Abstract:
A fluid injection device for ejecting fluid droplets in response to electrical signals comprises an oscillating surface that has one or more tapered apertures, each aperture having a first and second opening. The first opening of each aperture is larger than the second opening. The first opening is in surface tension contact with the fluid to be ejected. The fluid interaction with the tapered aperture wall creates cycles of fluid compression and decompression inside the aperture, causing fluid to be drawn from the large opening and ejected out the small opening of the aperture. The device includes a fluid supply nozzle that transports fluid to the oscillating surface at the large opening of the apertures. A discharge valve controls the fluid supply. An electronic wave generator induces oscillation in the tapered aperture containing surface. The device is used to great advantage for fluid atomization and fluid spray.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of patent application Ser. No. 07/726,777 filed on Jul. 8, 1991 now abandoned, which is a continuation-in-part of patent application Ser. No. 07/691,584 filed on Apr. 24, 1991, now U.S. Pat. No. 5,164,740. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the fields of liquid spray and atomization of liquids of all kinds and, more specifically, finds utility in humidification and misting, industrial cleaning, surface coating and treatment, particle coating and encapsulating, fuel atomization, and medical spray applications. 
     2. Description of Related Art 
     Many types of ultrasonic fluid ejection devices have been developed for atomizing of water or liquid fuel. These atomizers can be classified into two groups. The first type atomizes liquid that forms a thin layer on an ultrasonically-excited plate. The first type is not capable of ejecting atomized fluid droplets. U.S. Pat. No. 3,738,574 describes an atomizer of this type. 
     The second type utilizes a housing defining an enclosed chamber. The housing includes a perforated membrane or a pinhole membrane as the front wall of the chamber. The apparatus further includes a means to vibrate the membrane or a side wall of the chamber, typically by a piezoelectric element affixed to the front face of the chamber. The piezoelectric element oscillates the fluid in the chamber. As a result, pressure waves are generated in the chamber, forcing fluid through the open pinholes. All the devices of the second type require fluid to be kept inside the chamber next to the discharge opening. When volatile fluids are used, problems arise. The volatile fluids escape through the discharge opening. The discharge opening will clog, restricting or stopping further discharge. These problems are prevalent with volatile fluids such as fuel, paint, or other coating materials. To overcome these problems, U.S. Pat. No. 4,632,311 utilizes a chamber with a suction pump in communication with the chamber. The pump is energized after operation to drain the liquid from the chamber, leaving it dry during nonworking periods. This is supposed to prevent otherwise solid substances from clogging the nozzle opening. U.S. Pat. No. 4,533,082 uses a vacuum pump to ensure that the liquid in the chamber is kept under negative pressure during nonuse. In these devices it is particularly difficult to feed fluid into the chamber without causing the fluid to uncontrollably flow out of the discharge opening. 
     Other variations of apparatus for ejecting atomized liquid, utilizing one of the above two types, are disclosed in U.S. Pat. Nos. 3,812,854, 4,159,803, 4,300,546, 4,334,531, 4,465,234, 4,632,311, 4,338,576, and 4,850,534. 
     SUMMARY OF THE INVENTION 
     The present invention provides an ejection device that includes a free oscillating surface having microscopic tapered apertures of a selected conical cross-sectional shape. The apertures draw fluid into their large openings and eject the fluid from their small openings to a great distance. The ejection action is developed by the aperture, regardless of the amount of fluid in contact with the oscillating surface, and without any fluid pressure. Both sides of the oscillating surface are operating under the same ambient pressure. Therefore, the ejection device can operate equally well in vacuum or high-pressure environments. When only a thin film of fluid is allowed to adhere, in surface tension contact, to the large opening of an aperture, the supplied liquid continuously adheres to the large opening by surface tension. The film of fluid oscillates with the surface while it is being drawn into the large opening of the aperture and ejected forwardly. This continues until all the fluid is drawn from the surface, leaving the surface dry and free of liquid during the time that the device is not in use. 
     If the cross-section of the aperture is chosen with respect to the fluid to be ejected, the oscillation required to produce ejection is kept small, and the film of fluid on the oscillating surface appears to be dynamically at rest during ejection. By supplying only enough fluid to continuously form a thin film, in surface tension contact with the oscillating surface, to the side containing the large openings of the tapered apertures, neither clogging nor uncontrolled emission or leakage through the apertures occurs. The device can operate under any pressure conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The general purpose and advances of the present invention will be more fully understood hereinafter as a result of the detailed description of the preferred embodiments when taken in conjunction with the following drawings, in which: 
         FIG. 1  is a schematic illustration of a preferred embodiment of a device according to the present invention; 
         FIG. 2  is the schematic illustration of the present invention of  FIG. 1  shown in its oscillating configuration; 
         FIG. 3  is a top view of a vibrating surface according to the present invention; 
         FIG. 4  is a bottom view of a vibrating surface according to the present invention; 
         FIG. 5  is an enlarged cross-sectional view of the center area of the membrane shown in FIG.  2  and labelled “ 5 ”; 
         FIG. 6  is an enlarged elevational view of the center area of the vibrating surface of the present invention showing a preferred aperture shape; 
         FIG. 7  is a schematic illustration of the fluid characteristic within a tapered aperture during half of an oscillation cycle; 
       .  FIG. 8  is a schematic illustration of the fluid characteristic with a tapered aperture during half of an oscillation cycle; 
         FIG. 9  is a side view of an alternate preferred embodiment of the fluid ejection device according to the present invention; 
         FIG. 10  is a front view of the fluid ejection device of  FIG. 9 ; 
         FIG. 11  is an enlarged cross-sectional side view of the free end of the fluid ejection device of  FIG. 9 ; 
         FIG. 12  illustrates the ejector of  FIG. 9  provided with a fluid supply system; 
         FIG. 13  illustrates an alternative apparatus for preventing accidental overflow in the fluid supply system of  FIG. 12 ; 
         FIG. 14  illustrates the ejector of  FIG. 9  provided with an alternative fluid supply system; 
         FIG. 15  is an enlarged cross-sectional side view of the fluid supply tube of  FIG. 14  including a discharge nozzle attached at a side wall of the supply tube; 
         FIG. 16  is an enlarged cross-sectional side view of the discharge nozzle of  FIG. 14 ; 
         FIG. 17  is a side view of another alternative preferred embodiment of the fluid ejection device according to the present invention; and 
         FIG. 18  is a front view of the fluid ejection device of FIG.  17 . 
     
    
    
     INTRODUCTION 
     The present invention provides a new fluid ejection device that is especially advantageous in applications that require ejection of fluid droplets without fluid pressure and without a propellant and in ambient pressure environments. 
     A particularly important application for the present invention is industrial spray systems. The ejector is capable of ejecting viscose liquid such as paint and coating materials without the use of compressed air. 
     The use of air as a propellant in paint spray application causes overspray, in that part of the paint droplets escape to the atmosphere and cause air pollution. The transfer efficiency, that is, the percentage amount of coating material, such as paint, that reaches the target, is significantly increased when ejection is without air. 
     Another important application of the present invention is for consumer products such as deodorant and hair spray. The use of propellants in conventional aerosols, commonly known as volatile organic chemicals (VOCs), has a negative effect on the environment and on human health. There is an ongoing trend to find ways to atomize fluid without using such propellant gases. 
     The present invention provides a device that ejects fluid from microscopic tapered apertures. The fluid is transported to the ejecting surface at the large opening of the tapered aperture. A cohesive attraction force (surface tension) exclusively causes the liquid to adhere to the tapered aperture. The solid/fluid interaction of the fluid with the tapered aperture wall causes fluid to be drawn into the large opening of the aperture and ejected from its small opening. This ejection action is attributed to the geometry of the aperture, as well as the fluid characteristics such as viscosity, density, and elasticity. The fluid supply to the surface is tightly controlled to prevent overflow of liquid from the supply side of the oscillating surface. A flow control valve or a two-way valve is provided to control the amount of fluid that is transported to the surface. The valve may have a built-in electrical contact that activates oscillation simultaneously with the flow of fluid. 
     During ejection, fluid is supplied to the oscillating surface from a discharge nozzle that is in close proximity to the oscillating surface. The fluid is held by surface tension forces in the small gap between the front face of the fluid supply nozzle and the oscillating surface. When the fluid supply is stopped, the surface with the tapered apertures is allowed to oscillate for a period of time sufficient for the apertures to draw all the fluid from the oscillating surface and the gap. When not in use, the gap, as well as the oscillating surface and the aperture, remain free of fluid. 
     The discharge nozzle is preferably made of elastomer material having a cut through its thickness. The cut is normally closed due to the elasticity of the elastomer. The cut opens under slight pressure when fluid is transported from the supply container. This arrangement keeps the fluid in the container hermetically sealed during periods of nonuse. 
     An electronic wave generator with a circuit that can turn the oscillating action on and off sequentially at a very high speed is preferred. The ratio of the “on” period versus the “off” period controls the duty cycle of ejection and, therefore, the ejection mean flow rate. Maximum flow is achieved when the oscillator is continuously “on.” 
     Fluid is preferably supplied to the oscillating surface at a rate that is lower than the maximum ejection rate of the aperture. If the fluid supply exceeds the maximum ejection rate of the apertures, excessive fluid may overflow from the supply side of the oscillating surface. When the fluid used is paint or ink, overflow is undesirable. To prevent overflow, a system to collect liquid overflow may be used. This system includes a ring provided with a slot at its circumference which is connected to a pump. If fluid accidentally escapes from the oscillating surface and reaches the slot, it is drawn and returned to the supply container. 
     Another method of preventing accidental overflow is provided by an electronic flow control valve. It has been found that as the amount of liquid over the surface increases, the current draw by the piezoelectric element decreases. If the current draw reaches a predetermined level which indicates that an overflow is about to occur, the electronic circuit transmits a signal to the flow control valve to reduce the flow of liquid to the surface. Thereby, overflow is avoided. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , it will be seen that the fluid ejection device  10  of the present invention comprises a vibrating surface  12  having a perimeter area  14  and a center area  16 . The perimeter  14  of vibrating surface  12  is affixed to an oscillator  18  which may, for example, be piezoceramic. The center area  16  of vibrating surface  12  is provided with a planar surface  15  through which there are apertures  22 . The portion of center  15  having the apertures is in surface tension contact with a fluid film  19  at the back side of planar surface  15  to produce an ejection of fluid droplets  20 . 
     The oscillatory motion of the vibrating surface  12  is shown in FIG.  2 . It will be seen therein that the perimeter  14  of the vibrating surface  12 , by virtue of its contact with the oscillator  18 , oscillates in a vertical direction, as viewed in  FIG. 2 , with an oscillating characteristic shown in the graph at the rightmost portion of FIG.  2 . As also seen in  FIG. 2 , the center  16  of vibrating surface  12  oscillates at the same frequency as the perimeter  14 , but with a much larger amplitude, as seen in the graph on the leftmost portion of FIG.  2 . The graphs of  FIG. 2  are for purposes of illustration and are not necessarily drawn to scale. 
     The significantly larger oscillation amplitude of the center of the vibrating surface in  FIG. 2 , as compared to the perimeter, is due primarily to two factors. One is the shape of the vibrating surface  12  and the other is the frequency of oscillation that is selected for activation of the oscillator  18 . More specifically, vibrating surface  12  is configured so that its cross-section is reduced toward the center. The vibrating surface configuration may be understood best by referring to  FIGS. 2 ,  3 , and  4 , which illustrate a preferred embodiment thereof. The apertures  22  in vibrating surface  12  may be understood best by referring to  FIGS. 5 and 6 . As seen therein, the center portion  15  ( FIG. 5 ) of the vibrating surface  12  is provided with apertures  22 , each characterized by a tapered wall  24 , forming a large opening  26  on one side of the center portion  15  and a small opening  28  on the opposite side thereof. The thickness of the center portion  15  of the vibrating surface  12  is preferably 0.003-inch. Each aperture  22  is positioned at or near the center of the vibrating surface and is circular in shape with large opening  26  having a radius of 0.006-inch and the small opening  28  thereof having a radius of 0.0025-inch. 
     The shape of vibrating surface  12  and, in particular, the reduction in cross-section of the vibrating surface between its perimeter  14  ( FIG. 3 ) and its center  16 , is selected to provide a significant increase in amplitude of oscillation between the perimeter and the center of vibrating surface  12 . This increase in oscillation amplitude has been found to occur at particular frequencies of oscillation of the vibrating surface  12  such as at the second harmonic of the natural oscillating frequency of the vibrating surface. In the preferred embodiment of the present invention, it is desirable to have a damping ratio of at least 10 percent and to provide an amplitude ratio between the center area and the perimeter of the vibrating surface of at least 10, depending upon the voltage applied to the oscillator  18  and its mechanical responsiveness thereto. 
     When the center of the vibrating surface oscillates with an amplitude which exceeds a preselected threshold, fluid droplets are ejected from aperture  22  ( FIG. 1 ) at the frequency of oscillation of oscillator  18 . Thus, by controlling the amplitude of the perimeter oscillation and, thus, the amplitude of the center oscillation so that it is either above or below this threshold ejection level, the ejection of fluid droplets may be readily controlled. 
     In one embodiment that has been reduced to practice, the oscillation amplitude is 0.0001-inch at the perimeter. The frequency of oscillation is approximately 60,000 Hz, which corresponds to the second modal frequency of the vibrating surface  12 . The fluid droplet ejection level, that is, the level above which the amplitude of oscillation of the center  15  of the vibrating surface  12  causes fluid droplets to be ejected therefrom, is approximately 0.0016-inch. The perimeter oscillation is adjusted so that the center oscillation varies in amplitude from cycle to cycle, so that it is just above the ejection level and below the ejection level upon alternate cycles. The actual ejection level threshold, that is, the actual oscillation amplitude of the center of the vibrating surface which causes the ejection of fluid droplets, depends upon the characteristics of the fluid selected, as well as the shape and dimensions of the aperture  22 . In the particular preferred embodiment shown herein, the ejection level is achieved using gasoline. 
     As shown in  FIGS. 7 and 8 , fluid  19  continuously adheres through solid/fluid surface tension to the large opening  26  of aperture  22 . The fluid is compressed in the first half of the oscillation ( FIG. 7 ) when the vibrating surface strokes toward the fluid and decompresses in the second half of the oscillation cycle ( FIG. 8 ) when the vibrating surface strokes away from the fluid. Droplets are ejected each time the amplitude of oscillation of the aperture element  15  ( FIG. 5 ) exceeds the ejection level threshold. The number of droplets and spacing there-between are a function of the frequency of oscillation. In the preferred embodiment hereof, at a 60,000-Hz oscillation frequency, it has been found that when the ejection amplitude is continually above the threshold level, droplets are attached to each other and form a continuous stream. By altering the oscillation amplitude, such as by reducing it below the threshold level every second cycle, the droplets can be separated. This feature is particularly advantageous in fuel injection systems. It will be understood, however, that with selected changes in the shape of the vibrating surface  12 , the characteristic of the fluid, and in the shape and dimensions of aperture  22 , the selected frequency of operation may vary from that recited herein. Nevertheless, based upon the preferred embodiment disclosed herein, it will now be understood that ejection may be achieved by the present invention and that, in fact, fluid-droplet ejection at frequencies exceeding 60,000 Hz is readily achieved. 
       FIG. 9  illustrates an alternate preferred embodiment of the fluid ejection device  30  of the present invention which comprises a cantilever beam  32  including a base portion  34  and a free end  36 . The base portion  34  is affixed to a piezoelectric oscillator  38 . The free end  36  of the beam  32  is provided with a planar surface through which there are nine microscopic tapered apertures. Fluid  42  is in contact with the free end  36  through which droplets  44  are ejected. 
       FIG. 10  provides a front view of the fluid ejection device  30  and best illustrates the apertures  40 .  FIG. 11  is an enlarged cross-sectional side view of the fluid ejection device  30  showing the free end  36  in contact with the fluid  42 . The large opening  46  of each aperture  40  is in surface tension contact with the fluid  42 . The piezoelectric element  38  ( FIG. 9 ) produces high-frequency oscillations at the base end  34  of the beam  32 . The planar surface  37  at the free end  36  oscillates at the same frequency as the base  34 , but with much greater amplitude. Such oscillation of the free end  36  is due primarily to two factors: the beam  32  is shaped such that its moment of inertia is reduced toward the free end  36 ; and the induced frequency is substantially the natural frequency of the beam  32 . 
     The oscillation of the planar surface  37  produces cycles of pressure fluctuation at the interface between the fluid  42  and the surface  37  and inside the apertures  40 . The pressure fluctuation inside the apertures  40  and, particularly, near the inside wall  48  of each aperture, is significantly more intense as compared to the pressure fluctuation near the planar surface  37 . This characteristic is exclusively attributed to the conical cross-sectional geometry of the apertures  40 . As a result, fluid cavitation is developed inside each aperture  40  at an oscillation amplitude that is too small to dynamically disturb the fluid  42  near the planar surface  37 . The cavitation inside the aperture  40  produces a negative pressure that draws fluid from the planar surface  37  into the large opening  46  of the aperture  40  and ejects a stream of droplets  44  from its small opening  47  to a great distance. The ultrasonic oscillations do not break up or nebulize the fluid  42  at the surface  37 , such fluid remaining dynamically at rest during the ejection of fluid  42  within the aperture  40 . Ejection continues until all the fluid  42  is drawn from the surface  37  and ejected forwardly as droplets  44 . In this preferred embodiment, the diameter of the large opening  46  of the aperture  40  is 0.006″ and the diameter of the small-opening  47  is 0.0025″. The thickness of the planar surface  37  is 0.003″ and the oscillation frequency is, 50 kHz, which is the third natural frequency of the beam  32 . 
     Referring now to  FIG. 12 , the ejector  30  described in the specification with respect to  FIGS. 9 ,  10 , and  11  is now provided with a fluid supply system  50  that continuously transports fluid  51  to wet the oscillating surface  37  via a supply tube  53  ending at a supply nozzle  54 . The fluid  51  is transported to the surface  37  at a rate which is lower than the maximum ejection rate of the apertures  40  to prevent overflow of fluid  42  from the supply side of the oscillating surface  37 . A pinch valve  56  controls delivery of the fluid  51  to the oscillating surface  37 . The fluid supply system  50  is connected to an electronic flow control valve  52  which, in the preferred embodiment, is made by ICS sensors. The valve  52  is connected to an electronic circuit that detects the amount of liquid  42  on the oscillating surface  37 . In the event of excessive delivery of fluid, the oscillation amplitude decreases and the current draw by the piezoelectric element  38  decreases. A current sensor circuit  39  senses the current draw and transmits an overflow signal  41  to the flow control valve  52  to reduce the delivery rate of liquid  51  to the surface  37  until the amount of fluid returns to a normal level. 
       FIG. 13  illustrates an alternative apparatus for preventing fluid overflow with the fluid supply system  50 . An additional ring element  58  including a slot  60  is installed near the oscillating surface  37  such that the slot  60  is positioned a predetermined distance from the boundary  62  of the fluid  42 . The preferred ring element  58  is manufactured by Clippard Instruments Laboratory, Inc. of Cincinnati, Ohio and is designated as Model No. 1022. The slot  60  is connected to a suction venturi pump (not shown) through an inlet  64 . A suction venturi pump, designated as Part No. 16480, is commercially available from Spraying Systems Co. of Wheaton, Ill. In the event of overflow, the boundary  62  of the fluid  42  expands toward the ring  58  and returns to the supply line  53 . 
       FIG. 14  shows the ejection device  30  of  FIG. 9 , further including an alternative fluid supply system  70  and an electrical wave generator  71  including a battery or external power inlet (not shown) to activate the piezoceramic element. The ejector device  30  is preferably attached to a platform  72  of the supply system  70  at the piezoelectric oscillator  38 . The supply system  70  includes a fluid supply container  74  which is preferably made from a flexible, disposable nylon material. A discharge nozzle  76  is affixed at a side wall of the supply container  74  providing fluid communication between fluid in the tube and the ejection device  30 . When force is applied to the side of the supply container  74 , the fluid inside the supply container  74  is pressurized and forced through the discharge nozzle  76 . 
     The supply system  70  further includes a discharge valve apparatus  80  which is preferably attached to the platform  72 . The preferred discharge apparatus  80  includes a spring-loaded plunger  82  acting on the external side wall of the supply container  74  against a rear opening of the discharge nozzle  76  to prevent unwanted discharge of fluid from the supply container  74 . When the plunger  82  is released, fluid is discharged toward the oscillating surface  37 . Fluid enters into a gap  84  between the nozzle  76  and the surface  37  and is held by surface tension contact. In the preferred embodiment this gap is 0.025″. 
     The alternative fluid supply system  70  additionally provides a means for applying mechanical pressure  90  on the nylon container  74  to force the fluid through the nozzle  76 . The pressure-applying means  90  includes a pressure plate  92  pivotally attached to a torsion spring  94  for applying a compressive force on a side wall  75  of the container  74 . As shown in  FIG. 14 , the pressure plate  58  can be rotated clockwise to a released position, facilitating the unloading and loading of fluid supply containers  74 . In operation, the pressure plate  92  applies a continuous pressure of approximately 10 psi to the fluid inside the nylon container  74 . 
       FIG. 15  provides an enlarged cross-sectional side view of the supply container  74  including an integrally-formed discharge nozzle  76  attached at a side wall of the container  74 . The nozzle includes a rear surface  77  in fluid communication with fluid inside the supply container  74  and a front surface  79  positioned in close proximity to the vibrating free surface  37 . 
       FIG. 16  provides an enlarged cross-sectional side view of the discharge nozzle  76 . As can be readily appreciated, a circumferential ridge  78  formed around the discharge nozzle  76  ensures that the gap  84  is maintained at its preferred distance. The nozzle  76  is preferably made of an elastomer material and includes a cut  96  through part of its thickness. The cut  96  is normally closed because of the natural elasticity of the elastomer material. Fluid pressure applied to the rear side of the nozzle opening  98  forces the cut  96  to open and allow passage of liquid to the oscillating surface  37 . The discharge nozzle  76  is designed to keep the fluid in the supply tube  76  hermetically sealed when the fluid ejection device  30  is not in use. 
       FIG. 17  illustrates another alternative preferred embodiment of the fluid ejection device wherein the oscillating surface comprises a curved member  100  with two piezoelectric elements  102   a ,  102   b  respectively affixed to front surfaces  104   a ,  104   b . The piezoelectric elements  102   a ,  102   b  impart oscillations to a thin angled surface  106  located centrally on the curved member  100 , causing fluid  108  to be ejected forwardly as a divergent stream of droplets  110 . A predetermined curvature characteristic of the angled surface  106  results in a wider distribution of the droplets  110  within an ejection angle  112 .  FIG. 18  provides a front view of the curved member  100  and further illustrates that the angled surface  106  is bound on its perimeter by a window opening  114 . Preferably, the angled surface  106  includes 45 apertures  116  in a 5×9 matrix. 
     It will now be understood that what has been disclosed herein comprises a novel and highly innovative fluid ejection device readily adapted for use in a variety of applications requiring the ejection of small droplets of fluid in a precisely controlled manner. 
     Those having skill in the art to which the present invention pertains will now, as a result of the Applicant&#39;s teaching herein, perceive various modifications and additions which may be made to the invention. By way of example, the shapes, dimensions, and materials disclosed herein are merely illustrative of a preferred embodiment which has been reduced to practice. However, it will be understood that such shapes, dimensions, and materials are not to be considered limiting of the invention which may be readily provided in other shapes, dimensions, and materials.