NL2026281B1 - Spray device - Google Patents

Abstract

A spray device for delivering a liquid spray, having a spray length (L), through air to a target, in particular a body mist spray, comprises a containerfor holding a liquid, pressurizing means for pressurizing a quantity of said liquid to an elevated operating pressure ( P), and comprises a spray nozzle member that comprises a nozzle plate with a plurality (N) of nozzle orifices of substantially identical size (D), extending through said nozzle plate and communicating with said pressurizing means to receive a quantity of pressurized liquid at said operating pressure, during operation. Said plurality of nozzle orifices discharge said liquid spray at an outlet surface of said spray nozzle member member over at least said spray length (L). Their substantially identical size (D) is between a lower limit (Dmin) and an upper limit (Dmax), ‚ LÀ wherein said lower limit is a pp roximatelV:Dmin z 1 /10 —JP—,/P wherein said Upper limit isa pproximatelV-'Dmax "~' 1/10 Æ\[P—/p' wherein M18 n/p, n representing a viscosity of said air and p representing a density of said liquid.

Description

Spray device The present invention relates to a spray device for delivering a liquid spray, having a spay fength{L), to a target, in particular a body, through air, comprising: a container for holding a liquid, pressurizing means for pressurizing a quantity of said liquid to an elevated operating pressure { P}, and a spray nozzle member, wherein said nozzle member comprises a nozzle plate with a number of nozzle orifices of substantially identical size {Drone} extending through said nozzle plate and communicating with said pressurizing means to receive, during operation, said quantity of pressurized liquid at said operating pressure, and wherein said nozzle orifices discharge said liquid spray at an outlet side of said spray nozzle member over at least said spray length {L]. Many body mist devices for perfume, body and/or deo sprays are being used to produce sprays having droplets typically in the 20-200 micron range.

Spray characteristics are determined by a number of factors including the dimension and geometry of the outlet nozzle and the pressure with which the fluid is forced through the nozzle.

Current perfume or body mist devices produce inherently sprays with a rather broad size distribution having a large Geometric Standard Deviation (GSD >2}). {tis known that sprays with uniformly sized small droplets are difficult to produce, Uniform sized droplets are however beneficiary if one aims at low impact sprays, that are homogeneously distributed over the target area hitting the target with a uniform velocity.

A droplet of 50 micron however has 8 times more mass than a droplet of 25 micron, and a droplet of 50 micron will therefore decelerate much less in air and hit the target with a larger velocity than a droplet of 25 micron.

This will lead to sprays hitting the target with large droplets at a high velocity and small droplets at a low velocity.

Current sprays having a rather broad size distribution, typically between 20 and 200 micron are thus also characterized by droplets having a broad velocity distribution.

The generation of a uniform spray having a low and homogeneous impact at the target is becoming particularly desirable when it would be enabled by low pressure pumps, such as the manually-operable pump or trigger sprays being used for perfumes and many other over-the- counter (OTC) sprays.

However current perfume or body mists producing droplets with a typical size of 20-200 micron are generated with so called pressure swirl nozzles or hollow cone nozzles.

A stationary core inside the nozzle induces a rotary fluid motion which causes the swirling of fluid in a swirl chamber.

A liquid sheet film is discharged from the perimeter of the

-2- outlet orifice producing a characteristic hollow cone spray pattern. Air or other surrounding gas is drawn inside the swirl chamber to form an air core within the swirling liquid. Many geometries of fluid inlets are used to produce this hollow cone pattern depending on the nozzle capacity and materials of construction. These OTC nozzles still produce droplets with a rather broad size distribution sizes typically between 20-200 micron, and a mean drop size of 50-80 micron. Therefore, to generate a cone spray with uniformly sized droplets at a low working pressure below e.g.10 bar has thus been proven quite difficult. Moreover, the minimum volume flow or discharge rate of sprays generated with swirl nozzles is high, typically larger than 200 ul/sec, and the corresponding impact of the spray liquid on the target area is therefor always rather high. An area where uniform low impact sprays are advantageous is in the delivery of perfume, deo and body mists on the skin or cloths. The application of these fluids, especially with the use of high impact swirl nozzles, with an inherent high discharge rate typically more than 200 ul/sec, lead to a large spoilage of the precious fluid due to the creation of turbulent air streams that transport different sized droplets with different velocities away from the target. Additionally, a substantial volume of the perfume or body mist droplets bounces from the target area into the open air due to the high impact velocity of especially the larger droplets. This is becoming increasingly problematic with high surface tension water-based perfumes or body mists. As a result, the use of swirl nozzles is inaccurate and wasteful. Moreover, the present swirl nozzle technology does not provide a satisfactory way of controlling the droplet size nor the droplet velocity, nor does it provide a way of ensuring that the liquid that is dispensed actually lands on the target area and remains there.

An object of the present invention is, therefore, to provide an improved uniform low impact perfume, deo or body mist device which is capable of producing small uniformly sized droplets to be preset between 20 and 80 micron with a tunable preset diverging ray angle between e.g. 5°- 25°. Another object of the invention is to create a cone or diverging spray without the need of high impact swirl nozzles.

The present invention particularly relates to a spray device for generating a so-called micro-jet spray. A micro-jet spray consists of a number of concurrently emitting jets, in which each jet will initially breakup into a mono-disperse primary droplet train according to a jet breakup mechanism. As a result, consecutive primary droplets have a same size and propagate from the

-3- nozzle orifice in a same direction, typically the diameter of the primary droplet is between 1,85 and 2 times the diameter of the nozzle orifice. To that end a spray device of the type as described in the opening paragraph, according to the present invention, is characterized in that said nozzle orifices have a substantially identical size {Dnoze) that is between a lower limit (Dmin} and an upper limit {Dmax), wherein said lower limit is approximately: Dmin = 1/10 2 , and wherein said upper limit is approximately: Dmax & 1/10 [22 with A=18 n/p and n representing a viscosity of said air and p representing a density of said liquid, P the said operating pressure and L the spray length of the device. Said size of an orifice is being defined in the present application as representing the diameter of a circle having a same surface area as a cross sectional surface area of said orifice.

It is an insight according to the invention that bouncing of the larger droplets and the creation of turbulent air streams carrying away the smaller droplets can be prevented, by the use of more uniformly sized droplets and by lowering the impact of the spray liquid on the target area, especially when the chosen droplet size is substantially less than 50-100 micron and that the volume flow or discharge rate is substantially less than 200 ul/sec, in particular less than 100 ul/sec with a preferred discharge time larger than typical 1 second. A preferred embodiment of the spray device is thereby characterized in that said size of said orifices is less than 20 micron, in that said pressurizing means pressurize a quantity of between 50 and 150 microliter of said liquid to an operating pressure of between 2 and 5 bar, and in that nozzle orifices discharge said quantity of pressurized liquid over a period of at least half of a second. The invention is thereby based on the recognition to set a maximum on the concurrent impact of the spray liquid on the target area. Surprisingly it has been found that not only the discharge rate is a key parameter, but also that the Force of Spray Impact on the target has been found highly crucial. The Force of Spray Impact on the target is here defined as the total quantity of deposited mass multiplied with the spray velocity hitting the target divided by the discharge period of the spray device.

Concerning the force of spray impact, it is an insight according to the invention that, depending on their size, the spray droplets will decelerate substantially from their initial velocity before they hit the target at their terminal velocity. Especially, it has been found that small droplets decelerate much faster in air than larger droplets. When a droplet diameter is reduced by half, it turns out that the deceleration of such droplet will be about four times stronger. This will greatly reduce the terminal velocity of the droplets on impact on the target. At the same time the decelerated droplets in a droplet train may also tend to coalesce with subsequent droplets that are in their slip stream. The inventors have recognized that the latter may lead to a growth on average five times an initial size of the droplets, which will also increase the spray length of the device. The operating pressure and orifice size of the spray device together determine the droplet size (mass) and initial velocity and, hence, their initial and terminal momentum. The upper limit Dmax ensures that the latter will not exceed a threshold that will trigger an unpleasant skin sensation or a wet spot on the target area.

On the other hand the initial velocity of the droplets need be sufficiently high to reach the target area. To that end the initial momentum of the droplets should be sufficiently high, which is assured by the lower limit Dmin of the orifice size that inter alia determines the initial velocity as a result of the spray pressure ( P) and initial droplet size produced by the orifice diameter.

It has been recognized that the time dependent travelling distance x{t) and velocity v{t) of the droplets scale exponentially in time, At a typical initial velocity of v(t=0)= vo m/s; their time dependent droplet velocity v(t) and time dependent travel distance x(t} are given by: v(t) ~ v, e M/Darop” and x(t) x(t=0)= Peli (1 eM /Darop™y , by solving v(t) in Stokes law: i = 3nDgropv(l) that describes the movement of a single droplet in a surrounding fluid like air wherein A = 18 n/p with 1 the air viscosity, p the liquid density and Dg. the droplet diameter, For a single droplet with diameter Dg,,=25 micron, A= 32.4x10 mst and at an initial velocity vo =15 m/s this leads to a maximum travel distance Lmax & ZoDdroy” of about 3 cm.

For micro-jet sprays the maximum travel distance Lmax will be co-determined due to the traveling of the droplets in a droplet train, due to droplet coalescence inside the travel train, and also to a contribution from entrained air flow around the droplet train. To compensate for

5.

this effect a practical approach is that a spray consisting of many interacting droplets in a train with initial diameter Drum is assumed to behave as a spray of non-interacting single droplets with an effective Stokes diameter Dsngte. In practice the ratio Dsngte/ Drain = 5. In other words, the propagation of a droplet train of primary droplets with diameter Dia=25 micron can be considered as the propagation of a single droplet with diameter 125 micron. The maximum travel distance Lmax of a droplet train of primary droplets with diameter Dian=25 micron at an initial velocity ve =15 m/s would be (Dsingie/ Dea}? x3 cm = 75 cm. An effective operating pressure P over the nozzle plate is typically about 2.5 bar. If all the operating pressure is transferred to kinetic energy, we may apply Bernoulli equation, stating P= pvo?, and find v, =15 m/s. Hence, the initial velocity of the jet ejected from the nozzle is typically about 15 m/s.

The preferred droplet velocity v‚ for low impact on the target should be below 5 m/s at a target spray length L of between 10 and 50 cm, that is typical for a perfume or body mist. For a given maximum spray length Laax the initial velocity v‚ has dropped a factor 2 at half the distance Lmax/2. So if the aim is to deliver a low impact spray by decelerating the initial velocity to below 10 m/s, i.e by at least by said factor 2, then the target should be placed at a distance L=Lms/2, corresponding to single drops with a size Dsingte = = . However smaller drops are still being able to reach the spray length L, although it with much lower velocity. The smallest single drops that are still able to travel a distance L is given by Dsingte = = . So all single drops inthe size range = < Dsingte < I= are able to reach the target placed at a spray length L with a velocity less than half of the initial velocity, and this formula thus defines the concept of spray length L and also implies an optimum required size range for the droplets.

For droplets travelling in a train as in a micro-jet spray the said ratio Dsngie/ Drain = 5 should be used.

At an operating pressure of 2.5 bar and setting the maximum travel length L to 50 cm {with A=

3.24x103 mist, p=1000 kg/m?) and using a ratio Dsinge/ Drain = 5, then substitution in above equation yields 20 um < Dain < 30 um for a body mist spray. Droplets smaller than 20 um will never hit the target and droplets larger than 30 um will not decelerate sufficiently to below 5 m/s, so that vi < vo/2.7, whereas a typical preferred range for the droplet velocity at the target is

0.1ve<vi<0.5 vo. For a micro-jet spray the droplet size is about two times the nozzle diameter Dnozzie thus Dtrain= 2 Drozzte. Thus for a low impact body spray with L =50 cm we get then 10 um < Dnozzie < 15 Um at an operating pressure of 2.5 bar.

-6- Hence, more generally, at an operating pressure P over the nozzle plate, the present invention provides a spray device for delivering a liquid spray to a target at a given spray distance L of droplets emanating from one or more orifices having a substantially identical diameter Dnozae, while securing a velocity at spray distance L less than typically around 5 m/s if the diameter Drozze is chosen in between the above range of Dmin to Dmax at an operating pressure of 2.5 bar.

In a particular embodiment the device according to the invention is characterized in that said nozzle orifices are of a substantially identical size with a diameter Dnczz1e less than 20 micron, in that said pressurizing means generate an operating pressure P over the nozzle plate, in that said number N of orifices discharge said quantity V of pressurized liquid at a rate of between 10 and 100 microliter per second during at least T>500 microseconds, and that the said nozzle diameter Droze is chosen between with n the air viscosity and p the liquid density. A = Dgnge/ Drain has typically a value of 5, but in practice will also depend on the amount of entrained air, the number of droplets and the amount of divergence of the droplet train, etc between 3-7.

With preference the pressurizing means comprises a manually operable pump having a piston to pressurize said liquid, wherein said quantity of liquid V is pressurized to an operating pressure P over the nozzle plate in one stroke of said piston. This method is seen as most cost, user and environment friendly for the OTC market.

The body mist device according to the invention is advantageously used to create a low impact spray for body mists and other beauty and home care applications. It has been found that, not only the discharge velocity, but also the total Force of Spray Impact on the target is important. The Force of Spray Impact {FSi} on the target is here defined as the total quantity of deposited mass map V multiplied with the spray velocity hitting the target with velocity v, divided by the discharge period T of the perfume or body mist, so FSI=p V/T.

To create a more uniform spray at a reduced Force of spray impact, it is an insight according to the invention to decelerate substantially the initial velocity of the ejected droplets by using

WE droplets that all have a substantially identical size, so that all droplets have the same terminal velocity when hitting the target.

Perfume spray trials with test persons have revealed that inducing a Force of Spray Impact less than 1x10 kgm/s? on the skin is sufficient to prevent wet spots, whereas the chance for irritation and allergic reactions was also substantially reduced. Therefore, a preferred embodiment of the spray device of the invention is characterized by means that maintain a Force of spray impact (FSI) below about 1x103 kgm/s2.

For comparison conventional perfume swirl nozzles typically operate at a discharge rate well over 100 microliter per 0.2 second, thus at a discharge rate of more than 500 microliter per second. The Force of Spray Impact on the target of swirl nozzles exceeds therefore 5x10% kgm/s? at impact velocities of typical >5 m/s herewith easily creating unpleasant feelings or wet spots on the skin.

There is a delicate balance between several relevant parameters. In particular changing the mean droplet size and droplet size distribution of the spray has a profound influence on the impact force of the spray droplets at the target. Sprays with a narrow droplet size distribution are more suited for the creation of uniform low impact sprays.

Normally the droplet size distribution may be characterized in terms of volume as DVX, with X% being the total volume of liquid sprayed drops with a specific diameter expressed in micrometres (um) smaller than DVX, and 100-X% of droplets with a larger diameter than DVX. A DV10 of 10 micron means that 10% of the spray volume has droplets with a diameter smaller than 10 micron. DV50 is also defined as the Volume Mean Diameter. The droplet size distribution is characterized by the Relative Span (RS) as RS= (DV90-DV10)/DV50. Satisfactory uniform low impact body mist sprays are delivered with a particular embodiment of the spray device according to the invention, characterized in that RS < 2, in particular RS < 1. Measured droplet size distributions by a further particular embodiment of a spray device according to the invention are further characterized by a Geometric Standard Deviation GSD < 1.8, in particular GSD < 1.6. These body mist sprays can be considered as nearly monodisperse.

The invention, moreover, relates to a method for delivering a liquid spray to a target at a distance, in particular to the skin or clothed skin, comprising: pressurizing a quantity of said

-8- liquid to an elevated operating pressure, forcing said liquid at said elevated pressure at an initial velocity through a plurality {N) of nozzle orifices that are provided in a nozzle plate of a spray nozzle, herewith generating a micro-jet spray, consisting of {(N) concurrently emitting jets that breakup in droplets, characterized in that a substantial part of the droplets hit the target at a substantial equal velocity, which an (average) value between 10% and 50% of the initial velocity of the emitted jets, and that N is typical between 10 and 100, and in particular between 20 and

100. Further investigations and experiments revealed that for perfumes and body mists, according to the invention, the nozzle orifices preferably have a nozzle orifice opening size of between 8-16 micron in size, creating primary ejected droplets between 16-32 micron and downstream droplets with a typical size of 40-80 micron. Preferably the spray emanating from the spray device is homogenously distributed over a specific target area, such as a specific clothed or unclothed skin area, and that the user should be able to direct the spray to the target area. A further preferred embodiment of the spray device according to the invention, to that end, is capable of producing directed diverging rays characterized by a diverging ray angle that is typically between 5 and 25 degrees, said ray angle preferably being tunable.

Increasing the number of orifices will balance the required discharge rate, the given initial droplet velocity and the preferred optimum droplet/orifice size to provide a uniform spray of sufficiently low impact. For body mist spray purposes, typically 20 -100 diverging rays appear to provide this balance,

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a body mist spray device for delivering a liquid spray of droplets. Fig. 2 illustrates a spray nozzle unit having a nozzle plate with a number of nozzle orifices. Fig. 3 shows a movie frame of a high speed camera of a body mist spray.

Fig. 4 shows a velocity profile of a body mist spray.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates a body mist spray device (1) for delivering a liquid spray of droplets (2) with an initial velocity vo larger than 10 m/s to the skin (3) at a given distance L, hitting the skin (3) at a

-9- strongly reduced velocity vi. The spray device (1) comprises a container {4) for holding the spray liquid, pumping means {5) for pressurizing a quantity V = 15 microliter, a spray nozzle unit (6) having an inlet {7) and having an outlet {8}, wherein the spray nozzle unit {6) produces a spray {2}, during operation for a period T= 1 sec.

Fig. 2 illustrates a spray nozzle unit (6} having an inlet (7) and outlet (8), and comprising a nozzle plate {9) having a number (10) of nozzle orifices, each having an entrance (11) in open communication with said inlet (7) and having an exit {12} in open communication with said outlet (8). The nozzle orifices (10) have a substantially identical size, here of 9 micron producing a droplet train (13) with a droplet diameter Dyan= 18 micron. The pressurizing means at an operating pressure P= 2.5 bar over the nozzle plate (9) generate jets with an initial velocity of v,=16 m/s originating from 50 nozzle orifices (10). In T=0.5sec a total quantity V=25 microliter at a rate of about 50 microliter per second is then discharged. With an ultra-high speed Shimadzu camera the velocity profile of the 50 diverging jets as depicted in Fig. 3 has been obtained and is plotted in Fig. 4. The from this experiment derived Stokes diameter Dsingie is here ca. 60 micron, thus ca 4 times the initial droplet train diameter. At a distance of 25 cm from the nozzle plate the velocity has dropped about with a factor e = 2.7 from 16 m/s toca 4 m/s. Thus for this configuration according to the invention the target should be placed at about 25-50 cm. In order to prevent wet spots when the spray is directed towards the skin the discharge rate should be less than 100 ul/sec and the Force of Spray Impact FSl=p Vv/ T should be equal or less than 1x10 kgm/s2, In this case the discharge rate is 50 ul/sec and at a distance of 50 cm the Force of Spray Impact FSI is much smaller than 1x10° kgm/s’. Below some other results of the perfume spray devices with healthy females are put in a table. Test Number Device type Distance | Discharge Discharge Force of Creation Allergic or Number | Volunteer to Skin Rate in Time Spray Impact | of Wet Irritation incm uliter/sec sec kem/s? Spots Reaction Count Count 1 10 20 | 400 2 10 Drain = 20 100 1.0 1x10% 1 0 15 micron 3 10 Dyain= 20 100 1.0 1x10°3 1 0 30 micron 5 12 Drain = 40 100 1.0 5x10 15 micron 6 12 Drain = 40 100 1.0 5x10 30 micron

-10- Three different perfume spray devices have been used, two according to the invention and one conventional commercial swirl nozzle type device with a typical volume of 100 ul per stroke and a discharge time of 0.25 sec.

According to the invention we have used two nozzle diameters, 7.5 and 15 micron respectively and two different number of nozzle orifices 50 and 100 respectively.

The table shows that when both the Discharge Rate and Force of Spray Impact are relatively high that wet spots on the skin are easily triggered.

To prevent the occurrence of wet spots it can be derived from the table that the Force of Spray Impact should be lower than 1x10° kgm/s? and also that the Discharge Rate is preferably substantially lower than 400 microliter per second.

Claims (18)

-11- Conclusions: A spray device for delivering a liquid mist over a spray length (L) through air to a target, in particular a body spray mist, comprising a container for holding a liquid, pressure means to release an amount of said liquid under increased working pressure {P), and comprising a spray nozzle comprising a nozzle plate having a plurality (N) of nozzle orifices of substantially identical size (D 2e), extending through said nozzle plate and communicating with said pressure means to, during use receiving a quantity of pressurized liquid at said operating pressure, said plurality of nozzle openings dispensing said liquid mist on a discharge side of said spray nozzle over at least said spray length (L), wherein the nozzle openings of said plurality of nozzle openings have an have substantially identical dimensions (De) lying between a lower bound (Din) and an upper bound (Dax), where at said lower limit is approximately equal to: Don = ef where said upper limit is approximately equal to: Dy, = Vo: IE 7 and where A =18 n/p, where n stands for a viscosity of the air and p for a density of the liquid. Spraying device according to claim 1, characterized in that said nozzle openings have a substantially identical size (D,,,.} of less than 20 microns, that said pressure means have an amount of between 50 and 150 microliters of said bringing liquid under operating pressure of between 2 and 5 bar, and that the nozzle openings deliver said quantity of liquid of increased pressure during a period of at least 500 microseconds. -12- Spraying device according to claim 1 or 2, characterized in that the pressure means comprise a manually operable pump with at least one piston for pressurizing said liquid, said pump being adapted to pressurize said quantity of liquid V. up to said working pressure P across the nozzle plate in one stroke of the at least one piston. A spraying device according to claim 1, 2 or 3, characterized in that the quantity (N) of nozzle openings is configured to deliver a corresponding number of micro-jets of the liquid mist mutually diverging from the discharge side, the quantity of nozzle openings in particular comprises between 10 and 100 openings, and more particularly between 20 and 100 openings. Spraying device according to claim 4, characterized in that said plurality of micro-jets form a cone with a spray cone angle of between 5° and 25°. Spraying device according to one or more of the preceding claims, characterized in that the spray nozzle is configured to provide a spray impact force of the magnitude of FSI = p.V.v, /T, which is smaller than 5x10° kg.m/ s', in particular smaller than 1x107 kg.m/s?, where v, represents the impact velocity at spray length L. The spray device of claim 6, wherein the spray nozzle is configured to deliver a delivery rate of less than 100 microliters per second, at a spray length (L) of between 10 and 50 cm. Spraying device according to any one of the preceding claims, characterized in that said openings have a diameter of between 5 and 20 microns, in particular between 8 and 16 microns. Spraying device according to any one of the preceding claims, characterized in that adjacent to said openings a thickness of said nozzle plate is smaller than three times a diameter of said openings, preferably smaller than said diameter. -13- A method of applying a liquid mist to a remote target, particularly skin or coated skin of a subject, comprising: pressurizing an amount of said liquid to an elevated working pressure, and one at the elevated pressure with a initial velocity forcing the liquid through a plurality of (N) nozzle openings arranged in a nozzle plate of a spray nozzle, generating a micro-jet spray composed of (N) simultaneously emitting jets which break up into droplets, characterized in that a substantial part of the droplets hits the target with a substantially equal velocity, of which an (average) value is between 10% and 50% of the initial velocity of the emitted jets, and that N is between 10 and 100. A method of delivering a liquid mist to a target at a distance {L), according to claim 10, comprising: pressurizing an amount of said liquid to an elevated working pressure {P), forcing the liquid at the increased pressure through a plurality (N) of nozzle orifices of substantially identical size (D-spray nozzle) arranged in a nozzle plate of a spray nozzle, a delivery of said liquid mist into air on a delivery side of said nozzle plate, and a targeting the liquid mist, said substantially identical magnitude (D) lying between an oncer boundary (D‚;) and an upper boundary (Dx), said lower boundary being approximately equal to: Doin ee JP where said upper limit is approximately equal to: Don = Yo IE /p where A =18 n/p, where n stands for a viscosity of the air and p for a density of the liquid, and where an amount of between 15 and 150 microliters of said liquid is delivered substantially continuously over a period of at least about 500 microseconds. -14- A method according to claim 10 or 11, wherein the liquid is pressurized to an operating pressure of between 2 and 5 bar. A method according to claim 10 or 11, wherein said quantity of liquid is pressurized by means of a manually operable pump having at least one piston to pressurize said quantity of liquid, and wherein said quantity of liquid V is pressurized to said quantity working pressure P in a single stroke of said at least one piston. A method according to any one of claims 10 to 13, wherein the spray nozzle is operated to provide a spray impact force of the magnitude of FSI = p.V.v, /T, which is smaller than 5x10 kg.m/s°, in particular smaller. than 1x10° kg.m/s*, where v, represents the impact velocity at spray length L. The method of claim 14, wherein the spray nozzle is operated at a discharge rate of less than 100 microliters per second. A method according to claim 13 or 14, wherein the spray nozzle is kept at a distance L of between 10 and 50 cm from said target, in particular from the user's body. A method according to claim 10 or 11, wherein the vice-spray is characterized by a droplet size distribution with a Relative Span (RS) that is less than 2, in particular less than 1. A method according to claim 10 or 11, wherein the liquid mist is characterized by a droplet size distribution with a geometric standard deviation (GSD) of less than 2, in particular a GSD of less than 1.6.