GB2586534A - Acousto-optical device - Google Patents

Abstract

An acousto-optical device comprising a composite of liquid crystalline (LC) capsules dispersed in a binder material, whereby an ultrasound waves are applied to the composite. The acousto-optical device may be an acousto-optical sensor for non-destructive testing (NDT) utilising the acousto-optic effect. The liquid crystal (LC) capsule may comprise: a polymeric shell; a core containing mesogenic medium; a binder selected from water-soluble polymers or waterborne latex binders, where the binders may also coprise polyvinyl alcohol. The capsule may have a size in the range of 1 nano-metre (1 x 10^-9 metres) to 10 micro-metres (1 x 10^-6 metres) [1nm – 10µm], with a film thickness in the range of 0.5 micro-metres (5 x 10^-7 metres) to 50 micro-metres (5 x 10^-5 metres) [0.5µm – 50µm].

Description

Acousto-optical device Ultrasounddiagnostic tools are part of non-destructive testing (NDT) techniques for inspection of materials and defect analysis in components.

Ultrasound diagnostics has recently been demonstrated in polymer dispersed liquid crystal (PDLC) films by 0. Trushkevych, T. J. R. Eriksson, S. N. Ramadas, S. Dixon, and R. S. Edwards in Appl. Phys. Lett. 107, 054102 (2015). The PDLC films are used to image two displacement profiles of air-coupled flexural transducer's resonant modes at 295 kHz and 730 kHz and the results are further verified using laser vibrometry. In this regard, the regions on the transducers with the largest displacement are clearly imaged by the PDLC films, with the resolution corroborating well with laser vibrometry scanning technique.

However, vibration sensing by using laser vibrometry techniques or laser doppler displacement, as described above, requires point by point scanning which may need several hours to produce a complete image profile of the subject under study, especially if a large area structure is being studies. Moreover, the above described method suffers from limited processability of the utilized PDLC film, which restricts its use on less-accessible surfaces.

Furthermore, laser vibrometry uses relatively large equipment that require space and thus smaller, inaccessible spaces that may require vibration inspection cannon be satisfied by this approach. In contrast to that, nanocapsule approach employes ultra thing layers of acousto-optically switchable materials which do not require large footprints".

Consequently, there is a need in the art for acousto-optical devices with improved, and optionally tunable, acousto-optical and physical properties.

Moreover, there exists a need for an more holistic non-destructive testing (NOT) technique for inspecting a wide range of materials to determine their structural status.

An object of the present invention is therefore to provide an improved holistic acousto-optical device or sensor or non-destructive testing (NOT) -2 -technique; utilizing the acousto-optic effect in liquid crystal device structures.These new acousto-optic devices additionally offer tunablity in both frequency and material physical properties such as Can be tuned by the LC composition and properties, capsule size, binder type, film thickness.

Further objects of the present invention are immediately evident to the person skilled in the art from the following detailed description.

The objects are solved by the subject-matter defined in the independent claims, while preferred embodiments are set forth in the respective dependent claims and are further described below. The present invention provides the following items including main aspects, preferred embodiments and features, which respectively alone and in combination contribute to solving the above object and eventually provide additional advantages.

A first aspect of the present invention provides an acousto-optical device, preferably an acousto-optical sensor, for non-destructive testing (NDT) techniques utilizing the acousto-optic effect, comprising a composite of liquid crystalline (LC) capsules dispersed in a binder material, whereby ultrasound is applied to the composite.

Ultrasound in the context of the instant application is defined as a sound at frequencies greater than 20 kHz. In air at atmospheric pressure, ultrasonic waves have wavelengths of 1.73 cm or less.

Suitable capsules comprise a polymeric shell, and a core containing a mesogenic medium and can preferably be obtained or are obtainable from a composition for encapsulation, wherein the composition comprises (i) a mesogenic medium which comprises one or more compounds of formula I R-A-Y-A'-R' -3 -wherein R and R' denote, independently of one another, a group selected from F, CF3, OCF3, CN, and straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by ON or 0F3 or mono-or polysubstituted by halogen, preferably F, and wherein one or more CH2groups may be, in each case independently of one another, replaced by -0-, -S-, -CO-, -000-, -000-, -0000-or -CEC-in such a manner that oxygen atoms are not linked directly to one another, A and A' denote, independently of one another, a group selected from -Cyc-, -Phe-, -Cyc-Cyc-, -Cyc-Phe-, -Phe-Phe-, -Cyc-Cyc-Cyc-, -Cyc-Cyc-Phe-, -Cyc-Phe-Cyc-, -Cyc-Phe-Phe-, -Phe-Cyc-Phe-, -Phe-Phe-Phe-and the respective mirror images thereof, wherein Cyc is trans-1,4-cyclohexylene, in which one or two non-adjacent 0H2 groups may be replaced by 0, and wherein Phe is 1,4-phenylene, in which one or two non-adjacent CH groups may be replaced by N and which may be substituted by one or two F, and denotes single bond, -000-, -CH2CH2-, -CF2CF2-, -CH20-, -CF20-, -CH=CH-, -CF=CF-or -CEC-, (ii) one or more polymerizable compounds, and 30 (iii) one or more surfactants.

A typically applied process for the production comprises thereby preferably the steps of -4 - (a) providing an aqueous mixture which comprises the composition as described above, (b) agitating, preferably mechanically agitating, the provided aqueous mixture to obtain nanodroplets comprising the composition, and the mesogenic medium, according to the invention dispersed in an aqueous phase, and (C) subsequent to step (b) polymerizing one or more polymerizable compounds according to the invention to obtain nanocapsules each comprising a polymeric shell and a core which contains the mesogenic medium as set forth above and below.

In a preferred embodiment, also other processes utilizing commonly known interfacial polymerisation techniques like it is disclosed in US 2016/0178941 Al or methods utilizing the "Ouzo-Effect" comprising the steps of (a) dissolving 0.01 to 5% w/w of a LC mixture comprising one or more mesogenic compounds of formula I and one or more polymerizable compounds, in 0.5 to 75 % w/w of a fully water-miscible organic solvent, (b) mixing the organic solution of step (a) with 25 to 99% w/w of water, and optionally, (c) depleting, removing or exchanging the aqueous phase, or even combinations of both methods can be applied.

In a preferred embodiment the one or more polymerizable compounds are selected from vinylchloride, vinylidenechloride, acrylnitriles, methacrylnitriles, acrylamides, methacrylamides, methyl-, ethyl-, n-or tert.-butyl-, cyclohexyl-, 2-ethylhexyl-, phenyloxyethyl-, hydroxyethyl-, hydroxypropyl-, 2-5 C-alkoxyethyl-, tetrahydrofurfurylacrylates or methacrylates, vinylacetates, -propionates, -acrylates, -succinates, N-vinylpyrrolidones, N-vinylcarbazoles, styrenes, divinylbenzenes, -5 -ethylenediacrylates, 1,6-hexanediolacrylates, bisphenol-A-diacrylates and -dimethacrylates, trimethylylpropanediacrylates, trimethylolpropanetriacrylates, pentaerythrittriacrylates, triethyleneglycoldiacrylates, ethyleneglycoldimethacrylates, tripropyleneglycoltriacrylates, pentaerythritoltriacrylates, pentaerythritoltetraacrylates, ditrimethylpropanetetraacrylates or dipentaerythritolpenta-or hexaacrylates. Also thiol-enes are preferred such as, for example, the commercially available product Norland 65 (Norland Products). It is also possible to use silane-based or siloxane-based reactive monomers.

The polymerizable or reactive group is preferably selected from a vinyl group, an acrylate group, a methacrylate group, a fluoroacrylate group, an oxetane group or an epoxy group, especially preferably an acrylate group or a methacrylate group.

Preferably the one or more polymerizable compounds are selected from acrylates, methacrylates, fluoroacrylates and vinyl acetate, wherein the composition more preferably further comprises one or more direactive and/or trireactive polymerizable compounds, preferably selected from diacrylates, dimethacrylates, triacrylates and trimethacrylates. In a preferred embodiment one or more polymerizable compounds of the polymerizable compounds are fluorinated, wherein particularly preferably the acrylate compounds and the methacrylate compounds are fluorinated acrylates and fluorinated methacrylates.

In an embodiment the one or more polymerizable compounds (ii) as set forth above comprise polymerizable groups selected from one, two or more acrylate, methacrylate and vinyl acetate groups, wherein the compounds preferably are non-mesogenic compounds.

In a preferred embodiment the composition according to the invention comprises one or more monoacrylates, preferably added in an amount, based on the overall composition, from 0.1% by weight to 75% by weight, more preferably from 0.5% by weight to 50% by weight, in particular from 2.5% by weight to 25% by weight. Particularly preferred monoreactive -6 -compounds are selected from methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, nonyl acrylate, 2-ethyl-hexyl acrylate, 2-hydroxy-ethyl acrylate, 2-hydroxy-butyl acrylate, 2,3-dihydroxypropyl acrylate, hexafluoroisopropylacrylate, 1,1-dihydroperfluoropropyl acrylate, perfluorodecylacrylate, pentafluoropropylacrylate, heptafluorobutylacrylate, 1H,1H,2H,2H-perfluorodecylacrylate, 3-tris(trimethylsiloxy)silylpropyl acrylate, stearylacrylate and glycidyl acrylate.

Additionally or alternatively vinyl acetate may be added.

In another preferred embodiment the composition according to the invention comprises, optionally in addition to the above monoacrylates, one or more monomethacrylates, preferably added in an amount, based on the overall composition, from 0.1% by weight to 75% by weight, more preferably from 0.5% by weight to 50% by weight, in particular from 2.5% by weight to 25% by weight. Particularly preferred monoreactive compounds are selected from methyl methacrylate, ethyl methacrylate, propyl methacrylate, ispropyl methacrylate, butyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, nonyl methacrylate, 2-ethyl-hexyl methacrylate, 2-hydroxy-ethyl methacrylate, 2-hydroxy-butyl methacrylate, 2,3-dihydroxypropyl methacrylate, hexafluoroisopropylmethacrylate, 1,1-dihydroperfluoropropyl methacrylate, perfluorodecylmethacrylate, pentafluoropropylmethacrylate, heptafluorobutylmethacrylate, 1H,1H,2H,2H-perfluorodecylmethacrylate, 3-tris(trimethylsiloxy)silylpropyl methacrylate, stearylmethacrylate, glycidyl methacrylate, adamantyl methacrylate and isobornyl methacrylate.

It is particularly preferred that at least one crosslinking agent is added to the composition, i.e. a polymerizable compound containing two or more polymerizable groups. Crosslinking of the polymeric shell in the prepared particle can provide additional benefits, especially with respect to further improve stability and containment, and to tune or respectively reduce susceptibility to swelling, in particular swelling due to solvent. In this respect direactive and multireactive compounds can serve to form polymer -7 -networks of their own and/or to crosslink polymer chains formed substantially from polymerizing monoreactive compounds.

Conventional crosslinkers known in the art can be used. It is particularly preferred to additionally provide direactive or multireactive acrylates and/or methacrylates, preferably added in an amount, based on the overall composition, from 0.1% by weight to 75% by weight, more preferably from 0.5% by weight to 50% by weight, in particular from 2.5% by weight to 25% by weight. Particularly preferred compounds are selected from ethylene diacrylate, propylene diacrylate, butylene diacrylate, pentylene diacrylate, hexylene diacrylate, glycol diacrylate, glycerol diacrylate, pentaerythritol tetraacrylate, ethylene dimethacrylate, also known as ethyleneglycol dimethacrylate, propylene diamethcrylate, butylene dimethacrylate, pentylene dimethacrylate, hexylene dimethacrylate, tripropylene glycol diacrylate, glycol dimethacrylate, glycerol dimethacrylate, trimethylpropane trimethacrylate and pentaerythritol triacrylate.

The ratio of monoreactive monomers and di-or multireactive monomers can be favourably set and adjusted to influence the polymer make-up of the shell and its properties.

The composition according to the invention comprises one or more surfactants. In an embodiment, the surfactant(s) can be prepared or provided separately in an initial step, and then added to the other components. In particular, the surfactant(s) can be prepared or provided as an aqueous mixture or composition, which is then added to the other components comprising the mesogenic medium and the polymerizable compound(s) as set forth above and below. Particularly preferably, the one or more surfactants are provided as aqueous surfactant(s).

The surfactant(s) can be useful in lowering the surface or interfacial tension and in promoting emulsifying and dispersion.

Conventional surfactants known in the art can be used, including anionic surfactants, for example sulfate, e.g. sodium lauryl sulfate, sulfonate, -8 -phosphate and carboxylate surfactants, cationic surfactants, for example secondary or tertiary amine and quaternary ammonium salt surfactants, zwitterionic surfactants, for example betaine, sultaine and phospholipid surfactants, and nonionic surfactants, for example long chain alcohol and phenol, ether, ester or amide nonionic surfactants.

In a preferred embodiment according to the invention nonionic surfactant is used. The use of nonionic surfactant can provide benefits during the process of preparing the nanocapsules, in particular with respect to dispersion formation and stabilization as well as in PIPS. It is furthermore recognized that it can be advantageous to avoid charged surfactants in case surfactant, for example residual surfactant, is comprised in the formed nanocapsules. The use of nonionic surfactant and the avoidance of ionic surfactant can thus be beneficial in terms of stability, reliability and the electro-optical characteristics and performance of the nanocapsules, also in the composite system and electro-optical devices.

Particular preference is given to polyethoxylated nonionic surfactant. Preferable compounds are selected from the group of polyoxyethylene glycol alkyl ether surfactants, polyoxypropylene glycol alkyl ether surfactants, glucoside alkyl ether surfactants, polyoxyethylene glycol octylphenol ether surfactants such as Triton X-100, polyoxyethylene glycol alkylphenol ether surfactants, glycerol alkyl ester surfactants, polyoxyethylene glycol sorbitan alkyl ester surfactants such as polysorbate, sorbitan alkyl ester surfactants, cocamide monoethanol-amine, cocamide diethanolamine and dodecyldimethylamine oxide.

In a particularly preferred embodiment the used surfactant(s) is (are) selected from polyoxyethylene glycol alkyl ether surfactants, which comprise commercially available Brij® agents. Particular preference is given to a surfactant which comprises, more preferably consists of, tricosaethylene glycol dodecyl ether. In a very particularly preferred embodiment the commercially available Brij® L23 (Sigma-Aldrich), also referred to as Brij 35 or polyoxyethylene (23) lauryl ether, is used. -9 -

Preferably, surfactant is provided in the composition in an amount, based on the overall composition, of less than 25% by weight, more preferably less than 20% by weight, and in particular less than 15% by weight.

When, in accordance with a preferred embodiment, the surfactant is provided as a prepared aqueous mixture, the amount of water is not considered to contribute to the overall composition in terms of weight, i.e. water is excepted in this respect.

Also in the process for preparing the nanocapsules according to the invention polymeric surfactants or surface active polymers or block copolymers can be used.

In a particular embodiment the use of such polymeric surfactants or surface active polymers is however avoided.

According to an aspect of the invention polymerizable surfactant, i.e. surfactant comprising one or more polymerizable groups, can be used.

Such polymerizable surfactant can be used alone, i.e. as the only surfactant provided, or in combination with non-polymerizable surfactant. In an embodiment, a polymerizable surfactant is provided in addition and in combination with a non-polymerizable surfactant. This optional provision of polymerizable surfactant can provide the combined benefits of contributing to suitable droplet formation and stabilization as well as to the formation of stable polymeric capsule shells. Therefore, these compounds act at the same time as surfactant and polymerizable compound. Particular preference is given to polymerizable nonionic surfactants, in particular to nonionic surfactants which additionally have one or more acrylate and/or methacrylate groups. This embodiment which includes the use of polymerizable surfactant can have an advantage in that the template properties at the amphiphilic interface may be particularly well preserved during polymerization. Furthermore, the polymerizable surfactant may not only take part in the polymerization reaction, but may be favourably incorporated as a building block into the polymer shell, and more preferably also at the shell surface such that it may advantageously -10 -influence the interface interactions. In a particularly preferred embodiment silicone polyether acrylate is used as polymerizable surfactant, more preferably cross-linkable silicone polyether acrylate. It is also possible to add poly(ethylene glycol) methyl ether methacrylate.

In a preferred embodiment, the composition according to the invention is provided as an aqueous mixture, wherein more preferably the composition comprising the components (i), (ii) and (Hi) are dispersed in an aqueous phase. In this respect the provided surfactant(s) can favourably contribute to form and stabilize the dispersion, in particular emulsion, and to promote homogenization.

In case aqueous mixtures are provided, the amount of water is not considered to contribute to the overall composition in terms of weight, i.e. water is excepted in this respect.

Preferably water is provided as purified water, in particular deionized water.

In a particularly preferred embodiment the composition according to the invention is provided as nanodroplets dispersed in an aqueous phase.

The composition may contain additional compounds such as one or more pleochroic dyes, in particular dichroic dye(s), one or more chiral compounds and/or other customary and suitable additives.

Pleochroic dyes preferably are dichroic dyes and can be selected from for example azo dyes and thiadiazol dyes.

Suitable and preferred LC capsules and corresponding processes are known from WO 2017/178419 Al which content is incorporated by reference.

Preferably, the utilized LC capsules have an average size, as determined by dynamic light scattering analysis, in the range of 1 nm to 10 pm, more preferably in the range 10 nm to 5 pm and most preferably in the range of nm to 3 pm. Dynamic light scattering (DLS) is a commonly known technique which is useful for determining the size as well as the size distribution of particles in the submicron region. For example, a commercially available Zetasizer (Malvern) may be used for the DLS analysis.

If the capsule size becomes very small, in particular approaching the molecular size of the LC molecules, the functionality of the capsules may become less efficient, considering that the amount of enclosed LC material decreases and also the mobility of the LC molecules becomes more limited.

The thickness of the polymeric shell or respectively wall, which forms a discrete individual structure, is chosen such that it effectively contains and stably confines the contained LC medium, while at the same time allowing for relative flexibility and still enabling excellent acousto-optical responsiveness of the LC material.

Typically, the capsule shell or wall thickness is below 100 nm. Preferably, the polymeric shell has a thickness of less than 50 nm, more preferably below 25 nm, and in particular below 15 nm. In a preferred embodiment, the polymeric shell has a thickness from 1 nm to 15 nm, more preferably form 3 nm to 10 nm, and in particular from 5 nm to 8 nm.

Microscopy techniques, in particular SEM and TEM can be used to observe the capsule size, structure and morphology. Wall thickness can e.g. be determined by TEM on freeze-fractured samples. Alternatively, neutron scattering techniques may be used. Moreover, for example AFM, NMR, ellipsometric and sum-frequency generation techniques can be useful to study the nanocapsule structure.

The capsules utilized in an acousto-optical device according to the invention typically have spherical or spheroidal shape, wherein the hollow spherical or spheroidal shells are filled with or respectively contain the LC medium.

-1 2 -The acousto-optical device according to the present invention thus comprises a plurality of discrete spherical or spheroidal bodies or particles of LC which are each encapsulated by a polymeric shell and which each individually but also collectively are operable in acousto-optical devices in at least two states.

In a preferred embodiment the utilized LC medium has a birefringence of An 0.15, more preferably 0.20 and most preferably 0.25.

Surprisingly, by suitably providing and setting the birefringence, even the small volume of LC is sufficient to effectively and efficiently modulate light, wherein ultrasound can be used to effect or respectively change alignment of the LC molecules in the capsules as it will be described below.

By obtaining substantially uniform capsule sizes, i.e. a low polydispersity can be achieved, which can favourably provide a uniform acousto-optical performance of the capsules in acousto-optical devices of the present invention. Moreover, the capsules obtained by or respectively obtainable from the controlled and adaptable process according to WO 2017/178419 Al can be adjusted and tuned in terms of capsule size, which in turn allows to tune the acousto-optical performance as desired.

For an acousto-optical device in accordance with the present invention the discrete LC capsules are mixed with a binder material, wherein the mixed LC capsules substantially maintain, preferably fully maintain, their integrity in the composite while however being bound, held or mounted in the binder.

In this respect, the binder material can be selelcted from the same material as the polymeric shell material or a different material. Therefore, the capsules can be dispersed in a binder made from the same material as or a different material from that of the capsule shell. Preferably, the selected binder is a different or at least modified material.

The binder can be useful in that it can disperse the LC capsules, wherein the amount or concentration of the capsules can be set and adjusted.

-13 -Typically, the LC capsules are contained in the composite in a proportion from about 2% by weight to about 95% by weight. Preferably, the composite contains the LC capsules in a range from 10% by weight to 85% by weight, more preferably from 30% by weight to 70% by weight, even more preferably in the range from 40 to 68% by weight and in particular in the range from 50 to 65% by weight.

The binder material can furthermore improve or influence the coatabilty or printability of the LC capsules and the film forming ability and performance. The binder preferably exhibits suitable and adequate transparency to visible spectrum. In the context of the present application, the visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light, VIS light or simply light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. as defined above.

Preferably, the binder is a polymeric material. Suitable materials may be synthetic resins such as, for example, epoxy resins and polyurethanes which, for example, are thermally curable.

Furthermore, vinyl compounds and acrylates, in particular polyvinyl acrylates and polyvinyl acetates may be used. Furthermore, polymethyl methacrylate, polyurea, polyurethane, urea formaldehyde, melamine formaldehyde, melamine urea formaldehyde can be used or added.

It is also possible to use thiol-ene based systems or waterborne binders, such as, waterborne latex binders based on polyurethane, polyacrylate, which both also might be crosslinkable. An example for thiol-ene based systems is the commercially available thiol-ene based system Norland Optical Adhesive 65 (Norland Products).

Particularly preferably water-soluble polymers are used, such as, for example, polyvinyl alcohol (PVA), starch, carboxyl methyl cellulose, methyl cellulose, ethyl cellulose, polyvinyl pyrrolidine, gelatin, alginate, casein, -14 -gum arabic, or latex-like emulsions. The binder can for example be chosen in view of setting the respective hydrophobicity or hydrophilicity.

By using water soluble binder, it is possible to easily remove the coated layer from the surface after testing by washing with water or aquoes solutions.

In a particularly preferred embodiment the one or more binders comprise polyvinyl alcohol, which includes partially and fully hydrolyzed PVA.

Favourably, water solubility and hydrophilicity can be adjusted by varying the degree of hydrolysis. Thus water uptake may be controlled or reduced. The properties, such as mechanical strength or viscosity, of the PVA may be favourably set by e.g. adjusting the molecular weight, the degree of hydrolysis or by chemical modification of the PVA.

The binder properties can also be favourably influenced by cross-linking the binder. Therefore, in particular when PVA is provided as the binder, in an embodiment the binder is cross-linked, preferably by cross-linking agents such as dialdehydes, e.g. glutaraldehyde, formaldehyde and glyoxal. Such cross-linking may e.g. favourably reduce any tendency for undesirable crack-formation.

The composite may further comprise customary additives such as stabilizers, surfactants, antioxidants, free radical scavengers and/or plasticizers.

One or more further surfactant(s) can be useful in further lowering the surface or interfacial tension and in promoting emulsifying and dispersion or improving film formation.

Conventional surfactants known in the art can be used, including anionic surfactants, for example sulfate, e.g. sodium lauryl sulfate, sulfonate, phosphate and carboxylate surfactants, cationic surfactants, for example secondary or tertiary amine and quaternary ammonium salt surfactants, zwitterionic surfactants, for example betaine, sultaine and phospholipid -15 -surfactants, and nonionic surfactants, for example long chain alcohol and phenol, ether, ester or amide nonionic surfactants.

According to an aspect of the invention polymerizable surfactant, i.e. surfactant comprising one or more polymerizable groups, can be used.

Such polymerizable surfactant can be used alone, i.e. as the only surfactant provided, or in combination with non-polymerizable surfactant. In an embodiment, a polymerizable surfactant is provided in addition and in combination with a non-polymerizable surfactant.

For the binder, in particular PVA, ethylene glycol can be used as a preferable plasticizer. It is also possible to add glycerol to the binder, in particular PVA-based binder. Furthermore, to favourably influence film forming properties film-forming agents, for example polyacrylic acid, and anti-foaming agents may be added.

Such agents may be used to improve film formation and substrate wettability. Optionally, degassing and/or filtration of the coating composition can be carried out to further improve film properties. Likewise, setting and adjusting binder viscosity can have a favourable influence on the forming or respectively formed film.

The binder can be provided as a liquid or paste, wherein a carrier medium or solvent, such as water, aqueous solvent or organic solvent, can be removed from the composite mixture, for example during or after film formation, in particular by evaporation at an elevated temperature.

The binder preferably mixes and combines well with the capsules, while further avoiding aggregation of capsules. Moreover, the binder can be chosen such that a high density of capsules can be provided in the composite, for example in a film formed of the composite. Furthermore, in the composite the structural and mechanical advantages of the binder can be combined with the favourable acousto-optical properties of the LC capsules.

The capsules can be applied to a large variety of different environments, in particular by (re)dispersing them. They are favourably dispersed in or respectively mixed with the binder. The binder cannot only improve film -16 -forming behaviour but also film properties, wherein in particular the binder can hold the capsules relative to an applied substrate. Typically, the capsules are randomly distributed or respectively randomly oriented in the binder.

The composite comprising the capsules and the binder material may be suitably applied or coated to a large variety of different environments or workpieces.

A workpiece in the term of the present application is a piece of metal or other material that is in the process of being worked on or made or has been cut or shaped by a hand tool or machine and which is suitable for non-destructive testing (NDT) techniques utilizing ultrasound.

As commonly known ultrasonic or ultrasound testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material or workpiece tested. In most common UT applications, very short ultrasonic pulse-waves with centre frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials.

Ultrasonic testing is typically performed on steel and other metals and alloys; however, it is likewise preferred that ultrasonic testing is also performed on both plastics and complex composite. It is used in many industries including steel and aluminium construction, metallurgy, manufacturing, aerospace, automotive and other transportation sectors.

Typically, in ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is pressed against body of an object being inspected.

The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required. There are two methods of receiving the ultrasound waveform: reflection and attenuation.

In reflection (or pulse-echo) mode, the transducer functions as a transceiver of the pulsed waves, as the "sound" is reflected back to the -17 -device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal in frequency (or spatial) domain, with an amplitude representing the intensity of the reflection and the distance representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the magnitude of the ultrasound energy that is received at the receiver end. Imperfections or other conditions in the space between the transmitter and receiver attenuates the ultrasound energy, thus revealing their presence.

Using the couplant increases the efficiency of the process by reducing the losses via acoustic impedance matching.

With the acousto-optical device in accordance with the present invention ultrasound can directly be applied to the workpiece and direct visual inspection of the workpiece enables the user to assess the material's structural health.

The frequency of operation for the present invention depends on several factors including the mechanical properties of the binder, the size of the capsules, and the overall thickness of the composite film. For example, for diagnostic applications involving low frequency Ultrasonic radiation around 20kHz, it would be advantageous for the sensor in the present invention to use larger capsules. Alternatively, careful balance between capsule size and the overall film thickness can be designed to match with the region of acoustic radiation of interest.

For low amplitude ultrasound radiation applications, employing thin film as well as using comparatively stiffer binder materials (high rigidity) for the composite fabrications in the said invention may facilitate good ultrasound energy transfer form source to the acousto-optic sensor. Accordingly, the frequency of operation is thus defined by both the liquid crystal mixtre and capsule size Preferably, the composite can be applied onto a workpiece by conventional coating techniques such as spin coating, blade coating, -18 -spray coating, or drop coating. Alternatively, they can also be applied to the onto a workpiece by conventionally and known printing methods, like for example, ink-jet printing.

It is also possible to dissolve the capsules or the composite in a suitable solvent. The resulting solution is then coated or printed onto a workpiece, for example by spin-coating or printing or other known techniques, and the solvent is evaporated off. In many cases it is suitable to heat the mixture to facilitate the evaporation of the solvent. However, evaporation at ambient temperatures such as room temperature is also possible. As solvents for example water, aqueous mixtures or standard organic solvents can be used. The resulting film of the composite after drying can be polymerised by, for example, UV or thermally using commonly known photo-or thermal initiators.

It is preferred that the material applied to the workpiece is the composite, i.e. that it also contains binder. In a preferred embodiment a film made of the composite has a thickness of from 0.5 pm to 50 pm, very preferably from 1 pm to 40 pm, from 2 pm to 20 pm.

The optical properties of the LC material, the polymeric capsule shell and the binder are favourably and preferably matched or aligned in terms of compatibility and in view of the respective applications.

Furthermore, excellent physical and chemical stability is obtainable, for example by preferably and favourably reducing water uptake. In particular, good stability and resistance to heat or mechanical stress can be achieved while providing suitable mechanical flexibility.

In an embodiment the capsules are dispersed in the binder, wherein the capsules in the binder exhibit a random orientation relative to each other. Regardless of any possible absence or presence of alignment or orientation of the LC material within each individual capsule, this random orientation of the capsules with respect to each other can result in the LC material as a whole giving an observed average refractive index (navg).

-19 -In the initial state of the acousto-optical device the capsules scatter VIS light, in this embodiment the subsequent application of an ultrasound forces (re)alignment of the LC material, that can modulate the phase shift, or retardation, of the transmitted, or reflected, light. The composite systems advantageously allow for a high degree of adaptability and for setting and adjusting several degrees of freedom, especially in view of tuning the acousto-optical properties and functionality.

For example, the layer or film thickness can be set, adapted or varied while being able to independently vary the density of the LC material in the film, wherein furthermore the size of the capsules, i.e. the amount of LC material in each individual capsule can be preset and thus also adjusted. Furthermore, the LC medium can be chosen to have specific properties, e.g. suitably high values of An.

In a preferred embodiment the amount of LC in the composition, in the capsules and in the composite is suitably maximized to achieve favourably high acousto-optical performance.

Preferably and favourably the capsules and composite systems as provided show sufficient processibility such that aggregation during concentration and filtration of the capsules, mixing with the binder, film formation and optional drying of the film is kept at a minimum.

The capsules and the composites are useful in acousto-optic devices, such as acousto-optic modulators, acousto-optic tunable filters, acousto-optic deflectors, in acousto-optic sensors, or even privacy screens.

The invention thus advantageously provides acousto-optic devices. These devices comprise the capsules, wherein preferably the capsules are mixed and dispersed in the binder.

Moreover, provided there is an acousto-optic device, in particular an acousto-optic sensor, which makes advantageous use of the capsules and/or the composite system as described both in previous sections and -20 -the proceeding sections. In the device a plurality of the capsules are provided.

The functional principle of such device will be explained in more detail as given below. Firstly, the composite is applied to the body of the workpiece.

The liquid crystals in the capsules embedded in the binder are initially aligned randomly, scattering light thus the film is opaque. By applying ultrasoundradiation to a pristine or unimpaired workpiece, the liquid crystals' alignment changes, allowing to pass through( clear film). Different mechanisms have been proposed to explain the effect of acoustic waves on changing the molecular alignment of the liquid crystals(C. Vitoriano, Fur. Phys. J. E (2017) 40: 48, such as acoustic streaming (W. Helfrich, Phys. Rev. Lett. 1972, 29, 1583; K. Miyano, Y. R. Shen, Phys. Rev. A 1977, 15, 2471) and minimum entropy generation (L. Dion, A. D. Jacob, Appl. Phys. Lett. 1977, 31, 490).

Accordingly, the present invention relates also to a method of ultrasonic testing of a workpiece comprising the steps of applying the composite as described above and below to the surface of a workpiece; curing of the composite; and applying a transducer acting as the ultrasound source to the workpiece.

A further aspect is also the use of an acousto-optical device as described above and below for non-destructive testing (NDT) techniques utilizing the acousto-optic effect, preferably for ultrasonic testing.

Throughout this application, unless explicitly stated otherwise, all concentrations are given in weight percent and relate to the respective complete mixture, however excluding water solvent or water phase as indicated above.

All temperatures are given in degrees centigrade (Celsius, °C) and all differences of temperatures in degrees centigrade. All physical properties and physicochemical or electro-optical parameters are determined by generally known methods, in particular according to "Merck Liquid Crystals, Physical Properties of Liquid Crystals", Status Nov. 1997, Merck -21 -KGaA, Germany and are given for a temperature of 20 °C, unless explicitly stated otherwise.

Above and below, An denotes the optical anisotropy, wherein An = no -no, and AE denotes the dielectric anisotropy, wherein Ac = EH -1. The dielectric anisotropy AE is determined at 20°C and 1 kHz. The optical anisotropy An is determined at 20°C and a wavelength of 589.3 nm.

The Ac and An values and the rotational viscosity (71) of the compounds according to the invention are obtained by linear extrapolation from liquid-crystalline mixtures consisting of 5% to 10% of the respective compound according to the invention and 90% to 95% of the commercially available liquid-crystal mixtures ZLI-2857 or ZLI-4792 (both mixtures from Merck KGaA).

Besides the usual and well known abbreviations, the following abbreviations are used: C: crystalline phase; N: nematic phase; Sm: smectic phase; I: isotropic phase. The numbers between these symbols show the transition temperatures of the substance concerned.

In the present invention and especially in the following examples, the structures of the mesogenic compounds are indicated by means of abbreviations, also called acronyms. In these acronyms, the chemical formulae are abbreviated as follows using Tables A to C below. All groups CnH2n+1, CmH2m+1 and C11-121+1 or CnH2nr1, CrnH2m_i and CIH21-1 denote straight-chain alkyl or alkenyl, preferably 1-E-alkenyl, each having n, m and I C atoms respectively. Table A lists the codes used for the ring elements of the core structures of the compounds, while Table B shows the linking groups. Table C gives the meanings of the codes for the left-hand or right-hand end groups. The acronyms are composed of the codes for the ring elements with optional linking groups, followed by a first hyphen and the codes for the left-hand end group, and a second hyphen and the codes for the right-hand end group. Table D shows illustrative structures of compounds together with their respective abbreviations.

-22 -Table A: Rina elements

G

U

C

P

D

A

Y

M DI o

Al -n-

GI Ul

MI

NI _ 11(11 2) _

N tH

K

L

F Nf

-23 -Np N3f tH2f

LI N3fI tHI tH2f1

KI Fl Nfl dH

-24 - Table B: Linkina aroubs Z -00-0- ZI -0-00- E -CH2CH2- V -CH=CH- X -CF=CH- 0 -CH2-0- XI -CH=CF- 01 -0-CH2- B -CF=CF- 0 -CF2-0- T -CC- 01 -0-CF2- W -CF2CF2- -25 -

Table C: End groups

Left-hand side Right-hand side Used alone -n- CnH2n+1- -n --CrH2n+1 -n0- CrH2n+1-0- -n0 -0-CnItn+1 -V- CH2=CH- -V -C H=C H2 -nV- CnH2n+1-CH=CH- -nV -Cn H2n -CH=C H2 -Vn- CH2=CH-Cr 1-12n+i - -Vn -CH=CH-Cr H2n+i -nVm- CrH2n+1-CH=CH -Cm H2m--nVm -Cr 1-12n-CH=C H-CmH2m+1

-N- NEC- -N -CEN

-S- S=C=N- -S -N=C=S

-F- F- -F -F

-CL- Cl- -CL -Cl -M- CFH2- -M -CFH2 -D- CF2H- -D -CF2H -T- C F3- -T -C F3 -MO- CFH20 - -OM -0CFH2 -DO- CF2H0 - -OD -0CF2H -TO- CF30 - -OT -0CF3 -FX0- CF2=CH-0- -OXF -0-CH=CF2

-A- H-CEC- -A -CEC-H

-nA- Cr1-12n+1-CEC- -An -CEC-CnH2n-F1

-NA- NEC-CEO- -AN -CEC-CEN

Used together with one another and with others CH=CH- -CH=CH- -00-0- -CO-O- -0-CO- -...ZI... -0-CO- -00- -CO- -CF=CF- -...W... -CF=CF- -26 -wherein n and m each denote integers, and the three dots "..." are place-holders for other abbreviations from this table.

The following examples are merely illustrative of the present invention and they should not be considered as limiting the scope of the invention in any way. The examples and modifications or other equivalents thereof will become apparent to those skilled in the art in the light of the present disclosure.

15 20 25 30 -27 -

Examples

In the Examples,

no denotes extraordinary refractive index at 20°C and 589 nm, no denotes ordinary refractive index at 20°C and 589 nm, An denotes optical anisotropy at 20°C and 589 nm, and cl.p., T(N,I) denotes clearing point [00].

Utilized LC media A liquid-crystal mixture B-1 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

Base Mixture B-1 CPGP-5-2 5.00% Clearing point [°C]: 102.0 CPGP-5-3 5.00% An [589 nm, 20°C]: 0.249 PGUQU-3-F 6.00% no [589 nm, 20°C]: 1.761 PGUQU-5-F 8.00% PGU-3-F 8.00% PUQU-3-F 17.00% PCH-301 10.00% P0101-3-F 6.00% PPTUI-3-2 10.00% PPTUI-3-4 15.00% PTP-102 5.00% PTP-201 5.00% 100.00% Furthermore the commercially available LC mixture E7 (commercially availably from Merck KGaA, Darmstadt, Germany) is utilized in the following experiments.

15 20 25 30 -28 -Materials and method: Ultrasound transducer: 033 piezoelectric transducer (Dimension: 27mm (diameter) 0.52 mm(height)) commercially available (Tektronic AF03021B). A 20Vpp 97kHz sine-wave is applied to the transducers using a function generator connected to a x20 voltage amplifier (F20A, FLC electronics). Periodically, the function generator is turned on at t=Osec and turned off at t=90sec, and the resulting effect on the said invention is recorded, accordingly Temperature and photo recordina: A FLIR camera is used to continuously monitor the clearing sate of the films. Additionally, the maximum, average and minimum temperatures of the nanocapsule film for 180sec are recorded. Optical photographs are acquired every 15secduring measurements. The FLIR and optical camera are fixed to stands, to ensure both the point of view and parasitic reflections are constant throughout the measurements (15cm from the objective) -29 -Preparation of Nanocapsules LC capsules are prepared in accordance with the method disclosed in WO 2017/178419 Al and characterized with respect to its general physical properties, having the composition and properties as indicated in the

following table.

Batch NC1 NC2 LC material E7 1.2g B-1 1.2g Surfactant Brij 98 40 mg Brij 98 40 mg EGDMA 300 mg 300 mg HEMA 40 mg 100 mg MMA 100 mg 100 mg Hexadecane 75 mg 175 mg 2nd Surfactant TegoWet@ 270 50 mg TegoWet® 270 50 mg Initiator 12 mg 12 mg Polydispersity 0.128 0.181 Zetasize 448 339 zetanumber 393 255 General procedure LC mixture, hexadecane, methyl methacrylate, hydroxyethyl methacrylate, ethylene glycol dimethacrylate and Tego wet 270 are weighed into a 250 ml tall beaker. Brij® L23 is weighed into a 250 ml conical flask and water (150 g) is added. This mixture is then sonicated for 5 to 10 minutes.

The Brij® L23 aqueous surfactant solution is poured directly into the beaker containing the organics. The mixture is turrax mixed for 5 minutes at 10,000 rpm. Once turrax mixing is complete, the crude emulsion is passed through a high-pressure homogenizer at 30,000 psi four times. The mixture is charged into a flask and fitted with a condenser, and after adding AIBN (35 mg) is heated to 70°C for three hours. The reaction -30 -mixture is cooled, filtered, and then size analysis of the material is carried out on a Zetasizer (Malvern Zetasizer Nano ZS) instrument.

The obtained capsules have an average size of 213 nm, as determined by dynamic light scattering (DLS) analysis (Zetasizer).

The respective particle suspensions are then concentrated by centrifugation, wherein the centrifuge tubes are placed in a centrifuge (ThermoFisher Biofuge Stratos) and centrifuged at 6,500 rpm for 10 minutes and at 15,000 rpm for 20 minutes. The resulting pellets are respectively redispersed in 1 ml of the supernatant.

Preparation of a 30% solid content PVA binder The PVA (molecular weight Mw of PVA: 31k; 88% hydrolysed) is first washed to remove ions in a Soxhlet apparatus for 3 days.

46.66 g of ion-free water are added to a 150 mL bottle, a large magnetic stirrer bar is added, and the bottle is placed on a 50°C stirrer hotplate and allowed to come to temperature. 20.00 g of the solid washed 31k PVA are weighed into a beaker. A vortex is created in the bottle and gradually the 31k PVA is added over approximately 5 minutes, stopping to allow the floating PVA to disperse into the mixture. The hotplate is turned up to 90°C and stirring is continued for 2-3 hours. The bottle is placed in oven at 80°C for 20 hours. The mixture is filtered whilst still warm through a 50 pm cloth filter under an air pressure of 0.5 bar. The filter is replaced with a Millipore 5 pm SVPP filter and the filtration is repeated.

The solid content of the filtered binder is measured 3 times and the average is calculated by weighing an empty DSC pan using a DSC microbalance, adding approximately 40 mg of the binder mixture to the DSC pan and recording the mass, placing the pan on a 60°C hotplate for 1 hour followed by 110°C hotplate for 10 min, removing the pan from the hotplate and allowing to cool, recording the mass of the dry pan, and calculating the solid content.

Film Coating: After concentration and re-dispersion the capsule solution solid content is measured using a PerkinElmer Simultaneous Thermal Analyzer 6000. A 50% capsule 50% Binder (by mass) mixture is then formulated. The formulation is placed on a Ratek RM5 Heavy Duty Roller to allow for -31 -complete mixing of the two components. A wet film of this mixture is then coated onto a surface of piezoelectric transducer with a coating machine (RK PrintCoat Instruments) K Control Coater using a Red K Bar. The film is placed on a hotplate at 60°C for 10 minutes to evaporate the water. For UV or thermal curing binder the film is cured under UV irradiation or by heat.

Film Thickness Measurement: The capsule film thickness is measured using a DetaktXT surface profiler.

A small area of each film (coated onthe surface of the transducer) is removed. The surface profile of this region is then generated by the profilometer using the Vision64 software. The profile revealed the height of the capsule film in comparison to the surface of the transducer in the area where the film had been removed. The capsule film thickness is measured from this surface profile.

Sensor performance -Example 1: The composite is prepared with E7 containing capsules and PVA as a binder. The composite is coated on surface of the piezoelectric transducer.

The clearing point of E7 capsules film in PVA binder is determined to be 50°C. The applied thickness of the film is 30pm. The LC capsule film is opaque in the absence of an applied ultrasound radiation. By using ultrasound waves, the liquid crystals' alignment changes, allowing light to pass through. This results transparent film clearing. The results are summarized in the following table: Transducer off Transducer on Transducer on ( t=0s) ( t=30s) ( t=90s) Clearing level ++ Film clearing: no clearing -, partially clearing +, fully clearing ++ -32 -Sensor performance -Example 2: The composite is prepared with B-1 containing capsules and PVA as a binder. The mixture is coated on surface of the piezoelectric transducer. The clearing point of B-1 containing capsules film in PVA binder is 68°C.

The thickness of the applied film is 15pm. The composite film is opaque in the absence of an applied ultrasound radiation. On exposure to ultrasound radiation, the liquid crystals' alignment changes, allowing transmission by light in an optically transparent film in clear film. The results are summarized in the following table: Transducer off Transducer on Transducer on ( t=0s) ( t=43) ( t=90s) Clearing level ++ Film clearing: no clearing -, partially clearing +, fully clearing ++ Partial clearing of the NC-2 film occurs after 43s after urning on the transducer. Maximum temperature after the elapsed time (43s) is recorded to be 45.3°C, which it is below the clearing point of NC-2 film (68°C). Full clearing of the film occurs after 90s with maximum temperature of 52.2°C which is well below the clearing point of NC-2 film (68°C). base on these observations we can safely conclude that the clearing of the film is the result of the ultrasounds radiation.

-33 -Example 3: Effect of amplitude The composite is prepared with B-1 containing capsules and PVA as a binder. The mixture is coated on surface of transducer. The clearing point of B-1 containing capsules film in PVA binder is 68°C. The thickness of the applied film is 15pm. The composite film is opaque in the absence of an applied ultrasound radiation. A sine-wave is applied to the transducers using a function generator connected via a x20 voltage amplifier at a fixed frequency of 97kHz. The amplitude of the applied signal is subsequently varied from 1 to 20Vrms.

The results are summarized in the following table: Voltage (Vrms) Current (A) Max. Temp. after 90s Clearing level 1.4 23°C 2.8 - 23°C - 4.3 - 23°C - 5.7 - <30°C - 7.2 0.07 <30°C - 8.5 0.08 <30°C 9.9 0.09 36°C + 11.4 0.10 39°C + 12.9 0.11 41°C + 14.3 0.12 43°C + 15.3 0.12 46°C + 16.9 0.13 49°C ++ 18.4 0.13 53°C ++ 19.9 0.14 58°C ++ Film clearing: no clearing -, partially clearing +, fully clearing ++ In conclusion partial clearing is observed at amplitudes above 11.4Vrms (>39°C) and full clearing above 16.9Vrrns (>49°C).

20 25 30 -34 -Example 4: Effect of frequency: The composite is prepared with B-1 containing capsules and PVA as a binder. The mixture is coated on surface of transducer. The clearing point of B-1 containing capsules film in PVA binder is 68°C. The thickness of the applied film is 15pm. The composite film is opaque in the absence of an applied ultrasound radiation. A sine-wave is applied to the transducers using a function generator connected to a x20 voltage amplifier. The impedance response of transducers are recorded to determine their resonance frequencies, where acousto-optic devices may be excited.

The results of variation of frequency on clearing film at fixed amplitude (20Vrms) are summarized in the following table: Frequency (kHz) Power (W) Max. Temp. after 90s Clearing level 32 1.61 37°C + (resonance) 58 2.38 39°C + (resonance) 97 3.58 53°C ++ (resonance) 123 2.81 42°C + (resonance) 169 2.18 30°C (low resonance) 310 0.70 23°C - (non-resonant) 354 0.29 36°C + (resonance) Film clearing: no clearing -, partially clearing +, fully clearing ++ 20 25 30

Claims (12)

1. -35 -Claims 1. An acousto-optical device comprising a composite of liquid crystalline (LC) capsules dispersed in a binder material, whereby an ultrasound radiation is applied to the composite.
2. 2. The acousto-optical device according to claim 1 characterized in that it is an acousto-optical sensor for non-destructive testing (NDT) techniques utilizing the acousto-optic effect.
3. 3. The acousto-optical device according to claim 1 or 2, characterized in that the utilized liquid crystalline (LC) capsules are obtainable from a composition for encapsulation, comprising (i) a mesogenic medium which comprises one or more compounds of formula I R-A-Y-A'-R1 I wherein R and R' denote, independently of one another, a group selected from F, CF3, OCF3, CN, and straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by ON or CF3 or mono-or polysubstituted by halogen and wherein one or more CH2groups may be, in each case independently of one another, replaced by -0, S, CO, C00-, -000-, -0000-or -CEC-in such a manner that oxygen atoms are not linked directly to one another, -36 -A and A' denote, independently of one another, a group selected from -Cyc-, -Phe-, -Cyc-Cyc-, -Cyc-P he-, -Phe-Phe-, -Cyc-Cyc-Cyc-, -CycCyc-Phe-, -Cyc-Phe-Cyc-, -Cyc-Phe-Phe-, -Phe-Cyc-Phe-, -Phe-Phe-Phe-and the respective mirror images thereof, wherein Cyc is trans-1,4-cyclohexylene, in which one or two non-adjacent CH2 groups may be replaced by 0, and wherein Phe is 1,4-phenylene, in which one or two non-adjacent CH groups may be replaced by N and which may be substituted by one or two F, and Y denotes single bond, -000-, -CH2CH2-, -CF2CF2-, -CH20-, -CF20-, -CH=CH-, -CF=CF-or (H) one or more polymerizable compounds, and (Hi) one or more surfactants.
4. 4. The acousto-optical device according to one or more of claims 1 to 3, characterized in that the liquid crystalline (LC) capsules respectively comprise a polymeric shell, and a core containing a mesogenic medium which comprises one or more compounds of formula I as set forth in claim 3.
5. 5. The acousto-optical device according to one or more of claims 1 to 4, characterized in that the utilized LC capsules have an average size, as determined by dynamic light scattering analysis, in the range of 1 nm to 10 pm.
6. 6. The acousto-optical device according to one or more of claims 1 to 5, characterized in that the binder is selected from water-soluble polymers or waterborne latex binders.
7. -37 - 7. The acousto-optical device according to one or more of claims 1 to 6, characterized in that one or more binders comprise polyvinyl alcohol.
8. 8. The acousto-optical device according to one or more of claims 1 to 7, characterized in that a film of the composite has a thickness in the range from 0.5 pm to 50 pm
9. 9. A method for ultrasonic testing of a workpiece comprising the steps of applying the acousto-optical device according to one or more of claims 1 to 9 to a surface of a workpiece; curing of the composite; and applying an ultrasound to the workpiece.
10. 10. Use of anan acousto-optical device according to one or more of claims 1 to 9 for non-destructive testing (NDT) techniques of a workpiece.
11. 11. Use of anan acousto-optical device according to claim 10 for nondestructive testing (NDT) techniques utilizing an acousto-optic effect.
12. 12. Use of anan acousto-optical device according to claim 10 or 11 for ultrasonic testing.