Abstract:
An orifice plate is vibrated up and down at high frequency while liquid is delivered to its lower surface so that the liquid is ejected up from the plate in the form of very small diameter droplets. The upper surface of the plate is constructed to resist wetting and buildup of a liquid film thereby to form smaller diameter liquid droplets which are ejected to greater heights. The upper surface of the plate may be treated with a surfactant such as a flurosurfactant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the atomization of liquids and in particular it concerns novel methods and apparatus for forming atomized liquid droplets of minimal size. 
     2. Description of the Related Art 
     U.S. Pat. No. 5,164,740 describes a vibrating plate atomizing device in which liquid which is supplied to one side of a vibrating orifice plate, passes through orifices in the plate and becomes atomized and ejected from the opposite side of the plate. Other U.S. patents which describe similar devices are U.S. Pat. No. 5,586,550, U.S. Pat. No. 5,297,734 and U.S. Pat. No. 6,296,136 B1. 
     Such devices may be used to disperse liquids, such as fragrances and insecticides into the atmosphere. When such liquids are formed into small droplets and ejected as droplets into the atmosphere, their high surface area to volume ratio improves their ability to evaporate. While it is preferred that each droplet evaporates entirely before falling back onto an adjacent surface, this does not always happen due to various factors, one being that the size of many of the droplets is so large that they do not have time to fully evaporate before reaching the adjacent surface. 
     SUMMARY OF THE INVENTION 
     The present invention helps to minimize the amount of unevaporated liquid from a vibrating plate atomizing device which falls back toward an adjacent surface. 
     According to one aspect of the invention, there is provided a novel method of generating droplets of minimal diameter by means of a vibratory atomization plate to which a liquid is delivered. This novel method involves the steps of treating the surface of the plate from which droplets are ejected during atomization to minimize liquid accumulation on said surface, and supplying the liquid to the plate while vibrating it to atomize the liquid. 
     According to a further aspect of the invention, there is provided a novel atomization device for converting a liquid into droplets of minimum diameter and ejecting said droplets into the atmosphere. This novel device comprises an atomization plate coupled to an actuator to be vibrated by the actuator and a liquid supply system arranged to supply liquid to the plate as it is being vibrated. The plate has a surface, from which droplets are ejected, which has been treated to minimize accumulation of liquid. 
     It has been found that by providing the vibrating plate with a finish on its ejection surface that eliminates or at least minimizes accumulation or buildup of liquid, the plate can eject droplets which are smaller and which are thrown up top a greater height than is possible with vibrating plates having conventional surface finishes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view taken in elevation showing the interior of a vibratory plate atomization device in which the present invention may be incorporated; 
     FIG. 2 is an enlarged section view of a piezoelectric actuator and vibratory orifice plate used in the atomization device of FIG. 1; 
     FIG. 3 is a further enlarged fragmentary view showing a portion of a vibratory orifice plate according to the prior art; and 
     FIG. 4 is a view similar to FIG. 3, showing a portion of a vibratory orifice plate according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an atomizer device  10  in which the present invention may be used. The atomizer device  10  comprises an outer hollow plastic housing  12  which rests on a surface  14  such as a table top or a shelf A reservoir  16  which contains a liquid to be atomized is mounted in the housing. An atomizing assembly comprising a ring shaped piezoelectric actuator  18  and an orifice plate  20  which extends across and is fixed to the actuator, is mounted in the housing just above the reservoir  16 . A liquid delivery system, such as a wick or capillary tube  22  delivers liquid from the reservoir  16  to the underside of the orifice plate while high frequency alternating electrical fields are applied across the piezoelectric actuator  18 . This causes the actuator  18  to expand and contract radially and force the orifice plate  20  to vibrate up and down at the high frequency. As the orifice plate moves up and down it causes the liquid from the capillary tube  22  to be forced through tiny orifices in the plate and ejected in the form of minute droplets into the atmosphere in the form of a cloud  24 . As the droplets which form the cloud  24  fall back toward the surface  14  they vaporize and are thereby dispersed into the atmosphere. 
     The specific construction of the atomizer  10  is not part of the invention, which may be used with any vibratory plate atomization device. The particular atomizer shown herein is described in detail in U.S. patent application Ser. No. 09/699,106, filed Oct. 27, 2000. 
     The configuration of the actuator  18  and the orifice plate  20  is shown in the enlarged section view of FIG.  2 . As can be seen the ring-shaped actuator has flat upper and lower surfaces which are metallized with an electrically conductive metal, for example silver or nickel, to form upper and lower electrodes  18   a  and  18   b . Electrical wires  26  are soldered to these electrodes and supply them with high frequency alternating electrical fields from a battery powered electrical supply system (not shown). These alternating electrical fields cause the piezoelectric material of the actuator  18  to expand and contract in directions perpendicular to the direction of the applied fields. That is, the actuator expands and contracts in radial directions as shown by the double headed arrow A in FIG.  2 . 
     The actuator  18  may be made of any of several different ceramic materials which exhibit a piezoelectric effect. By way of example, the material used for the actuator may be a ceramic material made from a lead zirconate titanate (PZT) or lead metaniobate (PN). The actuator  18  in the illustrated embodiment has an outer diameter of about 0.382 inches (0.970 cm), an inner diameter of about 0.177 inches (0.450 cm) and a thickness of about 0.025 inches (0.0635 cm). However, these particular materials and dimensions are not critical to this invention. 
     The orifice plate  20  has an outer flange  26  which is fixed to the lower metallized surface of the actuator  18 , preferably by soldering with a tin-lead solder, so that the orifice plate extends across the inner diameter of the actuator. The center region of the orifice plate is slightly dome-shaped as shown at  28 . The domed center region contains several (for example 85) small orifices which extend through the plate and which are spaced from each other by about 0.005 inches (0.130 mm). The orifices are preferably tapered from the lower to the upper surface of the plate. For dispensing fragrances and insecticides the orifices may taper from a diameter of 107 microns at the bottom surface of the plate to about 7 microns at the upper surface. These dimensions are not critical and the orifice diameters at the upper surface may vary from 3 to 10 microns or more. Again these specific dimensions are given only by way of example. 
     The orifice plate  20  is preferably made of nickel, although other materials may be used, provided that they have sufficient strength and flexibility to maintain the shape of the orifice plate while being subjected to flexing forces. Some examples of alloys that could be used are nickel-cobalt and nickel-palladium alloys. 
     The orifice plate  20  may be made by electroforming, with the perforations being formed in the electroforming process. However, the orifice plate may be made by other processes including rolling; and the perforations may be formed later. 
     As the actuator  18  expands and contracts radially, it alternately squeezes in on and pulls out on the plate  20 , causing the flange region  26  of the plate to flex, and its domed center region  28  to move up and down. This causes liquid, which is supplied to the underside of the plate by a liquid delivery system such as a wick, for example, to be drawn up through the orifices in the plate and ejected upwardly in the form of small droplets. By way of example, the actuator  18  is energized to cause the domed center region of the plate to vibrate up and down at a rate of about 120 to 160 kilohertz. 
     In the highly magnified fragmentary cross-section of FIG. 3 a portion of the orifice plate  20  is shown, along with one orifice  32  extending through the plate. The orifice  32  is shown tapered, with its smaller diameter at the upper side of the plate. This tapering provides improved atomization but is not necessary to the present invention. Also, because of the high magnification of FIG. 3, the region where the perforations  32  intersect with the upper and lower surfaces of the plate are shown slightly rounded. 
     As can be seen in FIG. 3, the liquid  30  which passes through the orifice  32  forms into a bulge  30   a  which, because of the momentum imparted to the liquid by the up and down movement of the plate, breaks away in the form of a droplet  30   b  which is thrown upwardly. 
     It will be seen that not all of the liquid which passes through the orifice  32  goes to forming the droplet  30   b . As a result, a portion of the liquid adheres to and wets the upper side of the plate so as to form a liquid layer  34  on the upper surface of the plate. The inventors have found that this liquid layer interferes with droplet formation in a number of ways. First, the inertia of the layer  34  imposes a load which interferes with the up and down movement of the plate, thus reducing the energy available for atomization of the liquid. Secondly, liquid from the layer  34  is added to liquid passing through the orifice  32  which adds to the diameter of the droplet  30   b . The large droplet, because of its size, cannot be thrown upward as high as a smaller droplet. Finally, the larger droplet requires a larger amount of time to become completely evaporated. As a result a portion of the droplet may fall back on adjacent surfaces in liquid form, This may cause chemical attack on those surface or may just result in a an unsightly appearance on these surfaces. 
     FIG. 4 illustrates how the present invention overcomes the above described problem. As can be seen in FIG. 4, little or no liquid remains on the upper surface of the orifice plate  20 . Accordingly, the liquid layer  34  in FIG. 4 is significantly thinner than the layer  34  in FIG.  3 . As a result the plate  20  can move up and down at maximum amplitude to project droplets to a greater height. Also, because there is less liquid in the layer  34 , the bulge  30   a  in FIG. 4 is significantly smaller than the bulge  30   a  in FIG.  3  and the size of the bubble  30   b  in FIG. 4 is determined essentially by the liquid which passes through the orifice  32  during each up and down cycle. 
     The invention involves preparing the upper surface of the orifice plate  20  so that it is not wetted by the liquid being atomized. It has been found that this wetting can be eliminated or greatly reduced by coating the upper surface of the plate with a coating comprising a surfactant, for example a fluorinated surfactant. Any treatment of the upper surface of the orifice plate  20  to lower wetting or spreading of liquid helps to reduce the size of the droplets that are produced by up and down vibratory movement of the plate. Any chemical which contains a fluorinated group, for example polymers, surfactant, fluorinated silanes, etc., may be used as a coating to reduce wetting of the upper surface of the plate. 
     Actually, it has been found that because the liquid from which the droplets are formed passes through orifices in an orifice plate (a solid) and into the atmosphere (a gas), which also is in contact with the upper surface of the plate, three interfaces are involved in droplet formation, namely gas/solid (g/s), solid/liquid (s/l) and liquid/gas (l/g). Further the interfacial surface tensions (σ) between these three phases must be in a particular relationship to minimize the formation of the liquid layer  34  on the upper surface of the plate. Specifically, it has been found that if 
     
       
         σ s/g &lt;σ s/l +(σ l/g  cos θ)  
       
     
     where θ is the angle between a line tangent to the surface of the orifice plate  20  and a line tangent to the surface of a droplet being formed on the plate, liquid will not tend to spread along the surface of the plate or to build up the layer  34 . 
     This invention is not limited to the use of a surfactant. Any surface or any surface treatment that has the ability to reduce wetting of the orifice plate and buildup of the layer  34  shown in FIG. 3 will result in a decrease in droplet size. 
     INDUSTRIAL APPLICABILITY 
     This invention improves the atomization characteristics of vibratory plate atomizers in a manner such that they use less energy and such that they produce smaller droplets which are ejected higher into the atmosphere, whereupon a greater portion of the liquid is evaporated into the atmosphere and less liquid rains down on adjacent surfaces in liquid form.