WO2007077424A1

WO2007077424A1 - Electrostatic spray device

Info

Publication number: WO2007077424A1
Application number: PCT/GB2006/004919
Authority: WO
Inventors: David Waterman; Alastair Pirrie
Original assignee: Aerstream Technology Limited
Priority date: 2006-01-05
Filing date: 2006-12-22
Publication date: 2007-07-12
Also published as: US20070194156A1

Abstract

An electrostatic spray atomisation device is disclosed. The device comprises a spray electrode (64) disposed in a first recess (2) in a dielectric surface (1) , the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid. Various techniques for dealing with problems of liquid deposition around the electrodes are described. The techniques may be used alone or in combination.

Description

ELECTROSTATIC SPRAY DEVICE

This invention relates to any electrostatic spray system, such as described in European Patent 1399265, which is incorporated by reference.

European Patent 1399265 provides in one embodiment for liquid to be fed (by any suitable means) to a spray electrode, where an electric field is created between the spray electrode and a discharging electrode by the generation of a potential difference between the two electrodes. European Patent 1399265 also provides a dielectric surface, which reshapes the electric field in the vicinity of the electrodes and dielectric to maintain an efficient production of droplets from the spray electrode that do not impinge on the dielectric material.

In experiments with devices following the teaching of European Patent 1399265, we have found it to be an excellent delivery method with high efficiency and low levels of self deposition of sprayed matter. Nevertheless, there are instances when the impact of some of the spray on the dielectric material is inevitable, for example in extremely high air currents, such as found in ventilation ducting or in high winds. In such cases the small amount of spray that lands on the dielectric surface will often evaporate and any residual will have a negligible effect on the performance of the spray system over the normal lifetime of the dispenser.

However, when attempting to dispense heavier liquids, or liquids containing heavier ingredients (where the term heavy is used to describe the compound's inability or low tendency to evaporate), we have found that this residual deposition evaporates too slowly, and overtime can begin to interfere with the proper functioning of the dielectric material and thereby the whole dispenser system.

The dispenser's applications may be limited by, for example, limiting the environments in which the dispenser is used, or limiting the specified lifetime of the dispenser in such environments, or limiting or restricting the use of such heavy materials in the liquid being dispensed. However, such limitations are commercially undesirable whereas some definite advantage can be gained from being able to operate in such difficult conditions or from being able to spray matter with a high vapour pressure. Means for wicking and directing the flow of liquid matter are known in the art in relation to electrostatic spray systems, but are limited to the supply of liquid matter to be sprayed from a spray electrode or to providing an additional surface from which to evaporate matter which has been sprayed. Wick-fed spray electrodes are taught in, for example, US Patent 6297499, US Patent application 2004023411 , US Patent 7081622, European Patent 1095704, US Patent 5503335 and Japanese Patent 5192224.

The device taught in US Patent 6893618 employs at least one porous-fibre trap in a non-homogenous electric field in order to capture and then dissipate by evaporation liquid which has been caused to flow through the device.

The wicking system of US Patent US6871794 provides a wick-on-wick device increasing the surface area available for evaporation of wicked liquid.

The liquid dispersion device which is the subject of US Patent 6729552 comprises wicking material which aids evaporation of condensed spray by providing a large surface area for evaporation. The wicking material, which is a fabric such as cotton or a thin woven sheet may be assisted in its evaporative function by the heating effect of an optional proximal lamp. This device merely limits the degree to which the device becomes sodden by spraying in harsh conditions and does not provide a means by which the adverse effects of deposition on spraying may be corrected.

In accordance with a first aspect of the invention, there is provided an electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the first recess is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode such that the deposited liquid establishes an electrical contact with the spray electrode and resists deposition of further liquid by virtue of electrostatic repulsion.

By encouraging liquid deposited on the surface to make electrical contact with the spray electrode, the electric field becomes distorted such that further deposition of liquid in the same area is prevented. The invention therefore provides a self-regulating means for further deterring droplets emitted from the spray electrode away from the area of deposition.

Any patch of residual liquid coming into contact with the spray electrode effectively biases the electric field such that liquid emanating from the spray electrode is repelled away from the area of previous residue due to the mutual electrical repulsion between the highly charged spray and the dynamically charged residue. The enormous benefits of this become apparent when considering the management of such a dispenser set-up in a strong updraft. The updraft will initially cause a small amount of liquid to be deposited on the spray surface above the spray nozzle. However, once this liquid spreads out and then makes a connection to the spray electrode the field distorts, tending to fire the spray downwards against the updraft. Should the updraft stop, then the residue will continue to spread and evaporate and migrate to the spray electrode, (where it could co-mingle with the existing formulation), and as such its distorting effects on the electric field will gradually decrease and return the dispenser to its normal condition.

The dielectric spray surface may be provided with a network of small-scale surface troughs (such as described below) which provide a path towards the spray electrode. This spray feedback path is designed to deliberately distort the local electric field under extreme dispersal conditions, and it does so dynamically. When any residual liquid first lands on the dielectric spray surface, it will first spread out - usually migrating radially outwards from the point of impact. In the first instance, this helps it to evaporate by spreading the liquid across a large surface area (as in the second aspect of the invention described below), which may be sufficient control if the residual deposition was transitory, such as when there is a sudden gust of air current next to an operating device. If the deleterious cause is chronic, such as when operating inside an air conditioning duct, a greater amount of residual liquid matter might collect and would begin to spread out. Once it has spread out sufficiently, one edge of the residual liquid matter will find a path back to the spray electrode. Whilst most formulations, particularly the component remaining as residual matter on the spray surface, are largely non-conducting, they are more conductive than the dielectric itself, or at least the combination of residue and dielectric provide an easier path for the conduction of charge than through or across the dielectric alone. As a result the electric field is biassed towards (strengthened at) the discharge electrode, thereby reducing the amount of liquid being sprayed and increasing the number of ions available to discharge them, and to carry the droplets away from the device.

The reference electrode is normally disposed in a second recess in the dielectric surface.

In a preferred embodiment, the second recess is provided with at least one channel formed in the side of the recess. Typically, the at least one channel is continuous and is formed around the side of the recess.

Alternatively or in addition, the dielectric surface may be provided with at least one channel formed in the surface. The channel may be continuous and it may be disposed around the second recess.

The at least one channel may be in fluid communication with a reservoir for storing excess liquid. The reservoir may be a closed reservoir, or it may simply be a drip tray or any other surface, such as the floor.

The at least one channel may provide a dispersion path for the liquid over a surface, such as the dielectric surface, from which the liquid can evaporate.

The at least one channel may have a v-shaped cross-section, a rounded cross-section, or a semi-circular cross-section.

The provision of the channel helps prevent migration of liquid to the discharging means (i.e. the reference electrode) because any liquid naturally wetting a path from the dielectric surface to the reference electrode will touch the channel, which would carry the liquid away by capillary action, preferably into a separate reservoir. If liquid came into contact with the reference electrode, this would at best counteract the benefits of drawing the liquid back to the spray electrode, and at worst create a short circuit of sorts, which would unnecessarily drain the voltage supply. It is also possible to provide a polished finish of the recess surfaces adjacent to the discharging electrode.

Typically, the first recess is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by capillary attraction. The capillary attraction may be provided by a channel formed in the side of the first recess, the channel running from the base of the first recess to the periphery of the first recess. Preferably, however, the capillary attraction is provided by an array of channels, each of which is formed in the side of the first recess, the channel running from the base of the first recess to the periphery of the first recess. The array of channels are normally uniformly spaced around the first recess.

The at least one channel or each of the array of channels may be v-shaped in cross- section, rounded in cross-section or semi-circular in cross-section.

In one embodiment, the at least one channel or each of the array of channels is wider at the periphery of the first recess than at the base.

Alternatively, the capillary attraction may be provided by a capillary running from the base of the first recess to the periphery of the first recess.

The first recess may be adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by gravity. This may be in addition to or instead of the capillary attraction referred to above. One way of achieving this is to provide a first recess having a flared profile such that the base of the recess has a narrower diameter than the periphery where it meets the surface. Clearly, the shape of the cross-section of the first recess is not critical. It could be circular, square, triangular, rectangular, elliptical or any other cross-section. Any of these cross-sections could have the flared profile mentioned above, wherein the cross-section in whatever shape is narrower at the base of the recess than at the periphery where it meets the surface.

The dielectric surface may be adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by chemical affinity between the liquid and the dielectric surface.

The chemical affinity may be provided by a oleophilic or oleophobic coating.

Alternatively, the chemical affinity may be provided by a lyophilic or lyophobic coating. In accordance with a second aspect of the invention, there is provided an electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface, thereby increasing the surface area and evaporation rate of the liquid.

The invention therefore provides a way for driving residual liquid migration by surface tension or other means which, regardless of the surface tension between the residual liquid and the dielectric material of the spray surface, promotes the spreading of the residual liquid in order to maximise the opportunity for it to evaporate, or in the case of more heavy compounds provides an efficient path for their clearance away from the critical area in the vicinity of the electrodes themselves.

A combination of the first and second aspects of the invention provides a means for an electrostatic spraying device to clear itself of any residual build-up, and in extreme situations where either the environmental conditions or the liquid matter itself would normally limit useful application there is provided a self-regulating electric-field-distortion system to steer the spray away from the troublesome area. This invention thereby expands the environments in which this dispenser may be used, and also extends the range of compounds that may be dispensed through it, providing a significant commercial advantage.

The dielectric spray surface may be provided with a rough surface finish, such as may be obtained by using raw sintered tooling to injection-mould the dielectric, or by chemical or laser etching, sand-blasting, sand-finishing, rough-shot-peening of a standard injection-moulding tool, or by chemical or laser etching, sand-blasting, sand-finishing, or rough-shot-peening of the dielectric surface after manufacture, or where the surface is a ceramic by using a coarse sinter. A characteristic of this type of surface is that it has many small sharp peaks, ridges, troughs and valleys whereby the base of the troughs and valleys are sharp and have a low radius of curvature less than 0.5mm, more preferably below 0.1mm or more preferably still below 10μm, and ideally below 1μm. These troughs and valleys are ideally acute angled. However, obtuse angle valleys are also useful provided that the radius of curvature of the base of the trough is sufficiently small. The distance between consecutive troughs is usually random, but normally distributed about a mean value, which may be called the pattern scale-length. In general this scale-length is of the order of 1 mm or below, but may be a few hundred microns or below.

The dielectric spray surface may be provided with a textured finish, such as may be obtained by impression from a textured injection-moulding tool, or laser cutting, hand sculpting or computer numerical control (CNC) post-processing, or branding with a hot textured tool, whereby the textured finish contains a systematic interconnecting network of ridges and troughs and the base of the troughs and valleys are as sharp as possible having a low radius of curvature less than 0.5mm, more preferably below 0.1mm or more preferably still below 10μm, and ideally below 1μm. These troughs are ideally acute angled. However, obtuse angle troughs are equally useful provided the radius of curvature of the trough is sufficiently small. The distance between consecutive troughs is not of particular importance, although ideally it is less than a distance representing a potential difference of under 100V in the electric field, more preferably 10V, or more preferably still 1V. In practice, this scale-length of the texture pattern therefore depends on the proximity to the electrodes, and is perhaps 1mm. However, between the electrodes it may be advantageous to have a scale length of under 10Oμm or even 10μm, whilst away from the electrodes it could be as much as 2, 5 or 10mm depending on the application and relative position of the electrodes and dielectric spray surface.

A combination of textured and surface finishes may be used, to provide both small-scale and large-scale trough networks. Either alone or in combination these patterned networks provide a means to spread liquid out on the dielectric surface regardless of the relative surface tension between the spray surface and any residue that might happen to land on it. Ideally the radius of curvature of the troughs is sufficiently small that liquid matter can spread out relatively easily regardless of orientation. In other words the sharpness of the troughs has sufficient 'wicking' power that the liquid matter can migrate upwards against gravity if the spray surface is vertical. Furthermore, the scale-length of the pattern should also be sufficiently small that liquid matter spreads out as evenly as possible so that any effect on the electric field is not localised to a particular trough. In a preferred embodiment, the reference electrode is disposed in a second recess in , the dielectric surface.

The second recess may be provided with at least one channel formed in the side of the recess. The at least one channel may be continuous and formed around the side of the recess.

The dielectric surface may be provided with at least one channel formed in the surface. The channel may be continuous and it may be disposed around the second recess.

The at least one channel may provide a dispersion path for the liquid over a surface, such as the dielectric surface, from which it can evaporate.

The at least one channel may have a v-shaped cross-section, a rounded cross-section or a semi-circular cross-section.

The provision of the channel helps prevent migration of liquid to the discharging means (i.e. the reference electrode) because any liquid naturally wetting a path from the dielectric surface to the reference electrode will touch the channel, which would carry the liquid away by capillary action, preferably into a separate reservoir.

Typically, the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by an array of troughs formed in the surface. The dielectric surface may be formed as a pattern of ridges such that each of the troughs lie between adjacent ridges.

The array of troughs may be formed in a repeating pattern to provide an interconnecting network of troughs. Each of the troughs may have a v-shaped cross- section. The troughs in the array of troughs may be disposed in a random pattern, thereby providing a random pattern of troughs to present a surface texture.

Then again, the troughs in the array of troughs may be disposed in a non-random pattern, thereby providing a non-random pattern of troughs to present a surface texture. Such a surface may be provided by a brushing process which develops the troughs where the bristles of the brush run.

Typically, at least some of the troughs are of microscopic dimension. In this case, the radii of curvature of the bases of the troughs are below 10μm, and preferably below 1μm.

Alternatively, the dielectric surface may be; adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by at least one channel running from the periphery of the first recess towards the periphery of the surface. The at least one channel may run to the periphery of the surface. The channel may follow a spiral path.

In another alternative, the dielectric surface may be adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by a series of concentric circular channels, each channel connected to adjacent channels by one or more linking channels.

In this alternative, the first recess is typically circular, and the linking channels may run radially relative to the centre of the first recess.

These two alternatives provide the dielectric spray surface with liquid migration paths designed to delay the migration of liquid towards the spray electrode by making the migration channels more circuitous. This reduces the directionality of the distortion of the electric field when the liquid comes into contact with the spray electrode, so the response to residual droplets is less directional. This might have advantage where the dispenser was subjected to occasional blasts of air which, while temporarily causing a residue, have no real effect on chronic performance, so the requirement for directional field distortion is not necessary. The dielectric surface may be adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by an oleophilic coating over at least part of the surface.

In this case, the surface may have an oleophobic coating around the second recess, thereby preventing liquid from migrating to the second recess.

A suitable oleophilic treatment is Oxygen plasma, provided by Porton Plasma Innovations (P2i) Ltd, Unit 14, Central 127 Milton Park, Abingdon, Oxfordshire, 0X14 4SA, UK. A suitable oleophobic coating is also provided by Porton Plasma Innovations (P2i) Limited.

The dielectric surface may be adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by a lyophilic and/or lyophobic coating over at least part of the surface.

In accordance with a third aspect of the invention, there is provided an electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the dielectric surface is in fluid communication with a reservoir to which any of the liquid deposited on the surface may be drawn.

By providing a path or paths from the dielectric surface into a separate, redundancy reservoir the invention is able to cope with the deposition of a large quantity of liquid. This reservoir therefore prevents the dielectric surface becoming entirely saturated with running liquid which could seriously limit the performance of the spray device by removing the liquid to a distal location.

The reservoir is typically either well away from or behind the spray surface, and in any case does not affect the electric field around the electrodes and spray surface should liquid matter gather there. This redundancy reservoir is normally only required in extreme situations where the residual liquid matter will not evaporate from the spray surface of its own accord. Under such circumstances the liquid matter, rather than remaining on the dielectric surface where it could begin to have a detrimental effect on the electric field, will migrate to the redundancy reservoir, where it is held harmlessly out of the way.

in a preferred embodiment, the reference electrode is disposed in a second recess in the dielectric surface. Typically, the second recess is provided with at least one channel formed in the side of the recess. The channel may be continuous and formed around the side of the recess.

The dielectric surface may be provided with a channel formed in the surface. The channel may be continuous and may be formed around the second recess.

Preferably, the at least one channel is in fluid communication with the reservoir for storing excess liquid. The reservoir may be a closed reservoir, or it may simply be a drip tray or any other surface, such as the floor.

The at least one continuous channel may have a v-shaped cross-section, a rounded cross-section or a semi-circular cross-section.

The provision of the channel helps prevent migration of liquid to the discharging means (i.e. the reference electrode) because any liquid naturally wetting a path from the dielectric surface to the reference electrode will touch the channel, which would carry the liquid away by capillary action, preferable into the reservoir.

The dielectric surface is normally in fluid communication with the reservoir by way of one or more conduits leading from the surface to the reservoir.

Liquid deposited on the surface may be drawn to the reservoir by virtue of gravity.

Alternatively or in addition, liquid deposited on the surface may be drawn to the reservoir by virtue of capillary attraction. The dielectric surface may be adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by chemical affinity between the liquid and the dielectric surface.

The chemical affinity may be provided by a oleophilic or oleophobic coating.

The chemical affinity may be provided by a Iyophilic or lyophobic coating.

The device typically further comprises a porous member within the reservoir to absorb liquid. The porous member is typically a sponge.

The first, second and third aspects of the invention may be used alone or in any combination. It is envisaged that there may be provided:

i) a combination of an electrostatic spray atomisation device according to the first aspect with an electrostatic spray atomisation device according to the second aspect;

ii) a combination of an electrostatic spray atomisation device according to the first aspect with an electrostatic spray atomisation device according to the second aspect;

iii) a combination of an electrostatic spray atomisation device according to the second aspect with an electrostatic spray atomisation device according to third aspect; and

iv) a combination of an electrostatic spray atomisation device according to first aspect with an electrostatic spray atomisation device according to the second aspect and with an electrostatic spray atomisation device according to the third aspect.

In each aspect, the invention therefore provides a specially configured dispenser system which addresses the management of build-up of residual heavy compounds on the dispenser dielectric spray surface.

There now follows a description of the invention by way of example with reference to the accompanying drawings, in which:

Figure 1 shows two views of one possible dispenser spray surface configuration which embodies this invention; Figure 2 shows another dispenser spray surface configuration which embodies this invention;

Figures 3a and 3b show surface profiles according to the invention;

Figures 4a to 4f show the spreading of oil droplets on different surfaces as described herein;

Figures 5a to 5f illustrate schematically the action of the barrier migration channel;

Figure 6a to 6c show exemplary views of a dispenser comprising one possible embodiment of this invention;

Figures 7a and 7b show possible textured finishes according to the invention which delay the migration of the residual liquid to the spray electrode; and

Figures 8a to 8c show different tool treatments, illustrating an advantageous tool design for achieving a preferred surface texture.

Figure 1 shows two illustrations of a dispenser with one possible dielectric spray surface configuration embodying the present invention. A dielectric spray surface 1 is shown having all the necessary features, including a spray electrode recess 2 and a discharging electrode recess 3 meeting with a dielectric spray surface 4, which has both a macroscopic textured finish and sand-blasted roughness. Also illustrated are radial liquid migration channels 5 which provide a passage for liquid residue from the spray surface 4 to the base of the spray electrode (not shown). There are also shown perpendicular barrier channels 6 which further prevent liquid from migrating from the spray surface 4 to the discharging electrode (not shown).

This invention may be used with an electrostatic dispenser such as the one described in European Patent 1399265, which provides a liquid reservoir feeding liquid to the spray electrode and a voltage generation means to create an electric potential between the spray electrode and the discharging electrode. Under normal use, a liquid exposed at the tip of the spray electrode is subjected to a strong electric field which acts against the surface tension of the liquid, causing it to break up into charged droplets. Similarly, the electric field also creates at the discharge electrode ions of opposite polarity to the sprayed liquid Through a combination of these two oppositely-charged entities in the space in front of the spray surface 4 the viscous drag of the droplets in the air becomes the dominant force upon them, and they are propelled away from the device under the gentle breeze generated by the initially rapid movement of the original charged entities.

Whilst the action of the dielectric spray surface 4 combined with these charged entities is usually sufficient to keep the spray surface free from impact of the droplets, this is not the always the case under certain extreme conditions. Such extreme conditions include: prolonged or rapid forced air flow towards or across the dispenser; the spraying of certain heavy, non-volatile compounds; extended use of a dispenser for periods of the order of a year or fractions thereof depending on the duty cycle of the dispenser; or a combination thereof. Under such conditions it is possible for some of the droplets, or the non-volatile fraction thereof to deposit on the spray surface 4. These residual droplets form an undesirable residue on the spray surface.

In many cases the small quantity of this droplet residue is negligible since it evaporates from the spray surface naturally. However, the dielectric spray surface 4 is provided with a surface finish as well as a textured finish which can be seen as small inter-connecting raised elements on the surface with sharp troughs where the interconnected raised elements meet. Both of these features act to draw the liquid away from where it first lands, causing it to spread out, thus increasing the surface area exposed to air and hence increasing the overall evaporation of the residue.

For many applications the enhanced action of spreading out the residue is sufficient to evaporate it and render its effect on the performance of the spray system negligible. However, for particularly sticky, heavy, non-volatile liquids this may not be sufficient, and spray feedback migration channels 5 are provided together with the barrier channels 6 to ensure that liquid is more likely to migrate towards the spray electrode. Once residual liquid reaches the spray electrode (and does not reach the discharge electrode) the residue distorts the electric field, by first increasing the field strength around the discharging electrode, thereby increasing the number of ions and attracting them towards the zone of deposition thus hindering and usually preventing further deposition, In addition, by means of the spray feedback migration channels, the field is distorted so that the droplets tend to be sprayed away from the original zone of deposition, attenuating any further deposition.

Finally, if these measures are still not sufficient to prevent some of the droplets from impacting on the spray surface, the excess liquid is drawn down into a redundancy reservoir 8 well away from the spray surface via an excess residue migration path 7.

Figure 2 shows another spray surface modified according to the present invention, where the spray surface 21 has been provided with a textured finish, and three spray feedback migration channels 22 are provided to make the necessary connection between any residual liquid and the spray electrode should any droplets land on the spray surface.

In this case the dielectric material is made from polypropylene, and has been injection moulded. Other possible materials for the textured spray surface include acrylic, acrylonitrile butadiene styrene (ABS), ceramics and ceramic blends, glass and glass blends, polyethylene terephthalate (PET), nylon (6, 11 and 66), polyether- and styrene-butadiene blends, polyacetyl-polytetrafluoroethylene (PTFE) blends, polybutylene terephthalate (PBT) blended with glass or mineral or mica or combinations thereof, polycarbonate blended with ABS or blends of ABS and glass, polyetherimide, polyethylene, polyketone, polyphenylene sulphide blended with glass, polyphthalamide, polypropylene, polystyrene, polysulfone blended with glass, polyvinylidene fluoride (PVDF), styrene acrylonitrile, as well as reinforced versions of the same plastics using, for example, glass, minerals, aluminium, zinc, steel, tin, copper, mica and blends thereof with each other or with glasses, organic molecules and minerals, for example, such as for changing the colour of the plastic.

Figures 3a and 3b illustrate magnified cross-sections of two different types of surface finish profile shown as sections through the dielectric spray surface 4. Figure 3a is a preferred surface finish since it has created sharp troughs in the spray surface 4, which act as microscopic wicking channels to draw any residue liquid away from the deposition zone and to spread it out. This surface finish works well even if the contact angle between the liquid residue and the dielectric spray surface is, high. Figures 4a to 4f illustrate how surface roughness helps to spread liquid out on a surface. Figures 4a and 4b show a drop of oil on a polished polypropylene surface immediately after deposition on the surface and after an interval of 30 seconds respectively. It is clear that there has been no appreciable spreading of the oil in this time. By contrast figures 4c to 4f show a rough polypropylene surface as described in this invention with a similar oil drop. Figure 4c shows the drop immediately after deposition on the surface, and figures 4d to 4f show the same drop at consecutive time intervals of 10 seconds after deposition. In the same 30 second interval the drop on the rough surface has spread out over an area approximately 16 times that at the start, improving evaporation of residue from the spray surface.

The traditional Roughness Average (RA) figures are of little use when specifying the finish of the spray surface, as this only indicates the amplitude of the roughness not the acuteness of the trough in the surface. This invention provides for roughness both on the microscopic scale, with a Roughness Average of say 1 μm, up to macroscopic textured finishes where the RA figure is of the order of 1mm. These extremes of RA figures and those in between them are all valid within the scope of this invention.

Methods used to create the required roughness on the dielectric spray surface generally fall into two categories. The first method is to create an untreated dielectric spray surface in a conventional way, such as CNC milling, casting or injection moulding, and then to provide a post-production process. The second method is to employ either a casting or injection moulding process, where the inverse of the required surface finish is produced on the tool, so that when the part is turned out the required finish is created on it by nature of its contact with the inverted pattern on the tool. The latter is likely to be the easier method, provided the quality of the tool does not deteriorate over the course of a production run.

Various post-production processes can be used to create the required surface roughness. They range in complexity from sand-papering, sand-blasting, shot-peening to laser etching.

Whilst these provide the rough surface required on the dielectric spray surface 4, these methods can only be used with limited success or ease to create the radial spray feedback migration paths. Sand blasting and shot-peening cannot be made directional, and so radial liquid migration channels can only be made in this way by employing a complex set of masks, which would usually be either too delicate or too coarse to be of much practical benefit. Similarly sand-papering cannot be used to create a migration pattern. Laser etching is sufficiently flexible to be able to provide a wide variety of patterns including the desired radial pattern, but this is an expensive process and therefore probably uneconomical for most applications.

One simple method of generating the migration path is to score lines in the dielectric spray surface using a sharp, pointed object, such as a razor, scalpel, pin, hook or needle, Such score lines need only be of the order of 0.1 mm to 1 mm deep, and can be less wide than they are deep, such that the width to depth ratio is anything from 1 to 10 or more. This method is particularly suitable for small production volumes or where the dispenser is being assembled by hand.

When casting or injection moulding is used to create the dielectric spray surface 4, the main requirement is to modify the tool so that the resultant dielectric has the required properties, which are a network of inter-connecting troughs with sharp bottoms, which means the tool must have a network of inter-connecting ridges with sharp peaks. These sharp peaks can be created by using a sintered metal surface in the appropriate region of the production tool, or by creating a textured finish directly on the tool by sanding, sand blasting, shot peening, scouring, scratching, milling, grinding, or chemical etching for example. All of these techniques can also be employed, either on their own or in combination as well as being combined with other surface treatments such as sputtering, chemical vapour deposition of another material, for example Titanium vapour deposition on Steel.

Figures 5a to 5c and Figures 5d to 5f demonstrate the fate of a droplet 51 sitting on a vertical plane 52, where the plane 52 of Figures 5d to 5f also has a barrier clearance trough 53 running vertically down the plane 52. Figures 5b and 5c then show how the droplet 51 in figure 5a spreads out gradually over the vertical plane. Note it is assumed here that the volume of the liquid residue contained in the droplet is small, and that correspondingly the gravity forces on the droplet 51 are negligible compared to the surface tension forces between the droplet 51 and the plane 52. In Figure 5d an identical droplet 51 is shown, but here the plane 52 contains a vertical clearance trough running from top to bottom 53, in a similar fashion to the perpendicular barrier channel 6 shown in Figure 1. Figures 5e and 5f then show that, although at first the droplet 51 expands and spreads out, as soon as it touches the vertical clearance trough 53 the capillary forces suck the liquid out of the droplet 51 and down the clearance channel 53, thus preventing the droplet from spreading across the clearance trough 53. In this way a barrier is set up to prevent residue from migrating off the dielectric spray surface 4 and back onto the discharging electrode. It is assumed here that the discharging electrode is situated somewhere in direct contact with the plane 52 to the right of each picture.

In practice these barrier clearance troughs 53 can be made by hand after the dielectric spray surface has been manufactured using a sharp scoring implement such as a razor, scalpel, pin, hook or needle. In production the surface surrounding the barrier troughs should be as smooth as possible, so any tool used to create the dielectric spray surface should be polished in this area. The trough itself can be a V-shaped notch, or a small step (which creates an approximately 90 degree notch), as indicated by feature 6 in figure 1.

Figures 6a and 6b show two views of a dispenser with an alternative dielectric spray surface 61 embodying this invention. Here the spray surface 61 has been given a microscopic rough finish that is not visible at this scale. A spray electrode recess 62 and discharge electrode recess 63 house a spray electrode 64 and discharge electrode 65 respectively. In this embodiment, only a single spray feedback migration channel 66 is provided together with a plurality of perpendicular barrier channels 67 to prevent any residue on the spray surface 61 from making contact with the discharge electrode 65.

A redundancy reservoir 68 sits below the main dielectric spray surface 61. Excess residue on the spray surface 61 drains via the redundancy conduit channels 69 to reservoir 68. Figure 6c shows the redundancy reservoir 68 on its own, and from this it is easy to see that liquid reaching the front of a redundancy conduit channel 69 will be drawn back into the redundancy reservoir 68 by a combination of gravity and capillary forces. Once there it can be prevented from spilling or leaking by an optional porous plug or sponge (not shown) which fills the redundancy reservoir 68. It should be noted that this particular design, instead of having a microscopic rough finish on the dielectric spray surface 61 , could be coated with a oleophilic coating which provides the same ability for residual droplets to quickly spread out across the front surface. In addition an oleophobic coating, such as supplied by Porton Plasma Innovations (P2i) Ltd, Unit 14, Central 127 Milton Park Abingdon, Oxfordshire 0X14 4SA, UK, could be applied around the discharging electrode to prevent or at least hinder the migration of residue from the spray surface 61 to the discharging electrode 65.

Without limiting the scope of texture patterns according to the invention, Figures 7a and 7b show examples of non-homogenous spray feedback migration texture patterns, where 7a forms a spiral arrangement and 7b a staggered arrangement of migration troughs. In both of these arrangements the textured finish is formed by the creation of narrow V-notch troughs, whose paths are given by the lines 71 and 72 in the drawing on the dielectric spray surface. These patterns have the added advantage of delaying the time for the liquid residue to reach the spray electrode, thus providing more time for the residue to simply evaporate from the spray surface. Note that whilst the inter-trough distance here is of the order of 1 mm, these patterns can be scaled down so this distance is as low as 0.1mm or so that one trough sits next to its neighbour.

Figures 8a to 8c illustrate a beneficial modification to the dimpled dielectric spray surface 4 of figure 1. Figure 8a shows a cross-sectional illustration of a tool 81 and resultant dielectric spray surface 82. The sharp peaks required in the tool (e.g. 83) are sometimes difficult to make leading to extra up-front manufacturing cost. In figure 8b the sharp peaks have become rounded 84, which leads to a corresponding rounding of the troughs formed in the dielectric spray front 85. Such rounded troughs are less suitable at causing liquid residue migration, and are therefore less desirable.

Figure 8c shows tool peaks 86 which have been modified to maintain troughs with ultra sharp twin edges 87, where the edges themselves (e.g. 88) have the required minimum radius of curvature. Such tools are also much easier to manufacture, as they can be created from a tool like that exemplified in Figure 8b and simply milled to create the finish shown in Figure 8c. If the sharp corners ever become slightly blunted, a simple milling operation is all that is required to restore the original sharpness. The invention described herein is suitable for the electrostatic dispersal of a wide range of liquids. Suitably such a liquid may comprise a component which is a fragrance oil. The liquid may comprise an insecticide component or components, for example, pyrethroid compounds including:

pyrethrins (such as pyrethrins 1 , 3 and 11), e.g. as 25 % or 50 % pyrethrum extract (the balance typically containing plant oils, other plant extracts and light paraffin oils);

- allethrins and related non-natural compounds, including d-allethrin, bioallethrin, esbiothrin (EBT), S-bioallethrin (S-Biol); resmethrin and bioresmithrin; bifenthrin, permethrin, deltamethrin, cypermethrin, alpha-cypermethrin, cyphenothrin, lambda-cyhalothrin, vaporthrin, transfluthrin, prallethrin and selected isomers such as Etoc, tefluthrin, pynamin, pynamin forte, neopynamin, metofluthrin, sumithrin, and imiprothrin.

Active insecticide ingredients of a liquid which may be dispersed by the invention described herein may also include organic phosphorus compounds including, but not limited to, fenitrithion, malathion, ciafos, diazinon, DDVP (dichlorvos), and carbamate compounds including carbaryi.

Active pest control ingredients include synergists, for example piperonyl butoxide which may suitably be used in combination with insecticides such as pyrethrins and other pyrethroids.

Active insect attractant ingredients which may be used include mosquito and tsetse fly attractants such as octenol.

Active insect repellant ingredients of a liquid which may be used include, for example, citronella, DEET (N,N-diethyl-meta-toluamide) or D-limonene.

Active air-cleaner or freshener ingredients, active anti-microbial ingredients, active anti-fungal ingredients and active anti-allergenic ingredients (which may include compounds known to neutralize or denature allergens), which may collectively be classed as active air sanitizers and may be dispersed by the invention, may include, for example, n-alkyl dimethyl benzyl ammonium chlorides, n-alkyl dimethyl ethyl benzyl ammonium chlorides, alkyl dimethyl 1-naphthylmethyl ammonium chloride, benzalkonium chloride, 1 , 2-benzisothiazolin-3-one, isopropanol, orthobenzylparachlorophenol, orthophenylphenol, paratertamylphenol, potassium peroxomonosulphate, sodium dodecylbenzene sulphonate, and sodium hypochlorite. Furthermore, active air sanitizers which may be used may include, for example, compounds which are known to have antioxidant, stabilising or other desirable properties (in particular those which are approved for human consumption), such as salts of acetic acid, salts of benzoic acid, salts of butyric acid, salts of citric acid, salts of lactic acid, salts of propanoic acid, salts of tartaric acid.

Glycols (including dipropylene glycol, propylene glycol, triethylene glycol) may also be used as active air sanitizer components when they are not included as or within the carrier.

Active medicament ingredients of a liquid to be dispersed by the invention may include, for example, ingredients which are diuretics (including bumatenide, furosemide, hydrochlorothiazide, melalazone, spironolactone, torsemide), hypotensive agents (including amiodarone, flecainide, procainamide, sotalol), vasodilators (including benazopril, captopril, enalapril, fosinopril, hydralazine, isosorbide dinitrate, isosorbide mononitrate, lisinopril, moexipril, perindopril, quinapril, ramipril), or otherwise suitable for treating cardiac conditions (including acebutolol, amlodipine, aspirin, atenolol, bepridil, caffeine, candesartan, irbesartan, losartan, valsartan, warfarin, busoprolol, carvedilol, diltiazem, digitoxin, dobutamine, esmolol, felodipine, labetalol, metoprolol, milrinone, nicardipine, nicotine, nifedipine, nisoldipine, propanolol, timolol, verapamil). Active medicaments ingredients of such a liquid may further include, for example, ingredients which are anti-microbial i.e. anti-bacterial or anti-viral (including, acyclovir, adefovir, amantidine, amoxicillin, amoxicillin-clavulanate, ampicillin, azithromycin, cefaclor, cefdinir, cefpodoxime proxetil, cefriaxone, cefotaxime, cefuroxime, cefuroxime acetil, clarithromycin, ciprofloxacin, clavulanate potassium, doxycycline, entecavir, famiciclovir, gatifloxacin, gentamicin sulphate, glycyrrhizin , interferon alfa-2b, lamivudine, loracarbef, minocycline, moxifloxacin hydrochloride, oseltamivir, peniciclovir, penicillins, riampin, ribavirin, rimantidine, telithromycin, trimethoprim/sulfamethoxazole, valacyclovir, valacycolvir hydrochloride, vancomycin, zanamivir, zidovudine) or anti-fungal (e.g. clotrimazole or fluconazole). Active medicament ingredients of such a liquid may additionally include, for example, ingredients which are suitable for treating bronchial, pulmonary or rhinal conditions (including albuterol, alupent, beclomethasone, beclomethasone dipropionate, bitolerol mesylate, bosentan, budesonide, cromolyn, ephedrine, epinephrine, flunisolide, fluticasone propionate, formoterol, guaifenesin, L-albuterol, mometasone, mometasone furoate, montelukast, nedocromil, omalzimab, pseudophedrine, salmeterol, tertbutaline sulphate, theophylline, triamcinolone, triamcinoline acetonide, zafirlukast, zielutin).

Active medicament ingredients of a liquid to be dispersed by the invention may further include, for example, ingredients which are anti-histamine or otherwise anti-inflammatory (including azelastine, cetirizine hydrochloride, chlorpheniramine maleate, desloratadine, dimetane, diphenhydramine hydrochloride, loratadine, phenylephrine).

Active medicament ingredients of such a liquid may also include, for example, ingredients which are anti-depressant (including amitriptyline, amoxapine, citalopram, clomipramine, desipramine, doxepin, fluoxetine, furazolidone, imipramine, linezolid, maprotiline, nortiptyline, paroxetine, phenelzine, protriptyline, selegiline, sertraline, tranylcypromine).

Active medicament ingredients of such a liquid to be dispersed may still further include, for example, ingredients which are suitable for treating glaucoma (including apraclonidine hydrochloride, betaxolol hydrochloride, brimonidine tartrate, carteolol hydrochloride, dipivefrin hydrochloride, dorzolamide hydrochloride, levobunolol hydrochloride).

Active medicament ingredients of such a liquid may also include, for example antineoplastic agents (including anastrozole, bicalutamide, carbiplatin, cisplatin, docetaxel, etoposide, exemestane, flutamide, gefitinib, gemcitabine, irinotecan, megestrol, letrozole, nilutamide, paclitaxel, tamoxifen citrate, vinorelbine).

Active medicament ingredients of a liquid which may be dispersed by the invention may yet further include, for example, ingredients which are anti-spasmodic and/or anti-Parkinsonian agents (including amomorphine, belladonna, benztropine mesylate, biperiden hydrochloride, bromocriptine, carbidopa, entacapone, glycopyrrolate, levodopa, orphenadrine citrate, pergolide, pramipexole, procyclidine hydrochloride, ropinirole, selegiline, talcapone, trihexyphenidyl hydrochloride).

In addition, active medicament ingredients of such a liquid may include, for example ingredients which have anaesthetic properties (including, benzocaine, bupivacaine, chloroprocaine, lidocaine, mepivacaine, nesacaine, prilocaine, procaine, tetracaine).

Active medicament ingredients of a composition may also include, for example keratolytic agents (including anthralin, calcipotriene, salicylic acid, tazarotene) and PDEV (Phosphodiesterase-V) inhibitors such as sildenafil, vardenafil, tadalafil and nicotinic receptor agonists e.g. nicotine.

Active medicament ingredients of such a liquid may further include, for example, ingredients which are analgesic (including, acetaminophen, capsaicin, celecoxib, choline magnesium trisalicylate, codeine, etodolac, hydrocone, ibuprofen, indomethacin, ketoprofen, ketorolac tromethamine, meloxicam, methyl salicylate, morphine, nabumetone, naproxen, naproxen sodium, oxaprozin, oxycodone, paracetamol, D-phenylalanine, piroxicam, propoxyphene, salsalate, sulindac, tramadol, valdecoxib). The invention described herein is suitable for the electrostatic dispersal of liquids into harsh environments where there is a strong cross wind, such as within the ventilation ducting or air conditioning or heating unit of a building (temporary or otherwise), car, aeroplane, helicopter, train, lorry, van, or other mode of transport or when the air around the dispenser is blustery, such as when a dispenser is situated outdoors.

The invention described herein is suitable for the electrostatic dispersal of liquids into excessively humid environments where there Is a possibility of water vapour condensing onto the dielectric spray surface. Whilst initially, such condensation or humidity, might adversely effect the electric field, the configuration described in this invention offers the best possible management of surface condensed water, and also the fastest route for system recovery, since the water will begin to collect and migrate according to the various surface treatments, so the system will dynamically and automatically compensate for the changes on the dielectric spray surface. Note that the system is equally suitable for the dispersal into other saturated vapour environments, where for example the dispenser is being used to atomise a liquid compound into a sealed environment, and where the vapour concentration of the compound would be expected to approach its saturation level.

Other applications and modifications thereof will be apparent to persons skilled in the art.

Claims

1. An electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the first recess is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode such that the deposited liquid establishes an electrical contact with the spray electrode and resists deposition of further liquid by virtue of electrostatic repulsion.

2. An electrostatic spray atomisation device according to claim 1 , wherein the reference electrode is disposed in a second recess in the dielectric surface.

3. An electrostatic spray atomisation device according to claim 2, wherein the second recess is provided with at least one channel formed in the side of the recess.

4. An electrostatic spray atomisation device according to claim 3, wherein the at least one channel is continuous and is formed around the side of the recess.

5. An electrostatic spray atomisation device according to any of claims 1 to 4, wherein the dielectric surface is provided with at least one channel formed in the surface.

6. An electrostatic spray atomisation device according to any of claims 3 to 5, wherein the at least one channel is in fluid communication with a reservoir for storing excess liquid.

7. An electrostatic spray atomisation device according to any of claims 3 to 6, wherein the at least one channel provides a dispersion path for the liquid over a surface from which it can evaporate.

8. An electrostatic spray atomisation device according to claim 7, wherein the surface is the dielectric surface.

9. An electrostatic spray atomisation device according to any of claims 3 to 8, wherein the at least one channel has a v-shaped cross-section.

10. An electrostatic spray atomisation device according to any of claims 3 to 8, wherein the at least one channel has a rounded or semi-circular cross-section.

11. An electrostatic spray atomisation device according to any of the preceding claims, wherein the first recess is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by capillary attraction.

12. An electrostatic spray atomisation device according to claim 11 , wherein the capillary attraction is provided by a channel formed in the side of the first recess, the channel running from the base of the first recess to the periphery of the first recess.

13. An electrostatic spray atomisation device according to claim 11 , wherein the capillary attraction is provided by an array of channels, each of which is formed in the side of the first recess, the channel running from the base of the first recess to the periphery of the first recess.

14. An electrostatic spray atomisation device according to claim 13, wherein the array of channels are uniformly spaced around the first recess.

15. An electrostatic spray atomisation device according to any of claims 12 to 14, wherein the at least one channel or each of the array of channels is v-shaped in cross- section.

16. An electrostatic spray atomisation device according to any of claims 12 to 14, wherein the at least one channel or each of the array of channels is rounded or semicircular in cross-section.

17. An electrostatic spray atomisation device according to any of claims 12 to 14, wherein the at least one channel or each of the array of channels is wider at the periphery of the first recess than at the base.

18. An electrostatic spray atomisation device according to claim 11 , wherein the capillary attraction is provided by a capillary running from the base of the first recess to the periphery of the first recess.

19. An electrostatic spray atomisation device according to any of the preceding claims, wherein the first recess is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by gravity.

20. An electrostatic spray atomisation device according to any of the preceding claims, wherein the dielectric surface is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by chemical affinity between the liquid and the dielectric surface.

21. An electrostatic spray atomisation device according to claim 20, wherein the chemical affinity is provided by a oleophilic or oleophobic coating.

22. An electrostatic spray atomisation device according to claim 20, wherein the chemical affinity is provided by a lyophilic or lyophobic coating.

23. An electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface, thereby increasing the surface area and evaporation rate of the liquid.

24. An electrostatic spray atomisation device according to claim 23, wherein the reference electrode is disposed in a second recess in the dielectric surface.

25. An electrostatic spray atomisation device according to claim 24, wherein the second recess is provided with at least one channel formed in the side of the recess.

26. An electrostatic spray atomisation device according to claim 25, wherein the at least one channel is continuous and is formed around the side of the recess.

27. An electrostatic spray atomisation device according to any of claims 23 to 26, wherein the dielectric surface is provided with at least one channel formed in the surface.

28. An electrostatic spray atomisation device according to any of claims 25 to 27, wherein the at least one channel is in fluid communication with a reservoir for storing excess liquid.

29. An electrostatic spray atomisation device according to any of claims 23 to 28, wherein the at least one channel provides a dispersion path for the liquid over a surface from which it can evaporate.

30. An electrostatic spray atomisation device according to claim 29, wherein the surface is the dielectric surface.

31. An electrostatic spray atomisation device according to any of claims 25 to 30, wherein the at least one continuous channel has a v-shaped cross-section.

32. An electrostatic spray atomisation device according to any of claims 25 to 30, wherein the at least one continuous channel has a rounded or semi-circular cross- section.

33. An electrostatic spray atomisation device according to any of claims 23 to 32, wherein the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by an array of troughs formed in the surface.

34. An electrostatic spray atomisation device according to claim 33, wherein the array of troughs are formed in a repeating pattern to provide an interconnecting network of troughs.

35. An electrostatic spray atomisation device according to claim 34, wherein each of the troughs has a v-shaped cross-section.

36. An electrostatic spray atomisation device according to any of claims 33 to 35, wherein the troughs in the array of troughs are disposed in a random pattern, thereby providing a random pattern of troughs to present a surface texture.

37. An electrostatic spray atomisation device according to any of claims 33 to 35, wherein the troughs in the array of troughs are disposed in a non-random pattern, thereby providing a non-random pattern of troughs to present a surface texture.

38. An electrostatic spray atomisation device according to any of claims 33 to 37, wherein at least some of the troughs are of microscopic dimension.

39. An electrostatic spray atomisation device according to claim 38, wherein the radii of curvature of the bases of the troughs are below 10μm, and preferably below 1μm.

40. An electrostatic spray atomisation device according to any of claims 23 to 32, wherein the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by at least one channel running from the periphery of the first recess towards the periphery of the surface.

41. An electrostatic spray atomisation device according to claim 40, wherein the channel follows a spiral path.

42. An electrostatic spray atomisation device according to any of claims 23 to 32, wherein the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by a series of concentric circular channels, each channel connected to adjacent channels by one or more linking channels.

43. An electrostatic spray atomisation device according to claim 42, wherein the first recess is circular, and the linking channels run radially relative to the centre of the first recess.

44. An electrostatic spray atomisation device according to any of claims 23 to 43, wherein the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by an oleophilic coating over at least part of the surface.

45. An electrostatic spray atomisation device according to claim 44 when dependent on claim 24, wherein the surface has an oleophobic coating around the second recess, thereby preventing liquid from migrating to the second recess.

46. An electrostatic spray atomisation device according to any of claims 23 to 45, wherein the dielectric surface is adapted such that any of the liquid deposited on the surface in use is caused to disperse across the surface by a lyophilic and/or lyophobic coating over at least part of the surface.

47. An electrostatic spray atomisation device comprising a spray electrode disposed in a first recess in a dielectric surface, the spray electrode being connected, in use, in an electric circuit with a reference electrode enabling a potential difference to be applied between the spray and reference electrodes to atomise a liquid, characterised in that the dielectric surface is in fluid communication with a reservoir to which any of the liquid deposited on the surface may be drawn.

48. An electrostatic spray atomisation device according to claim 47, wherein the reference electrode is disposed in a second recess in the dielectric surface.

49. An electrostatic spray atomisation device according to claim 48, wherein the second recess is provided with at least one channel formed in the side of the recess.

50. An electrostatic spray atomisation device according to claim 49, wherein the at least one channel is continuous and is formed around the side of the recess.

51. An electrostatic spray atomisation device according to any of claims 47 to 50, wherein the dielectric surface is provided with at least one channel formed in the surface.

52. An electrostatic spray atomisation device according to any of claims 47 to 51 , wherein the at least one channel is in fluid communication with the reservoir for storing excess liquid.

53. An electrostatic spray atomisation device according to any of claims 47 to 52, wherein the at least one channel provides a dispersion path for the liquid over a surface from which it can evaporate.

54. An electrostatic spray atomisation device according to claim 53, wherein the surface is the dielectric surface.

55. An electrostatic spray atomisation device according to any of claims 47 to 54, wherein the at least one continuous channel has a v-shaped cross-section.

56. An electrostatic spray atomisation device according to any of claims 47 to 54, wherein the at least one continuous channel has a rounded or semi-circular cross- section.

57. An electrostatic spray atomisation device according to any of claims 49 to 56, wherein the dielectric surface is in fluid communication with the reservoir by way of one or more conduits leading from the surface to the reservoir.

58. An electrostatic spray atomisation device according to any of claims 49 to 57, wherein liquid deposited on the surface is drawn to the reservoir by virtue of gravity.

59. An electrostatic spray atomisation device according to any of claims 49 to 58, wherein liquid deposited on the surface is drawn to the reservoir by virtue of capillary attraction.

60. An electrostatic spray atomisation device according to any of claims 47 to 59, wherein the dielectric surface is adapted to draw any of the liquid deposited on the surface in the region of the first recess towards the spray electrode by chemical affinity between the liquid and the dielectric surface.

61. An electrostatic spray atomisation device according to claim 60, wherein the chemical affinity is provided by a oleophilic or oleophobic coating.

62. An electrostatic spray atomisation device according to claim 60, wherein the chemical affinity is provided by a lyophilic or lyophobic coating.

63. An electrostatic spray atomisation device according to any of claims 49 to 62, further comprising a porous member within the reservoir to absorb liquid.

64. An electrostatic spray atomisation device according to claim 63, wherein the porous member is a sponge.

65. A combination of an electrostatic spray atomisation device according to any of claims 1 to 22 with an electrostatic spray atomisation device according to any of claims 23 to 46.

66. A combination of an electrostatic spray atomisation device according to any of claims 1 to 22 with an electrostatic spray atomisation device according to any of claims 47 to 64.

67. A combination of an electrostatic spray atomisation device according to any of claims 23 to 46 with an electrostatic spray atomisation device according to any of claims 47 to 64.

68. A combination of an electrostatic spray atomisation device according to any of claims 1 to 22 with an electrostatic spray atomisation device according to any of claims 23 to 46 and with an electrostatic spray atomisation device according to any of claims 47 to 64.