Abstract:
A method for delivering an aerosol, especially an aromatic aerosol, comprising the steps of contacting a capillary wick, comprising an EHD comminution site, with a liquid source, whereby at least a portion of the liquid transports to the EHD comminution site; applying a voltage to the liquid within the capillary wick at a location spaced apart from the liquid source and proximate the EHD comminution site; and applying a ground reference at a location external to the EHD comminution site, wherein at least a portion of the liquid EHD comminutes to form a spray having a generally-consistent flowrate and a device therefor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application claims priority to the following US Provisionals: “High Voltage Apparatus for Aerosol Delivery”, 60/652,059; “Apparatus for Aerosol Delivery Using Capillary Pumping from a Reservoir”, 60/652,060; “Apparatus for Aerosol Delivery Using Capillary Pumping”, 60/652,064; “Capillary Tip Geometries”, 60/652,057; and “Capillary Wick Aerosol Candle”, 60/652,067, the contents of each of which are fully incorporated herein. 
     
    
     STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A CD  
       [0003]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     This invention relates to a device and method for dispensing aerosol sprays in a manner which promotes rapid and consistent vaporization. In particular, the invention relates to dispensing aromas.  
         [0006]     2. Background  
         [0007]     There are various known techniques for dispensing or dispersing aromas or olfactory stimulants. For example, aromatic oils are often dispersed by application of heat to an evaporation surface. The heat may, however, detrimentally affect the aroma being dispensed. As well, where the aroma dispensing device comprises an aromatic candle, the vapors carrying the aroma are often denatured or oxidized in the candle flame, reducing the intrinsic or “natural” quality of the fragrance. Other aroma dispensing devices rely on the use of propellants or aerosols to enable the dispersion. However, such propellants and aerosols may also detrimentally affect the aroma being dispersed.  
         [0008]     In the conventional aroma delivery devices described above, it is difficult to consistently and precisely control delivery of the sprayed material. For example, in the case of an aromatic candle or other aroma delivery device that operates by using heat causing evaporation, some degree of evaporation will continue after the candle has been blown out or the device has been switched off. In addition, such devices generate an aroma a single aroma, continuously as long as the device is activated. This causes saturation of the olfactory senses and the perceived fragrance declines. Also, aerosol cans and pump sprays may produce large droplets which do not vaporize well and tend to rapidly fall under gravity and settle, also resulting in a continuous or lingering aroma which may degrade with time. Other devices, such as solid evaporative devices, experience a decay in aroma delivery rate over time.  
         [0009]     U.S. Pat. No. 5,196,171 to Peltier describes the generation of vapors and/or aerosols by applying a DC voltage to a wick-like, porous emitter. In this case, the wick comprises a porous “capillament assembly” in which is disposed a central conductive electrode. In operation, the liquid provides a means of conducting the charge from the center of the wick to the outer surface where vaporization takes place due to corona discharge. The greatest concentration of vapors is created at the corners and edges (points or sharp radius edges) where the corona discharge forms.  
         [0010]     Aerosols may also be created by the application of electrohydrodynamic (“EHD”) forces to a liquid. In doing so, the liquid forms a so-called Taylor cone at the EHD comminution site, becomes charged, and forms a jet or ligament which separates, or comminutes, into an aerosol. In utilizing EHD, it is desirable to keep voltages low to avoid corona discharge which is detrimental to the formation of aerosols. U.S. Pat. No. 5,337,963 to Noakes describes a spraying device which comprises a vertically-disposed capillary tube with one end immersed in a fragrance-producing oil. When an electrical potential is applied to the bulk liquid, generally near the submerged end of the capillary, the liquid is sprayed from the top end as a plurality of ligaments which break up into droplets. The applied electrical potential is reported to be in the range of 10-25 kV and must be high enough to cause EHD comminution at the top of the capillary. Liquid is fed by capillary action from a reservoir to the top end of the capillary for aerosolization. U.S. Pat. No. 5,503,335 to Noakes describes a similar spraying device, but which comprises a wick in place of the aforementioned capillary tube. The wick is fabricated from material having an open-celled structure. In this case as well, the high voltage is applied to the bulk liquid, generally near the submerged end of the wick. U.S. Pat. No. 5,810,265 to Cornelius et al. describes yet another similar spraying device, but which capillary structure comprises a hollow capillary tube having a convoluted inner surface to enhance capillary action. Similarly, the high voltage is applied to the bulk liquid, also generally near the submerged end of the capillary tube. Finally, U.S. Pat. No. 5,655,517 to Coffee describes a device for dispensing a comminuted liquid comprising a comminution site provided by fibers formed into a bundle projecting from an end surface or edge.  
         [0011]     In the delivery devices described above, it is difficult to consistently and precisely control delivery of the spray. While EHD spraying offers many advantages, including the ability to produce consistent sprays of aerosol particles having a narrowly-tailored size distribution, significant inconsistencies were observed in the delivery rate of the liquid to the surrounding air.  
         [0012]     It is, therefore, an object of the present invention to provide an aerosol delivery device that avoids or at least reduces adverse effects on an aroma resulting from the manner in which the aroma is delivered. It is another object of the present invention to provide an aerosol delivery device that enables improved control of delivery rate of the aerosol. It is yet another object of the invention to provide an aerosol delivery device that offers consistent aerosol delivery over the reservoir volume. It is a further object of the present invention to provide a method that offers the advantages of reduced adverse effects on the aroma, consistent aerosol delivery of the aroma, and improved capability for rapid vaporization.  
         [0013]     It is a further object of the present invention to provide a device and method for delivering other formulations that benefit from dispersion as an aerosol. These include, for example, anti-microbial agents; insect repellants; attractants; sterilizers; confusants; pheromones; fumigants; odor neutralizers; therapeutic agents, such as menthol and eucalyptus; animal mood control agents; household cleaning products, such as surface cleaning agents, surface modification agents for aesthetic benefits, surface protection agents, and sanitization/disinfectant agents; household laundry care products, such as stain-removing agents, fabric fresheners, and other fabric treatment agents for aesthetic benefits; personal cosmetic care products for body cleaning, body lotion, and sunscreen products for humans; and consumer adhesives. Formulations, especially for aromas, are oil-based, but other carriers may be used such as water, polymers, or organic solvents.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     In one aspect, the present invention provides a method of using EHD to create a spray having a generally-consistent flowrate, preferably an aerosol spray that rapidly vaporizes; wherein capillary action wicks a liquid from a liquid source to an EHD comminution site; a first electrical potential, preferably a high-voltage potential, is applied to a location away from the liquid source and near the EHD comminution site, preferably in or near a tapered portion of a capillary element; a second electrical potential, preferably a ground, is applied to a location external to the EHD comminution site, preferably to enhance the spray without directing the spray.  
         [0015]     In another aspect, the present invention provides a method of using EHD to create a spray, preferably an aerosol spray, wherein the spray is controllably emitted intermittently at a generally-consistent flowrate.  
         [0016]     In another aspect, the present invention provides a method of using EHD to maintain a desired/perceived level of fragrance over an extended period of time (e.g., weeks or months).  
         [0017]     In another aspect, the present invention provides a method of using EHD to create a spray, preferably an aerosol spray, by providing a length of capillary wick having a first and second segments contiguous at a first location, the second segment including at least one EHD comminution site; contacting the capillary wick first segment with a liquid source at a second location spaced from the first location; applying a first electrical potential to the capillary wick at the first location; positioning a reference electrode, preferably a ground, external to the capillary wick; and electrohydrodynamically producing a spray, preferably an aerosol spray, from the at least one EHD comminution site at a generally-consistent flowrate.  
         [0018]     In yet another aspect, the present invention provides an EHD apparatus for creating a generally-consistent flowrate spray, preferably an aerosol spray, comprising a reservoir for containing a source of EHD-comminutable liquid; a capillary element, preferably a capillary wick, comprising an EHD comminution site, positioned to contact the liquid source; a first charge source, preferably a high-voltage electrode, positioned in a spaced-apart relation to the liquid source and operably proximate the EHD comminution site; and a second charge source, preferably a ground, positioned external to the EHD comminution site.  
         [0019]     In yet another aspect, the present invention provides an EHD apparatus for creating a generally-consistent flowrate spray, preferably an aerosol spray, comprising a first charge source, preferably a high-voltage electrode, positioned in contact with the capillary element.  
         [0020]     In yet another aspect, the present invention provides an EHD apparatus for creating two or more generally-consistent flowrate sprays, preferably aerosol sprays, comprising two or more optionally curvilinear capillary wicks in liquid contact with two or more sources of EHD-comminutable liquid.  
         [0021]     In yet another aspect, the present invention provides an EHD apparatus for creating a generally-consistent flowrate spray, preferably an aerosol spray, comprising a housing formed to include an aperture, the aperture formed to include a charge source; a source of EHD-comminutable liquid; a capillary wick, comprising an EHD comminution site, the capillary wick at least partially within the aperture, the EHD site external to the housing, and the capillary wick in liquid communication with the liquid source; and a ground operably proximate the EHD comminution site. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0022]     The following detailed description of the embodiments of the invention will be more readily understood when taken in conjunction with the following drawings, wherein:  
         [0023]      FIG. 1  is a schematic cutaway of an embodiment of the present invention and illustrating its components.  
         [0024]      FIG. 2  is a partial detailed schematic cutaway of the embodiment of the present invention shown in  FIG. 1 .  
         [0025]      FIGS. 3   a - 3   h  are schematic cutaways of various capillary means with associated electrodes according to further embodiments of the present invention.  
         [0026]      FIG. 4  is a schematic cutaway of an embodiment of the present invention illustrating optional curvilinear capillary elements and an optional offset reservoir.  
         [0027]      FIG. 5  is a schematic cutaway of an embodiment of the present invention illustrating a spray device comprising a taper-like candle configuration and showing a narrow and deep liquid reservoir. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     EHD comminution entails the use of high voltages to charge liquids so that the electric charge on the liquid overcomes the surface tension of the liquid and the liquid is broken up, or comminuted, into a spray of fine aerosol droplets. In doing so, droplet size and droplet size distribution may be closely controlled. Droplet size may be in the sub-micron range, thus enabling rapid vaporization of the aromatics without denaturing and effecting a rapid onset of a perceived fragrance. Turning to  FIGS. 1 and 2 , to induce this action, the liquid must experience a high electric field, but preferably only at the point of comminution, known as the spray site, or EHD comminution site  35 . To accomplish this, the bulk liquid  29  in the reservoir  32  may be charged and the charge is conducted through the liquid to the EHD comminution site  35  at the tip of the delivery column  34 . To create the required electric field, an opposing electrode, often referred to as a reference or inducing electrode, and often a ground  26 , is spaced away from the spray site  35  to help generate a well-defined field. In applications where the liquid exhibits relatively high resistivity, such as with many aroma oils, or is semi-conductive, it is possible to induce a potential differential across the liquid itself if sufficiently high voltage is applied between the reservoir  32  and the reference electrode  26  and electric current flows through the highly-resistive liquid. As this happens, however, undesirably high levels of electric field pumping may occur.  
         [0029]     There are, in fact, two liquid movement mechanisms at play. The first is the capillary action associated with the liquid interaction (liquid surface tension, dynes/cm) with the surface energy of the capillary means (dynes/cm). The second is the electric field pumping due to the high voltage imposed on the liquid to induce aerosolization. It has been found that when a high-voltage charge sufficient to induce EHD spraying is applied to the bulk liquid, even near the capillary inlet for aroma and aerosol generation, a high degree of liquid delivery variability results at the EHD spray site at the opposite end of the capillary. It is believed that high-voltage pumping may contribute to the mechanism of liquid movement in the capillary at voltages necessary for EHD spraying, and that the voltage gradient along the capillary results in inconsistent movement through the capillary voids, particularly when fluid levels&#39; in a supply reservoir change over time.  
         [0030]     Turning again to  FIGS. 1 and 2 , an embodiment of the present invention is shown. The dispensing device  10  generally comprises a housing  12 ; a liquid source  29 , preferably contained within a reservoir  32 ; a voltage source, generally an electrode  31 ; a capillary element  34  terminating in an EHD comminution site  35 , generally, a capillary element with an associated electrode  31 ; and a reference electrode, or ground  26 ,  27 . Additionally, the dispensing device  10  may comprise a removable cap  14  that allows access to the internal components of the device  10 , a base  16  to further contain the internal components and to provide a stable platform for the device  10  when placed upon a horizontal surface, a battery  18 , a high-voltage power supply  22  to convert voltage (e.g., 9V) from the battery  18  to the higher, kV-level voltage required for operation of the device  10 , a circuit board  20  to handle the electronics functions such as timing, voltage control, operational indicators (e.g., lights, and control of intensity and delivery rate), a high-voltage lead  30  running between the output of the high-voltage power supply  22  and the electrode  31 , and a switch  24  to control operation of the device  10 . The optional light (not shown) may optionally contribute to a burning candle-like appearance for the device  10  or may be used to illuminate the spray, evoking a fountain-like effect. Optionally, the device may also comprise various control features to allow a user to adjust the spray and timing of the device.  
         [0031]     In operation, liquid is supplied to the delivery column  34 ,  64  (e.g.,  FIG. 3   c ) from the liquid source  29 . The delivery column  34 ,  64  generally comprises a capillary element  46 ,  66  ( FIG. 3 ) which may be formed from a capillary tube  46  ( FIG. 3   a ) or a wick  66  ( FIG. 3   c ) which will enable the liquid to be drawn toward the EHD comminution site tip  35  where a voltage charge causes the liquid to EHD comminute into an aerosol. As described above, placement of the electrode  31 ,  68  is important to providing consistent liquid and aerosol delivery rates. Capillary action has been shown to be an effective method for moving liquid from the reservoir  32  to the EHD comminution site  35 . However, there may be inconsistencies in delivery rate of the liquid to the site  35  and of the aerosol to the surrounding air, possibly caused by electric field pumping, the result of the high voltage imposed on the liquid to induce aerosolization causing electric current flow through the liquid. This high voltage over the entire length of the delivery column  34 ,  64 , however, is believed to cause electric field pumping to contribute to and result in inconsistent liquid flow rates. By minimizing this electric potential differential over the liquid path, consistent liquid delivery rates may be achieved. Advantageously, by using the capillary element  46 ,  66  to move liquid from the liquid source  29  to the EHD comminution site  35 , active pumping of any kind, including positive-displacement, is avoided. Importantly, too, the flowrate of the spray can remain generally-consistent over the delivery of the liquid in the reservoir  32 .  
         [0032]     Many capillary elements are possible. The important attribute is the ability to deliver the liquid from the liquid source  29  to the EHD comminution site  35 . The rate of capillary delivery must be sufficient to at least match the rate of EHD comminution or the EHD comminution site  35  will be starved of liquid and aerosolization will cease, or at a minimum aerosolization oscillates as liquid partially replenishes the EHD comminution site  35  and is sprayed away. Capillary tubes  46  ( FIG. 3   a ), capillary tubes  46  filled with a porous material  56  ( FIG. 3   b ), and fiber-like wicks  66  ( FIGS. 3   c - 3   h ) have been used successfully. A sample of common off-the-shelf paper towel material formed into a capillary element has been used successfully.  
         [0033]     Tubing materials include ABS, rigid PVC, polyester, polyamide, glass, Teflon® (poly-tetrafluoroethylene), PEEK, and polyimide. To maximize the capillary action using polymer tubes, an acceptable adhesion to the tube occurs when the surface energy of the polymer is greater than the surface tension of the liquid, preferably about 8-10 dynes/cm or more greater than the surface tension of the liquid. In spraying aromatic oils with surface tensions in the range of 27-30 dynes/cm, for example, preferred materials would include (with representative surface energy values) ABS (35-42 dynes/cm), rigid PVC (39 dynes/cm), polyester (41-44 dynes/cm), polyamide (ca. 36 dynes/cm), and polycarbonate (46 dynes/cm). While preferred, the capillary tube  46  need not be a single element. Multiple tubes and multiple tubes clustered together may be used. The capillary tube  46  need not be limited to a cylinder with a single opening. For example, two or more tubes may be coaxially combined to create a central aperture along with one or more annular apertures.  
         [0034]     In accordance with the present invention, open-cell, porous, or fiber-like wicks are most preferred for spraying aromatic aerosols. By way of example only, and not limitation, wicks include plotter pen wicks, felt nibs, china bristles, twisted nylon twine, braided shoelaces, foam materials, and candle wicks. Materials may be polymeric, such as polyester, or natural, such as cotton. Exemplary, the porous wicking material has an open cell structure with a porosity of about 40 percent. Preferably, the voids have consistent size and shape and the wicks exhibit uniformity from one wick to another. Preferably, each wick has a well-defined tapered, conical tip that is consistent from wick to wick. Preferably, each conical tip has a low height-to-diameter aspect ratio, but high enough to provide an effective EHD comminution site  35 .  
         [0035]     Further, the present invention enables flexibility in design. Multiple capillary elements or wicks ( FIG. 4 ) may be configured with multiple reservoirs (not shown) within the same dispensing device. As shown in  FIG. 4 , the capillary elements  166  may have curvilinear shapes to allow for placement of the spray sites, and positioning of the reservoir(s)  132  and other internal operational elements as required for a particular application. The size and shape of the other elements or desired placement of replaceable reservoirs may dictate non-symmetrical apparatus designs, irregularly-shaped reservoirs  132 , and curvilinear capillary elements  166 . There may be multiple spray sites drawing from a single reservoir. Where there are multiple reservoirs (not shown), multiple liquids may be sprayed either simultaneously or in a timed sequence. This latter capability enables the ability to dispense a first aroma and then cycle through separate aromas, thereby providing a continuous level of perceived fragrances and avoiding the phenomenon of olfactory saturation.  
         [0036]     A key element in the present invention is the placement of the electrode  48 ,  68  relative to the capillary element  46 ,  66 , specifically relative to the EHD comminution site  35 . As discussed above, if the electrode  48 ,  68  is placed in the bulk liquid in the reservoir  32 , even if placed near the bulk liquid end of the capillary element  46 ,  66  in the liquid, high voltages are required to effect aerosolization at the EHD comminution site  35 , especially with liquids having high resistivities, with resultant high levels of electric field pumping which produce inconsistent delivery rates.  
         [0037]     Viewing the capillary element  46 ,  66  as a column of liquid, it acts as a resistive element to the electric potential between the electrode  68 ′ ( FIG. 3   c ) and the EHD comminution site  35  at the end of the capillary element  46 ,  66 . A longer path effects a higher resistance and voltage drop which leads to the need for a higher electric potential and a less-consistent flowrate of aerosol. By reducing the distance between the EHD comminution site  35  and the electrode  68 , the resistance and voltage drop decrease, the required voltage decreases, and a more-consistent flowrate of aerosol results. Thus, the present invention limits electric field pumping to a smaller length of the capillary element  46 ,  66 . However, as shown in  FIG. 3 , the electrode  48 ,  68 ,  78 ,  88 ,  88 ′,  98 ,  98 ′,  108  does not extend beyond the EHD comminution site  35 ; some material of the capillary element covers or extends beyond the electrode. This is where the liquid gathers via capillary action to be available to the high-voltage charge to aerosolize it. As a result, the present invention improves delivery rates, allowing for consistent, repeatable delivery rates over time. In the field of aroma delivery, for example, this may be very desirable.  
         [0038]     The main factors in placing the electrode  68  to reduce or eliminate electric field pumping is proximity to the spray tip  35  and sharpness of the tip  35 . Minor factors include liquid resistivity, capillary uptake, aerosolization rate, and position of the ground  26 . In practice, the electrode  68  must be placed operably, or effectively, proximate the EHD comminution site  35 . That is, the position of the electrode  68  relative to the EHD comminution site  35  must be adjusted to produce a consistent aerosol delivery rate given the properties noted above. Not only may consistent delivery rates be achieved during each “on” cycle (discussed below), consistent delivery rates may be achieved over extended periods of “on” and “off” cycles.  
         [0039]     As a measure of consistency over a series of tests, the percent C v  was calculated by dividing the standard deviation by the mean. This measurement allows for comparing equally various wicks and configurations. The lower the C v , the more consistent the flowrate. By changing the charge location, for example from  68 ′ to  68 , the percent C v  improved (was reduced) in the range of three to 15 percentage points. For example, one wick improved from 28.7 percent C v  to 19.3 percent C v . For aromas, the preferred percent C v  is less than 20, more preferably less than ten. Much below ten percent is barely discernable by the average human olfactory senses. In general, the position of the charge electrode  68  has been found to be within the conical portion of the spray tip. As an example, for non-conducting wicks tested, the position of the charge source  68  has been in the range of 0.020 inches to 0.250 inches from the comminution site  35  to the charge point  68 . The measurement for a conducting wick would be virtually zero.  
         [0040]     Numerous electrode embodiments are feasible, all producing the same desired result of improved consistency of aerosol delivery rates with reduced electric field pumping. Importantly, as discussed above, the electrode  68  is placed away from the bulk liquid and the liquid uptake and nearer the EHD comminution site  35  to reduce the large voltage differential between the electrode  68  and the EHD comminution site  35 . Illustrated in  FIG. 3  are various possible electrode configurations relative to the capillary element  64  and the EHD comminution site  35 .  FIG. 3   a  shows a basic capillary element  44  comprising a capillary tube  46  with a voltage source electrode  48  positioned within the tube  46 . As liquid is drawn into the tube  46 , the electrode  48  provides a charge at the EHD comminution site  35  sufficient to aerosolize the liquid. The effects of electric field pumping are limited to that portion of the tube  46  between the voltage source electrode  48  and the EHD comminution site  35 .  FIG. 3   b  shows a similar arrangement, but with a porous material  56  disposed within the capillary tube  46 . Operation of the capillary element  54  shown in  FIG. 3   b  is similar to that of the capillary element  44  shown in  FIG. 3   a . As shown in  FIG. 3   c , a capillary element  64  may comprise a porous wick  66  into which is inserted a voltage source electrode  68 . The embodiment shown in  FIG. 3   c  provides a voltage source  68  inserted into the wick  66  near the EHD comminution site  35 . As in the previous embodiments, the electric field pumping is controlled and consistent aerosol delivery results. With just the voltage source electrode  68  operative, it is possible, in some embodiments, to have small amounts of undesirable electric field pumping downward and counter to the upward capillary action flow. This may be countered by positioning an additional voltage source electrode  68 ′ as shown. Thus, the electrical potential across the wick  66  may be equalized, or nearly so, and there is little or no counter electric field pumping. Turning now to  FIG. 3   d , yet another embodiment of a capillary element  74  is shown. Here, a voltage source electrode  78  comprises a helical coil positioned coaxial with the porous wick  66 . As long as the voltage source electrode  78  is positioned operably proximate the porous&#39; wick  66 , whereby a sufficient charge is imposed on the liquid, electric field pumping is controlled, and consistent aerosol delivery results. Counter electric field pumping is also minimized or eliminated.  FIGS. 3   e - 3   h  illustrate yet other embodiments of the capillary element  84 ,  94 ,  104 ,  114  of the present invention. The voltage source electrode  88 ,  98 ,  108 ,  118  may comprise a sheath surrounding the porous wick  66  as shown or, alternatively, the voltage source electrode may comprise arcuate tabs or the like (not shown) which may be positioned operably proximate the porous wick. Finally,  FIGS. 3   g  and  3   h  illustrate an embodiment wherein a portion of the housing  12  or reservoir cover  33  is formed to include the voltage source electrode  108 ,  118 . Plastic materials of construction (with their respective nominal ohm/square resistivities) for such voltage source electrodes  108 ,  118  include anti-static (E9-E12), static dissipative (E6-E9), and conductive plastics (E3-E6).  
         [0041]     To improve aerosol delivery, maximize aroma dispersion, and improve plume intensity and shape, it is preferred to place a ground or other reference electrode  26  operably proximate the EHD comminution site  35 .  FIGS. 1 and 2  show an example of the placement of the ground  26 . If the ground  26  is placed too far from the EHD comminution site  35 , the charge at the site  35  does not “see” the ground  25  and its effects are not noticeable. If the ground  26  is placed too close to the EHD comminution site  35 , the aerosol spray may be misdirected toward the ground  26 . Preferably, the ground  26  is placed to generate an electric field required to produce an aerosol without causing the spray to be attracted to the ground  26 . The optimal position of the ground  26  will depend upon the particular configuration of the device  10 . The liquid properties, particle size, spray site geometry, and corresponding electric field potential needed may affect the placement of the ground  26 . Referring to  FIG. 1 , the ground  26  is positioned off to the side and near the top of the delivery module  28  or just below the capillary element  34 . The ground  26  may be a neutral or opposite charge to the aerosol particles spraying from the EHD comminution site. Alternatively, depending upon the application, the electrode  26  may be disposed closer to the site  35  to purposely direct the aerosol spray. If desired, the ground  26  may comprise an adjustment (not shown) to allow varying of the position of the ground  26 . The materials used for the ground  26  may be any conductor, including, but not limited to, metals and plastics. The aerosol produced is preferably charged, but may be discharged and dispersed as neutral particles for selected applications. Alternatively, an external ground reference electrode  26  may be utilized. For example, an object or an animal, human or otherwise, may provide the ground reference.  
         [0042]     Preferably, various timing and control mechanisms are included as elements on the circuit board  20  or elsewhere. When the device  10  is initially activated in a room to emit an aroma(s), for example, it may be desirable to introduce a quantity of aroma sufficient to provide scent to the entire room after which the device  10  would shut off. Later, it may be desirable to periodically introduce a “maintenance” amount of scent to keep the level in the room constant and to counter the tendency of aromas to deaden or desensitize the sense of smell over time. This could be done by timing short sprays of perhaps several seconds duration with longer periods of quiescence. Such timing could also be used with multiple sprays having the same or different formulations. Depending upon the strength of the aroma, longer “on” times, upwards of one minute or more, may produce “hot spots” where the fragrance may become overwhelming. In these situations, the “off” time may be a minute or more.  
         [0043]     Having multiple fragrances in a single dispenser  10  enables several other types of operation. For example, fragrances that are related to each other may collectively produce a “bouquet” effect. By controlling dispensing to specific times of the day, one fragrance may induce an invigorating effect (morning), a calming, or stress-relieving effect (midday), and yet another, a relaxing effect (evening).  
         [0044]     Using a timing mechanism the device  10  may maintain a constant delivery rate even if fluid flow rate declines over time. For example, the delivery rate may be maintained through shorter spray intervals within a spray cycle time. Thus, if less liquid flows through the capillary wick  66  over time, a spray “on” interval may increase from, for example, five seconds to greater than five seconds. Alternatively, or in combination, the “off” time interval may decrease from, for example, 45 seconds to less than 45 seconds. These timing schemes can create an effective or perceived level of constant aroma delivery to the air.  
         [0045]     Many aromatic formulations have resistivities of greater than 5 E3 ohm-cm and surface tensions of between ten to about 50 dynes/cm. To produce an aerosol from these formulations, typical voltage levels are 3-10 kV and higher. For cost and battery-life considerations, it is preferred to maintain the required voltage to a minimum. Flowrates may be 0.005-0.100 μL/sec and delivery rates 5-50 mg/hr.  
         [0046]     While the invention has been described in connection with specific embodiments for the purposes of illustration and description, it is not intended to be exhaustive or to limit the invention to the precise form disclosed. Numerous modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.