of the second waveform of the second embodiment is illustrated in FIG. 5.

The waveform of Fig. 5 has a pure negative impulse due to the DC offset (Vd). Although the time period (t3) for applying the positive pulse is the same as the time period (t4) for applying the negative pulse, the DC bias is achieved by the difference in the pulse amplitudes. Specifically, the amplitude V3 of the positive pulse is smaller than the amplitude V4 of the negative pulse. This is caused by the DC voltage component (Vd) of the waveform. That is, the waveform illustrated in Fig. 5 has a DC offset.

The example drive waveform of Fig. 5 is a DC unbalanced waveform. However, one or more additional pulses may be included in the waveform of Fig. 5 of the opposite impulse, which may cause the overall waveform applied to the pixel to be DC balanced. This additional pulse (or additional pulses) may be applied prior to the DC unbalanced waveform (a pre-pulse). Also, the example waveform of Fig. 5 is a square AC waveform. Other examples of AC waveforms that may be used include sinusoidal waveforms, triangle waveforms, and sawtooth waveforms.

The AC waveform can have an amplitude of 10 V to 200 V and a frequency of 0.1 Hz to 6000 Hz. The AC waveform can have an amplitude of 15 V to 180 V, 20 V to 160 V, 25 V to 150 V, or 30 V to 140 V. The AC waveform can have a frequency of 0.5 Hz to 5000 Hz, 1 Hz to 4000 Hz, 5 Hz to 3000 Hz, 10 Hz to 2000 Hz, 15 Hz to 1000 Hz, 20 Hz to 800 Hz, or 25 Hz to 600 Hz. The ratio of the frequency of the AC waveform to the weight percent content of the charge control agent based on the weight of the electrophoretic medium can be from 400 Hz to 2000 Hz.

When the ICEO-induced motion of the electrically charged pigment particles is relatively low, the protruding structures of the microcell contribute to the effective operation of the tunable penetration device even when the device is driven using a DC balanced AC waveform. In the example where the protruding structure is a cone, any electrically charged pigment particles located on the surface of the cone will experience a net force that moves them toward the apex of the cone, as illustrated in FIG. 6 . FIG. 6 illustrates an electrically charged pigment particle (222) in contact with a protruding structure (617) (cone) in an electric field (602). In this case, the ICEO flow is illustrated by the curved arrows, which are more restricted on the "uphill" side of the cone than on the "downhill" side. This exerts a force on the particle, as illustrated by the dashed horizontal arrows. There will be an opposing force acting perpendicular to the cone, pushing the particle toward the apex of the cone. With an appropriate choice of AC field and frequency, the particles can be driven out of the channel region and up the side of the cone in this manner.

The electrophoretic medium of the variable optical transmittance device of the present invention comprises a first type of electrically charged pigment particles, a second type of electrically charged pigment particles, a charge control agent, and a non-polar liquid.

The first type of electrically charged pigment particles can be light reflective and the second type of electrically charged pigment particles can be light absorbing. A common example of the light reflective pigment particles is titanium dioxide, which has a white color. Typical examples of the light absorbing pigment particles include organic and inorganic pigment particles having black, blue, cyan, magenta, red, green, yellow and other colors. The first type of electrically charged pigment particles can have the same polarity as the second type of electrically charged pigment particles. The first type of electrically charged pigment particles can have an opposite polarity as the second type of electrically charged pigment particles.

In an electrophoretic medium having first type of electrically charged pigment particles having the same charge polarity and second type of electrically charged pigment particles, the zeta potential of the first type of electrically charged pigment particles may be lower than the zeta potential of the second type of electrically charged pigment particles. Additionally, the average particle size of the first type of electrically charged pigment particles may be larger than the average particle size of the second type of electrically charged pigment particles, which is determined by the average diameter of the pigment particles. An open optical state of the variable optical transmission device of this example is illustrated in FIG. 7A. In this example, upon application of an appropriate electric field between the first optically transparent electrode layer (202) and the second optically transparent electrode layer (207), both types of electrically charged pigment particles (222a (first type) and 222b (second type)) will migrate into the channels of the microcell to form an open state. However, since the first type of electrically charged pigment particles (222a) have a lower charge (and larger size), the second type of electrically charged pigment particles (222b) (light absorbing) will be positioned below the first type of electrically charged pigment particles (222a) (light reflective). In other words, the second type of electrically charged pigment particles (222b) will be positioned closer to the exposed microcell bottom inner surface (bottom of the channel) than the first type of electrically charged pigment particles (222a). The closed optical state of the variable optical transmittance device of this example is illustrated in FIG. 7b. Upon application of an appropriate electric field between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207), both types of electrically charged pigment particles (222a, 222b) will migrate toward the first light-transmitting electrode layer to achieve a closed optical state. However, since the first type of electrically charged pigment particles (222a) have a lower charge (and larger size), the second type of electrically charged pigment particles (222b) (light-absorptive) will be positioned closer to the sealing layer (206) than the first type of electrically charged pigment particles (222a) (light-reflective).

In another example, a variable optical transmission device has an electrophoretic medium including a first type of electrically charged pigment particles (light reflective) and a second type of electrically charged pigment particles (light absorbing), wherein the first type of electrically charged pigment particles and the second type of electrically charged pigment particles have opposite charge polarities. Two possible open optical states of this example are illustrated in FIGS. 8A and 8B , respectively. Upon application of an appropriate electric field between the first optically transparent electrode layer (202) and the second optically transparent electrode layer (207), depending on the polarity of the applied electric field, either the first electrically charged pigment particles (223a) (light reflective) or the second electrically charged pigment particles (223b) (light absorbing) will migrate toward the channel (open optical state). A closed optical state can be formed by application of an appropriate electric field (a second electric field) between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207) which causes electrically charged pigment particles of one type to move toward the first light-transmitting electrode layer (202) at a velocity having a lateral component. Depending on the applied electric field, either the first type of electrically charged pigment particles (223a) or the second type of electrically charged pigment particles (223b) will spread into the microcell region (microcell aperture) near the sealing layer (206).

The amount of the second type of electrically charged pigment particles, which may have a black color, will be selected such that it is sufficient to hide the white pigment within the channel when viewed from below (open optical state), but not so high as to cause too much light absorption in the closed state. By “viewed from below” is meant that the observer is positioned on the side of the variable optical transmission device that is closer to the second optically transparent electrode layer (207), as opposed to the side that is closer to the first optically transparent electrode layer (202).

The open optical state of a variable transmission device is preferably one having high transmittance and low haze. Additionally, in certain applications, such as building windows or vehicle sunroofs, it is desirable to manage heating of the building or vehicle. Thermal management is difficult when the variable transmission device comprises an electrophoretic medium including light-absorbing electrically charged pigment particles. In the closed optical state of such a device, light incident on the device is absorbed by the device, causing the device to heat up. The incident light can include wavelengths within the solar spectrum, i.e., ultraviolet, visible, and infrared. Another problem may be that the closed optical state is not completely opaque, i.e., some of the incident light penetrates into the building or vehicle, warming the interior of the building or vehicle.

Figure 9 provides a graph of light reflection, transmission and absorption versus layer thickness for a layer comprising a black pigment (light absorbing). That is, for each layer thickness, the graph provides the amount of light reflected, transmitted and absorbed as a percentage of the incident light.

Figure 10 provides a graph of light reflection, transmission and absorption versus layer thickness for a layer comprising white pigment (light reflective). That is, for each layer thickness, the graph provides the amount of light reflected, transmitted and absorbed as a percentage of the incident light.

Figures 9 and 10 show that the white pigment reflects significantly more light than the black pigment, but does not absorb incident light. In fact, the white pigment provides opacity by light reflection/scattering. As the layer thickness increases, more incident light is reflected and less incident light is transmitted. However, it is observed that a variable light transmission device comprising a medium having white pigment particles (reflective) may exhibit significant haze in the open optical state.

The inventors of the present invention have found that a variable light transmission device comprising an electrophoretic medium having electrically charged particles comprising a reflective pigment (first type) and electrically charged particles comprising an absorptive pigment (second type) provides significant advantages such as haze reduction. The reflective pigment can be titanium dioxide, and the absorptive pigment can be an inorganic black such as iron oxide black.

Figure 11 illustrates the effect of layer thickness on reflection, transmission and absorption of a closed optical state layer comprising a combination of white pigment (light reflective) and black pigment (absorptive). In the illustrated example, the weight ratio of black to white pigment is 0.1. That is, for each layer thickness, the graph provides the amount of light reflected, transmitted and absorbed as a percentage of the incident light.

As the layer thickness increases, the reduction in transmittance remains virtually unchanged, but the reflection from the white pigment is limited to 30%. The absorption increases with layer thickness, but is only about half that of the pure black pigment layer shown in Fig. 9. By stacking the white pigment in front of the black instead of mixing the black and white pigments, the absorption with the accompanying unwanted thermal effects can be reduced.

In an example of a tunable transmission device having an electrophoretic medium comprising a first type of electrically charged pigment particles (white or reflective with negative charge polarity) and a second type of electrically charged pigment particles (black or absorptive with positive polarity), the relative positions of the two types of electrically charged pigment particles in the open and closed optical states can be controlled by an applied electric field. For each type of electrically charged pigment particles, the polarizability and size of the particles determine the frequency required for optimal motion. The maximum ICEO velocity for these two types of oppositely charged pigment particles can be achieved by using electric fields comprising AC waveforms of different frequencies. In Example 7 below related to this scenario, the characteristic AC waveform frequency is much higher for the white pigment than for the black pigment. Therefore, as illustrated in the drawing, the black pigment can be switched into the channel using a relatively low AC frequency having an appropriate superimposed DC offset to move the black pigment into the channel of the microcell. Since the black pigment is positively charged, applying a positive offset voltage to the AC voltage of the first light-transmitting electrode layer while the second light-transmitting electrode layer is grounded will move the black pigment into the channel. The AC frequency is relatively low (10 Hz); at this frequency, both the white pigment and the black pigment have strong ICEO-induced lateral motion. When the black pigment is positioned in the channel, the AC frequency is increased to a higher value. At high frequencies, the ICEO-induced motion of the black pigment is reduced, but the motion of the white pigment is maintained. Therefore, the white pigment can be switched into the channel by a negative DC offset to the AC waveform. Both white and black particles are present in the channel (as shown in FIG. 7a). Under certain conditions, the electric field in the second stage may push some of the black pigment particles out of the channel, but will not move the black pigment laterally. Therefore, the black particles that can move out of the channel will undergo vertical motion, providing an open optical state similar to that illustrated in Fig. 8a.

Another approach for reducing haze of a variable light transmission device caused by light scattering effects by electrically charged pigment particles is illustrated in FIG. 12. FIG. 12 illustrates a side view of a variable light transmission device of the present invention comprising a first light-transmitting electrode layer (202), a second light-transmitting electrode layer, and an electrophoretic medium including electrically charged pigment particles (222), wherein the electrically charged pigment particles are light reflective. In this example, the electrophoretic medium comprises one type of electrically charged pigment particles. The variable light transmission device also comprises a first light-blocking layer (1202) on an exposed microcell bottom inner surface of the microcell. The first light-blocking layer (1202) can comprise black pigment particles that are light absorbing. In the open optical state of the device, the electrically charged pigment particles of the electrophoretic medium are contained within the channels of the microcell ( FIG. 12a ). When viewing the device from below, the first light blocking layer (1202) blocks the haze. Viewing from below means that the viewer views the device from the side closer to the second light transmissive electrode layer (207). However, when viewing the device from above, the haze still appears. Viewing from above means that the viewer views the device from the side closer to the first light transmissive electrode layer (202). Fig. 12b illustrates a closed optical state of the device. The variable light transmission device illustrated in Fig. 12 can be used in applications where the viewer is typically positioned on one side of the variable light transmission device, such as, for example, a sunroof.

A variable optical transmission device having improved performance in a closed optical state can also be achieved by using the device illustrated in FIG. 13. The variable optical transmission device illustrated in FIG. 13 includes a first optically transparent electrode layer (202); a second optically transparent electrode layer (207); and a microcell layer (203). The microcell layer is disposed between the first optically transparent electrode layer (202) and the second optically transparent electrode layer (207). The microcell layer includes a plurality of microcells and a sealing layer. Each microcell of the plurality of microcells includes an electrophoretic medium, wherein the electrophoretic medium includes electrically charged pigment particles in a fluid, a charge control agent, and a non-polar liquid. In this example, the electrophoretic medium includes one type of electrically charged pigment particles. Each microcell of the plurality of microcells has a microcell aperture, and the sealing layer spans the microcell apertures of the plurality of microcells. Each microcell of the plurality of microcells comprises a microcell bottom layer, a protrusion structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, and the microcell bottom inner surface comprises an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The protrusion structure has a protrusion base, a protrusion surface, a protrusion apex, and a protrusion height. The protrusion apex is a point or set of points of the protrusion structure that is shorter from the microcell opening than all other points of the protrusion structure, and the protrusion height is a distance between the protrusion base and the protrusion apex. The protrusion surface is a surface of the protrusion structure that does not include the protrusion apex, which contacts the electrophoretic medium. The microcell wall has a microcell inner wall surface, a microcell wall top surface, and a second light blocking layer (1311). The microcell inner wall surface is a surface of the microcell wall of the microcell that contacts the electrophoretic medium. The second light blocking layer (1311) is disposed between the microcell wall top surface and the sealing layer. The channel has a channel height, and the channel height is 50% of the protrusion height. The unexposed microcell bottom inner surface contacts the protruding base. The channel is a volume between the exposed microcell bottom inner surface, the protruding surface, and the microcell inner wall surface. The variable optical transmission device of FIG. 13 can further include a first optical blocking layer (1202), the first optical blocking layer (1202) disposed on the exposed microcell bottom inner surface. As described above, the first optical blocking layer (1202) mitigates haze when the device is viewed from below. The second optical blocking layer (1311) contributes to an improved closed state by increasing the opacity of the device, which may be caused by the partially optically transparent wall material. The second optical blocking layer (1311) can be black, white, or any other color. The second optical blocking layer (1311) can be electrically conductive, which can facilitate switching of the device. The variable light transmitting device of FIG. 13 may further include an auxiliary layer (1312) disposed between the sealing layer and the second light blocking layer (1311). The auxiliary layer (1312) may include an adhesive material. The auxiliary layer (1312) may include a light reflective pigment to further improve the opacity of the closed optical state. The auxiliary layer (1312) may also include an encapsulated electrophoretic layer including an electrophoretic medium including electrically charged pigment particles. Applying an electric field across the encapsulated electrophoretic layer of the auxiliary layer (1312) can switch the color (or image) of the auxiliary layer (1312), which can also affect the appearance of the variable light transmitting device of FIG. 13.

The charge control agent is typically an oligomeric or polymeric material that is soluble in the nonpolar liquid of the electrophoretic medium. The charge control agent is a surfactant type molecule having one or more polar functional groups (heads) and nonpolar portions (tails). The electrophoretic medium can comprise the charge control agent in a concentration of from 0.1 weight percent to 10 weight percent, based on the weight of the electrophoretic medium. The electrophoretic medium can comprise the charge control agent in a concentration of from 0.5 weight percent to 9 weight percent, from 0.7 weight percent to 8 weight percent, from 1 weight percent to 7 weight percent, or from 1 weight percent to 6 weight percent, based on the weight of the electrophoretic medium.

The nonpolar liquid of the electrophoretic medium may include an aliphatic hydrocarbon, an alicyclic hydrocarbon, an aromatic hydrocarbon, a halogenated aliphatic hydrocarbon, a polydimethylsiloxane, or a mixture thereof.

The electrophoretic medium may also include a flocculating agent, also referred to as a depletor. The depletor induces an osmotic pressure difference between the pigment-pigment particles and the pigment particle depletor molecules. As a result, the bistability of the optical states (open and closed) of the device is improved. The depletor is typically a polymeric material such as polyisobutylene and polydimethylsiloxane.

yes

Example 1 : A device was fabricated by co-laminating a polyethylene terephthalate (PET) sheet coated with an indium tin oxide (ITO) transparent conductor with an embossed microcell array on a second PET/ITO sheet comprising an electrophoretic medium. The structure of the device corresponds to the examples of FIGS. 2A to 2D, except that the sealing layer (212) is not incorporated. The structure of the embossed microcell array is illustrated in FIG. 14, which is a plan view of a microcell of the device. FIG. 15 shows a corresponding cross-sectional view of one microcell of the device. Table 1 shows the dimensions of the microcell.

element distance
(micrometer) Cavity pitch: Center-to-center distance between microcells 500 Height of the cylinder (bottom) below the cone protrusion 15.2 Maximum height of particle charge level (above base) 15.2 Minimum wall width on the first substrate 15 Slope angle for walls and cone base 8 　 　 Wall height 50 Wall recess/groove width in the first substrate 10 Wall recess/home slope angle 26.6 Wall recess/groove depth 5 Gap between the top of the projection and the top of the wall 9 Cone slope (degrees) 7.3

Table 1: Device Structure

The electrophoretic medium comprises a white pigment, a hydrocarbon solvent, a charge control agent (CCA) and a depleting agent. In the example, the electrophoretic medium sample was prepared by mixing 10 wt% of white pigment and 5 wt% of charge control agent (cationic charge control agent - CCA111 from Example 1 of US2020/0355978) in Isopar E solvent. The device was switched with a 50 V square wave AC waveform with 50% duty cycle. Three different pigment movement styles were observed by increasing the AC frequency from 0 Hz to 5000 Hz as shown in Fig. 16. The white pigment particles were prepared with a titanium dioxide pigment core and a polymer shell as described in Example 1 of US Pat. No. 8,582,196.

After multiple switching at low frequency of 10 Hz, the white pigment moved towards the edge (near the perimeter) of the microcell (Fig. 16a), probably due to slight pushing along the slope of the cone. Once positioned in the channel, the pigment switched up and down in the vertical direction. At this low frequency of 10 Hz, the motion was dominated by normal electrophoresis. By increasing the frequency to 100 Hz, the pigment tended to spread laterally into the region above the cone in the embossed microcell structure (Fig. 16b). At a frequency of 1000 Hz, the white pigments were completely spread into the circle of the embossed microcell structure, as shown in Fig. 16c. At the frequency of 1000 Hz, the pigment behavior is believed to be dominated by induced-charge lateral motion, probably as a result of ICEO. At a frequency of 5000 Hz, the white pigment particles tended to be oriented toward the center of the near-conical region of the embossed microcellular structures, as shown in Fig. 16d.

Example 2 : In Example 2, the effect of the concentration of charge control agent (CCA) on the movement of white pigments in an embossed microcell device was studied.

As described above, increasing the concentration of charge control agent in the electrophoretic medium would be expected to decrease the Debye length associated with the electrically charged pigment particle surface, and thus increase the frequency required for a particular ICEO flow. To test this hypothesis, three tunable optical transmission devices were fabricated having similar electrophoretic medium but using different charge control agent concentrations. The charge control agent used in this example is the cationic polymer-CCA111 disclosed in Example 1 of US2020/0355978. Three different electrophoretic media were prepared having charge control agent concentrations of 0.1 wt%, 1 wt% and 5 wt%, based on the weight of the electrophoretic medium. Each electrophoretic medium also included 10 wt% white pigment and Isopar E solvent. White pigment particles were prepared using a titanium dioxide pigment core comprising a polymer coating as described in Example 1 of U.S. Patent No. 8,582,196.

The switching performance of three samples in the embossed microcell devices was evaluated (as shown in FIGS. 14 and 15 ). The waveform is a square wave +/-50 V AC with 50% duty cycle. As shown in FIG. 17, as the concentration of the charge control agent increased, the frequency required to reach the closed optical state increased. That is, a frequency of 50 Hz was required to achieve the closed optical state for the electrophoretic medium containing 0.1 wt % CCA, a frequency of 100 Hz was required to achieve the closed optical state for the electrophoretic medium containing 1 wt % CCA, and a frequency of 1000 Hz was required to achieve the closed optical state for the electrophoretic medium containing 5 wt % CCA. The ratios of frequency to CCA concentration for the three experiments were 500 Hz, 1000 Hz, and 1000 Hz, respectively.

Devices having an electrophoretic medium containing 1 wt% CCA could be switched from the open optical state to the closed optical state using (a) a simple square wave AC of +/-100 V at 0.5 Hz, or (b) +/-50 V square wave AC at 50 Hz frequency with a superimposed DC voltage of -50 V. The open optical state was reached by +/-50 V square wave AC with 5% duty cycle, whereas the closed optical state required +/-50 V square wave AC with 95% duty cycle. The switching time in each case was approximately 1 s.

Example 3 : The effect of the charge of white pigment on the switching performance of a variable optical transmittance device was studied.

A tunable optical transmission device having an electrophoretic medium comprising 10 wt% positively charged white pigment particles in Isopar E was fabricated. The white pigment was functionalized with 1.6 wt% silane Z6030 and grafted with polylauryl methacrylate (PLMA). The zeta potential of the treated pigment was +35 mV as titrated with CCA (cationic polymer-CCA111 disclosed in Example 1 of US2020/0355978). The electrophoretic medium also comprised 1 wt% of CCA (cationic polymer-CCA111 disclosed in Example 1 of US2020/0355978). The waveform used to switch the device from open to closed optical state was DC superimposed AC, i.e., square wave waveform, +/-50 V AC at 500 Hz frequency. The waveform for switching the device from the closed optical state to the open optical state was +/-50 V AC with an offset of +50 V DC. Therefore, the behavior of the device of Example 3 was very similar to that of the device having the electrophoretic medium including 1 wt % CCA of Example 2, except that the DC offset required to achieve the open optical state was of opposite polarity.

Example 4 : In this example, the solvent of the electrophoretic medium was matched to the polymer that formed the embossed microcells.

"Haze" refers to the percentage of diffusely transmitted light relative to the total transmitted light. Diffusely transmitted light is light that is scattered as it is transmitted. To fabricate a tunable light transmission device having low haze, it was necessary to match the refractive index of the solvent of the electrophoretic medium liquid and the polymeric material used to fabricate the embossed microcells, as described in U.S. Patent No. 7,327,511.

Typically, solvents used in the electrophoretic medium have a low dielectric constant (preferably less than 10 and preferably less than 3), a low viscosity, a low vapor pressure, and a relatively high refractive index. Examples of solvents include, but are not limited to, aliphatic hydrocarbons such as heptane, octane and petroleum distillates such as Isopar® (Exxon Mobil) or Isane® (Total), terpenes such as limonene, such as 1-limonene, and aromatic hydrocarbons such as toluene. A particularly preferred solvent is limonene due to its combination of a low dielectric constant (2.3) and a relatively high refractive index (1.47). The refractive index of the electrophoretic medium can be adjusted by adding a refractive index matcher. For example, the aforementioned U.S. Patent No. 7,679,814 discloses an electrophoretic medium suitable for use in a variable optical transmission device wherein the nonpolar liquid of the electrophoretic medium comprises a mixture of partially hydrogenated aromatic hydrocarbons and terpenes, a preferred mixture being d-limonene and partially hydrogenated terphenyls, which is commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009.

To reduce haze, it is desirable that the refractive index of the encapsulating electrophoretic medium closely matches the refractive index of the encapsulating material. In most cases, it is advantageous to use an electrophoretic medium having a refractive index of 1.51 to 1.57 at 550 nm, preferably about 1.54 at 550 nm.

Example 4A : A tunable optical transmission device comprising microcells having an electrophoretic medium comprising 5 wt% of white pigment and 1 wt% of CCA (cationic polymer-CCA111 disclosed in Example 1 of US2020/0355978) in Cargille® 5040 and Isopar E solvent was fabricated. The waveform used was a square AC with an amplitude of +/- 50 V and a frequency of 10 Hz. To reach the closed optical state, a 5% duty cycle was used. To reach the open optical state, a 95% duty cycle was first employed and then the duty cycle was changed to 50%.

Example 4B : Another tunable optical transmission device comprising microcells was fabricated using an electrophoretic medium having a composition similar to Example 4A and further comprising polyisobutylene (PIB) as a depleting agent. The depleting agent is used to improve the bistability of the device, i.e., to maintain open and closed optical states when no electric field is applied to the device. The electrophoretic medium comprises 10 wt % white pigment, 1 wt % CCA (cationic polymer-CCA111 disclosed in Example 1 of US2020/0355978), 0.5 wt % polyisobutylene in Cargille® 5040, and Isopar E. solvent. The waveform used was +/- 50 V square AC with a frequency of 10 Hz. To achieve the closed optical state, 5% duty cycle alternated with 50% duty cycle was used. To reach the open optical state, a 95% duty cycle was employed, followed by a 50% duty cycle. The time required for complete switching was approximately 20 seconds. This time is significantly longer than the time required to switch a non-index-matched solvent containing no depleting agent (Example 4A).

Example 5 : Light-blocking layer containing black particles

A tunable transmission device is fabricated by co-laminating a PET sheet coated with an ITO transparent conductor with an embossed microcell array on a second PET/ITO sheet, wherein the embossed microcell array comprises an electrophoretic medium. The structure of the embossed microcell array is illustrated in FIGS. 2A to 2D , but the microcells in these examples did not include a sealing layer.

The light blocking composition used in the light blocking layer included a black pigment, a solvent, a charge control agent, and a depleting agent. In this example, the light blocking composition was prepared by mixing 10 wt % black pigment, 1 wt % CCA (cationic charge control agent - CCA111 from Example 1 of US2020/0355978), and 0.5 wt % polyisobutylene into a solvent mixture of partially hydrogenated terphenyl, limonene, Isopar M, and Isopar E, commercially available as Cargille® 5040 from Cargille-Sacher Laboratories (55 Commerce Rd, Cedar Grove N.J. 07009). The black pigment particles have a core comprising black iron oxide (Pigment Black 11) and a polymeric shell. As illustrated in FIG. 18 , the black pigment particles were dispersed throughout the entire electrophoretic medium when the device was fabricated. A 0.5 Hz, 50 V square wave waveform with a +50 V offset (i.e., switching between +100 V and 0 V) and 50% duty cycle was applied to the first optically transparent electrode layer, while the second optically transparent electrode layer was maintained at 0 V. As shown in Fig. 19, the electric field between the two electrodes induced electrophoresis to migrate the positive black pigment to the unexposed microcell bottom surface (inside the channel). The black pigment remained on the unexposed microcell bottom surface even after the voltage was released. The PET/ITO first electrode was then peeled from the device to allow the film to evaporate.

Example 6 : A tunable optical transmission device was prepared as in Example 5, except that the light blocking composition was prepared by mixing 10 wt% of white pigment and 5 wt% of CCA (cationic charge control agent - CCA111 from Example 1 of US2020/0355978) in an Isopar E solvent. The device readily switched between a closed optical state (0 V offset, FIG. 20 ) and an open optical state (-50 V offset, FIG. 21 ) under a 50 Hz/50 V square wave.

Example 7 : Switching of black and white pigments of opposite charge through channels of embossed microcells

A tunable transmission device was fabricated by co-laminating a PET sheet coated with an ITO transparent conductor with an embossed microcell array containing an electrophoretic medium on a second PET-ITO sheet. The structure of the microcell corresponds to the examples of FIGS. 2A to 2D, except that the device does not include a sealing layer. The structure of the embossed microcell array is illustrated in FIGS. 14 and 15.

First, an electrophoretic composition containing a black pigment was prepared and switched to the open optical state (black pigment in the channels). The first optically transparent electrode layer was then removed and the solvent was evaporated. After evaporation of the solvent, an electrophoretic medium containing a white pigment, solvent and a charge control agent (CCA) was prepared. In this example, the electrophoretic medium was prepared by mixing 10 wt % white pigment and 1 wt % CCA111 into a solvent mixture of partially hydrogenated terphenyl, limonene, Isopar M and Isopar E, commercially available as Cargille® 5040 from Cargille-Sacher Laboratories (55 Commerce Rd, Cedar Grove N.J. 07009). The white pigment particles were prepared using a titanium dioxide pigment core including a polymer coating as described in Example 1 of U.S. Pat. No. 8,582,196.

The electrophoretic medium was assembled into a microcell device, and the black pigment was switched into the channel and the solvent was evaporated. Accordingly, the device contained both white and black pigment particles of opposite charge. Both pigments could be switched electrophoretically. To switch both pigments into the channel, first, the black pigment was switched into the channel using a positive DC offset at a relatively low frequency AC voltage, and then the white pigment was switched into the channel using a negative DC offset at a relatively high frequency AC voltage.

In the first stage, a 10 Hz, 50 V square wave waveform with +2 V offset and 50% duty cycle was applied to the first light-transmitting electrode layer (202), while the second light-transmitting electrode layer was held at 0 V. The electric field between the electrodes induced electrophoresis, which superimposed the induced-charge electro-osmosis, to move the positive black pigment into the channel. In the second stage, a 500 Hz, 50 V square wave waveform with alternating +2 V and -2 V offsets and 50% duty cycle was applied to the first electrode, while the second electrode was held at 0 V. At higher frequencies (500 Hz versus 10 Hz in the first stage), the lateral motion of the black pigment was less than that of the white pigment. That is, the white pigment moved into the channel during the phase of the waveform with the negative DC offset. As shown in Figure 22, both white and black pigments were switched into the channels of the microcells.

200 A variable transmission device; 201 A first light-transmitting substrate; 202 A first light-transmitting electrode layer; 203 A microcell layer; 204 A plurality of microcells; 205 A microcell aperture; 206 A sealing layer; 207 A second light-transmitting electrode layer; 208 A second light-transmitting substrate; 209 An electrophoretic medium; 210 A microcell bottom layer; 211 A microcell bottom inner surface; 211a An exposed microcell bottom inner surface; 211b An unexposed microcell bottom inner surface; 212; A micro-wall; 213 A microcell inner wall surface; 214 A microcell wall top surface; 125 A channel; 216 A channel height; 217 A protrusion structure; 218 A protrusion base; 219 A protrusion apex; 220 A protrusion height; 221 A protrusion surface; 222 Electrically charged pigment particles; 222a first type of electrically charged pigment particles; 222b second type of electrically charged pigment particles; 223a first type of electrically charged pigment particles having an opposite charge polarity to the second type of electrically charged pigment particles (223b); 223b second type of electrically charged pigment particles having an opposite charge polarity to the first type of electrically charged pigment particles (223a); 1202 first light-blocking layer; 1311 second light-blocking layer; 1312 auxiliary layer.

Claims (20)

In a variable light transmission device (200),
First light-transmitting electrode layer (202);
a second light-transmitting electrode layer (207); and
A microcell layer (203) is included, wherein the microcell layer (203) is positioned between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207), and the microcell layer (203) includes a plurality of microcells (204) and a sealing layer (206).
Each microcell of the plurality of microcells (204) includes an electrophoretic medium (209), wherein the electrophoretic medium (209) includes electrically charged pigment particles of a first type, electrically charged pigment particles of a second type, a charge control agent, and a non-polar liquid,
Each microcell of the plurality of microcells (204) has a microcell opening (205), and the sealing layer (206) spans the microcell openings (205) of the plurality of microcells (204).
Each microcell of the plurality of microcells (204) includes a microcell bottom layer (210), a protruding structure (217), a microcell wall (212), and a channel (215).
The above microcell bottom layer (210) has a microcell bottom inner surface (211), and the microcell bottom inner surface (211) includes an exposed microcell bottom inner surface (211a) and an unexposed microcell bottom inner surface (211b).
The above protrusion structure (217) has a protrusion base (218), a protrusion surface (221), a protrusion apex (219), and a protrusion height (220), wherein the protrusion apex (219) is a point or a set of points of the protrusion structure (217), wherein the point or the set of points has a shorter distance from the microcell opening (205) than all other points of the protrusion structure (217), the protrusion height (220) is a distance between the protrusion base (218) and the protrusion apex (219), and the protrusion surface (221) is a surface of the protrusion structure (217) that does not include the protrusion apex and is in contact with the electrophoretic medium (209).
The above microcell wall (212) has a microcell inner wall surface (213) and a microcell wall top surface (214), the microcell inner wall surface (213) is a surface of the microcell wall (212) of the microcell that contacts the electrophoretic medium (209), and the microcell wall top surface (214) is a surface of the microcell wall (212) of the microcell that contacts the sealing layer (206).
The above channel (215) has a channel height (216), and the channel height (216) is 50% of the protrusion height (220).
The inner surface (211b) of the above-mentioned non-exposed microcell bottom is in contact with the above-mentioned protruding base (218),
A variable light transmission device, wherein the above channel (215) is a volume between the exposed microcell bottom inner surface (211a), the protruding surface (221) and the microcell inner wall surface (213). In claim 1,
A variable light transmission device, wherein the first type of electrically charged pigment particles are light reflective and the second type of electrically charged pigment particles are light absorbing. In claim 1 or claim 2,
A variable light transmission device, wherein the electrically charged pigment particles of the first type are white. In any one of claims 1 to 3,
A variable light transmission device, wherein the second type of electrically charged pigment particles are black. In any one of claims 1 to 4,
A variable light transmission device, wherein the first type of electrically charged pigment particles have the same charge polarity as the second type of charged pigment particles. In any one of claims 1 to 5,
A variable light transmission device, wherein the first type of electrically charged pigment particles and the second type of electrically charged pigment particles are positive, and the zeta potential of the first type of electrically charged pigment particles is lower than the zeta potential of the second type of electrically charged pigment particles. In any one of claims 1 to 6,
A variable light transmission device, wherein the first type of electrically charged pigment particles and the second type of electrically charged pigment particles are negative, and the zeta potential of the first type of electrically charged pigment particles is higher than the zeta potential of the second type of electrically charged pigment particles. In any one of claims 1 to 4,
A variable light transmission device, wherein said first type of electrically charged pigment particles have a charge polarity opposite to that of said second type of charged pigment particles. In claim 8,
A variable light transmission device, wherein the first type of electrically charged pigment particles are negatively charged and the second type of charged pigment particles are positively charged. In any one of claims 1 to 9,
A variable light transmission device, wherein the electrically charged pigment particles of the first type have an average particle size greater than an average particle size of the charged pigment particles of the second type. In any one of claims 1 to 10,
A variable optical transmittance device, wherein the content of the charge control agent in the electrophoretic medium is 1 weight percent to 8 weight percent based on the weight of the electrophoretic medium. In any one of claims 1 to 11,
A variable optical transmittance device, wherein the molecular structure of the charge control agent comprises a quaternary ammonium functional group and a nonpolar tail. In claim 1,
A variable optical transmission device, wherein the nonpolar liquid of the electrophoretic medium comprises a material selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, polydimethylsiloxane, or mixtures thereof. In any one of claims 1 to 13,
The above protruding structure is, (a) a cone, (b) a cone on a cylinder - the cylinder has a base, and the base of the cylinder is a protruding base of the protruding structure - , (c) a tetrahedron, (d) a tetrahedron on a triangular prism - the triangular prism has a triangular base, and the triangular base is a protruding base of the protruding structure - , (e) a triangular prism - the triangular prism has a square base, and the square base is a protruding base of the protruding structure - , (f) a square pyramid having a square base - the square base is a protruding base of the protruding structure - , (g) a square pyramid on a regular rectangular cuboid - the rectangular cuboid has a square base, and the square base is a protruding base of the protruding structure - , (h) a square pyramid on a rectangular parallelepiped - the rectangular parallelepiped has a right parallelogram base, and the right parallelogram is a protruding base of the protruding structure - , A variable light transmitting device, wherein the geometric solid is selected from the group consisting of: (i) a pentagonal pyramid, wherein the pentagonal pyramid has a pentagonal base, wherein the pentagonal base is a protruding base of the protruding structure; (j) a pentagonal pyramid on a pentagonal prism, wherein the pentagonal pyramid has a pentagonal base, wherein the pentagonal base is a protruding base of the protruding structure; (k) a hexagonal pyramid, wherein the hexagonal pyramid has a hexagonal base, wherein the hexagonal base is a protruding base of the protruding structure; (l) a hexagonal pyramid on a hexagonal prism, wherein the hexagonal prism has a hexagonal base, wherein the hexagonal base is a protruding base of the protruding structure; (n) a polygonal pyramid, wherein the polygonal pyramid has a polygonal base, wherein the polygonal base is a protruding base of the protruding structure; (o) a polygonal pyramid on a polygonal prism, wherein the polygonal pyramid has a polygonal base, wherein the polygonal base is a protruding base of the protruding structure. In claim 1,
A variable light transmitting device, wherein the protruding structure is a cone in the shape of a cylinder, the cylinder has a bottom, the bottom of the cylinder is the protruding base of the protruding structure, and the cone has an inclination of 5 to 20 degrees. In any one of claims 1 to 14,
A variable light transmitting device, wherein the above protruding structure is a geometric solid of a pyramid having a base with n sides, the base with n sides being the protruding base of the protruding structure, and n is an integer from 7 to 12, and (m) a pyramid having a base with n sides on a prism having a base with n sides, the base of the prism having n sides being the protruding base of the protruding structure, and n is 7 to 12. In any one of claims 1 to 16,
A variable optical transmission device, wherein switching of the variable optical transmission device to an open optical state occurs as a result of applying a first electric field between the first optically transparent electrode layer and the second optically transparent electrode layer via a first waveform. In any one of claims 1 to 17,
A variable optical transmission device, wherein application of a second electric field between said first optically transparent electrode layer and said second optically transparent electrode layer via a second waveform causes movement of said first type of electrically charged pigment particles towards said first optically transparent electrode layer at a velocity having a lateral component, resulting in a closed optical state, wherein said second waveform comprises at least one positive voltage and at least one negative voltage, wherein said second waveform has a pure positive or pure negative impulse, and wherein said closed optical state has lower optical transmission than said open optical state. In claim 18,
A variable optical transmission device, wherein the second waveform comprises an AC waveform, and the AC waveform has a duty cycle of 5% to 45%. In claim 18,
A variable optical transmission device, wherein the second waveform includes a DC offset waveform formed by superimposing a DC voltage component and an AC waveform.

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