Abstract:
A method of operation of an acoustic droplet emitter with an array of droplet emitting utilizing high liquid flow rates. The higher flow rates prevent excess heat absorption during the droplet emission process and allow for excess heat generated by control electronics to be transferred to the flowing liquid after droplet emission but before it leaves the droplet emitter. This prevents excess heat build-up within the droplet emitter and allows for higher more accurate droplet emission.

Description:
INCORPORATION BY REFERENCE 
     The following U.S. patents are fully incorporated by reference: 
     U.S. Pat. No. 5,786,722 by Buhler et al. titled “Integrated RF Switching Cell Built In CMOS Technology And Utilizing A High Voltage Integrated Circuit Diode With A Charge Injecting Node” issued Jul. 28, 1998. 
     U.S. Pat. No. 5,565,113 by Hadimioglu et al. titled “Lithographically Defined Ejection Units” issued Oct. 15, 1996. 
     U.S. Pat. No. 5,389,956 by Hadimioglu et al. titled “Techniques For Improving Droplet Uniformity In Acoustic Ink Printing” issued Feb. 14, 1995. 
    
    
     BACKGROUND 
     This invention relates generally to droplet emitters and more particularly concerns an acoustically actuated droplet emitter which is provided with a continuous, high velocity, laminar flow of liquid. 
     FIG. 1 shows a cross-sectional view of a standard droplet emitter  10  for an acoustically actuated printer such as is shown in U.S. Pat. No. 5,565,113 by Hadimioglu et al. titled “Lithographically Defined Ejection Units” and incorporated by reference hereinabove. The droplet emitter  10  has a base substrate  12  with transducers  16  on one surface and acoustic lenses  14  on an opposite surface. Attached to the same side of the base substrate  12  as the acoustic lenses is a top support  18  with channels, defined by sidewalls  20 , which hold a flowing liquid  22 . Supported by the top support  18  is a capping structure  26  with arrays  24  of apertures  30 . The transducers  16 , acoustic lenses  14 , and apertures  30  are all axially aligned such that an acoustic wave produced by a single transducer  16  will be focussed by its aligned acoustic lens  14  at approximately a free surface  28  of the liquid  22  in its aligned aperture. When sufficient power is obtained, a droplet is emitted. 
     FIG. 2 shows a perspective view of the droplet emitter  10  shown in FIG.  1 . The arrays  24  of apertures  30  can be clearly seen on the capping structure  26 . Each array  24  has a width W and a length L where the length L of the array  24  is the larger of the two dimensions. Arrow Lf shows the flow direction of the flowing liquid  22  through the top support  18 , which is in the direction of the length L and orthogonal to the width W of the channels formed by sidewalls  20  and is along a length L of the arrays  24 . This is due to the channels formed by sidewalls  20  being constructed such that the flowing liquid  22  flows in the direction of the length L of the each array. This configuration has many advantages. It is compact and allows the precisely aligned production of multiple arrays  24  of apertures  30  where each array is associated with a liquid having different properties. For instance, to enable a color printing application each array might be associated with a different colored ink. Furthermore, this configuration is easy to set up and attach to an ink pumping system. However, the pressure loss of the liquid  22  along the channel length L is dependent on the cross sectional area defined by sidewalls  20  and the channel length L. As the channel length L increases, the pressure loss along the flow direction increases. The portion of the pressure loss due to flow frictional losses is largely dependent upon and limited by the height h of the channel. 
     This pressure loss along the flow direction can become large and results in a limited flow rate. The pressure loss and the limited flow rate impacts the performance of the droplet emitter  10  by limiting the droplet emission rates possible in three ways. Firstly, the pressure loss will change the level of the free surface  28  of the flowing liquid in the apertures along the length L. At the very least, different liquid levels will contribute to focussing errors of the acoustic energy focussed by the acoustic lenses  14  and result in emitted droplets not landing in their target spots. For example, using a configuration of the type shown in FIGS. 1 and 2, with a length L of 1.7 inches and a flowing liquid having a viscosity of less than 1.3 centepoise, a flow rate which exceeds 10 ml per minute will exceed the focussing level tolerance of the acoustic lenses because the difference in meniscus position between the first and last emitter will exceed 5 microns. If the flow rate exceeds 35 ml per minute, the system can not sustain the free surface  28  of the flowing liquid  22  in the apertures  30 . At these flow rates both simultaneous spilling and air bubble ingestion occurs. 
     Secondly, the slow flow rate will also mean that the flowing liquid  22  and the substrate  12  will heat up from the portion of the acoustic energy that is absorbed in the flowing liquid  22  and the substrate  12  which is not transferred to the kinetic energy of the ejected drops. The liquid can sustain temperature increases by only a few degrees centigrade before emitted droplets show drop misplacement on the receiving media. In a worst case scenario, the flowing liquid  22  can absorb enough energy to cause it to boil. The practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept very slow to prevent the liquid from absorbing too much excess energy and heating up to unacceptable levels. 
     Using the example given above, with a configuration as shown in FIGS. 1 and 2 and a length L of 1.7 inches running under a maximum emission rate with all emitters emitting at approximately 30 watts, the temperature difference between the first and last emitter is approximately between 39 degrees centigrade and 75 degrees centigrade. This temperature differential is clearly above the preferred range of just a few degrees centigrade and affects the accuracy of droplet placement quality greatly. To correct this issue either the flow rate of the flowing liquid must be increased or the emission rate must be greatly reduced so that less heat energy is generated in the base substrate  12  and the flowing liquid  22 . However, using the design shown in FIGS. 1 and 2, increasing the flow rate of the flowing liquid  22  results in an unacceptable pressure loss and meniscus position variance as discussed above. Therefore, using the design shown in FIGS. 1 and 2, emission rates must be kept low to prevent excess heating of the flowing liquid  22  to achieve acceptable drop placement accuracy. 
     Thirdly, if the droplet emitter is emitting droplets at high emission rates, a greater volume of fluid will be lost to droplet emission than can be replaced by the slow flow rates. Again the practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept slow to allow sufficient replenishment times. 
     Therefore, it would be highly desirable if a droplet emitter  10  could be designed to maintain a substantially constant pressure along the emission portion of the liquid flow path and which also has a faster flow rate for a droplet emitter array of any arbitrary length L with a minimal rise of the liquid flow temperature at high emission speeds and has sufficient liquid replenishment rates. 
     Further advantages of the invention will become apparent as the following description proceeds. 
     SUMMARY OF THE INVENTION 
     Briefly stated and in accordance with the present invention, there is provided a method of operating an acoustic droplet emitter which utilizes high liquid flow rates. The droplet emitter has a first substrate which has been constructed to provide an array of focussed acoustic waves. The droplet emitter also has a second substrate which is spaced from the first substrate. The second substrate has an array of apertures which are so arranged such that each aperture may receive focussed acoustic waves. Further, there is a liquid flow chamber at least partially interposed between the first and second substrates. The liquid flow chamber has an inlet and an outlet and is constructed and arranged to receive a laminar flow of a liquid where a free surface of the liquid is formed by each of the apertures in the second substrate. The focussed acoustic waves received by each aperture are focussed substantially at the free surface of the liquid formed in the aperture. The laminar flow of liquid flows in through the inlet, out through the outlet at liquid flow rates of at least 35 ml per minute. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of a prior art droplet emitter for an acoustically actuated printer. 
     FIG. 2 shows a perspective view of a prior art droplet emitter shown in FIG.  1 . 
     FIG. 3 show a cross-sectional view of a droplet emitter according to the present invention. 
     FIG. 4 shows a perspective view of the droplet emitter shown in FIG.  3 . 
     FIG. 5 shows a cross-sectional view of the droplet emitter shown in FIG. 3 with a fluid manifold attached. 
     FIG. 6 shows a perspective view of the droplet emitter shown in FIG. 4 with the addition of liquid level control plate supports. 
     FIG. 7 shows a perspective view of cross-sectional view of the droplet emitter shown in FIG. 5 with additional thermally conductive components. 
     FIG. 8 shows an exploded view of the parts used to assemble an upper manifold. 
     FIG. 9 shows an exploded view of the parts used to assemble a droplet emitter with a lower manifold and flex circuitry. 
    
    
     While the present invention will be described in connection with a preferred embodiment and method of use, it will be understood that it is not intended to limit the invention to that embodiment or procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     ALPHA-NUMERIC LIST OF ELEMENTS 
     h height 
     Hf flow direction of heat 
     Lf flow direction of liquid 
     L length of an array 
     W width of an array 
       10  droplet emitter 
       12  base substrate 
       14  acoustic lens 
       16  transducer 
       18  top support 
       20  sidewall 
       22  flowing liquid 
       24  array 
       26  capping structure 
       28  free surface 
       30  aperture 
       40  droplet emitter 
       42  base substrate 
       44  acoustic lens 
       46  transducer 
       48  liquid level control plate support 
       50  flow chamber 
       52  flowing liquid 
       54  array 
       56  liquid level control plate 
       58  free surface 
       60  aperture 
       62  fluid manifold 
       64  sheet flow partition 
       66  manifold inlet liquid tube 
       68  manifold outlet liquid tube 
       70  heat sink 
       72  heat conductive back plane 
       74  thermally conductive connection 
       76  circuit component 
       78  spring clip 
       80  circuit chip 
       82  bridge plate 
       84  flexible seal 
       86  manifold inlet 
       88  manifold outlet 
       90  liquid sheet flow chamber 
       92  lower manifold 
       94  LLC gap protrusion 
       96  bond wire 
       98  upper manifold 
       100  flex 
       102  baffle 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to FIG. 3, there is shown a cross-sectional view of a droplet emitter  40  configured according to the present invention. The droplet emitter  40  has a base substrate  42  with transducers  46  on one surface and acoustic lenses  44  on an opposite surface. Spaced from the base substrate  42  is a liquid level control plate  56 . The base substrate  42  and the liquid level control plate  56  define a channel which holds a flowing liquid  52 . The liquid level control plate  56  contains an array  54  of apertures  60 . The transducers  46 , acoustic lenses  44 , and apertures  60  are all axially aligned such that an acoustic wave produced by a single transducer  46  will be focussed by its aligned acoustic lens  44  at approximately a free surface  58  of the liquid  52  in its aligned aperture  60 . When sufficient power is obtained, a droplet is emitted. 
     FIG. 4 shows a perspective view of the droplet emitter  40  shown in FIG.  3 . The array  54  of apertures  60  can be clearly seen on the liquid level control plate  56 . Arrow Lf shows the flow direction of the flowing liquid  52  between the base substrate  42  and the liquid level control plate  56 . Notice that the flow direction Lf is arranged such that the flowing liquid  52  flows along the shorter width W of the array  54  instead of along the longer length L of the array  54  as in FIGS. 1 and 2. In this configuration, the flow velocity of the liquid  52  is substantially independent of the distance between the sidewalls which define the channel. To further illustrate the point, notice in FIG. 2 that the length L of the array  24  and hence the length of the channel (the distance in the flow direction Lf) is much larger that the width W of the array  24  and hence the width of the channel (the distance transverse to the flow direction Lf). However, in FIG. 3, because the flow direction of the liquid has been rotated orthogonally to the length L of the array the distance in the flow direction Lf is much shorter. Therefore, as the array length increases, the flow rate and pressure loss along the flow direction is substantially independent of the array length, for the same flow velocities. 
     Much larger flow rates are achievable with this configuration. For instance, droplet emitters having a length L of 1.7 inches constructed with this configuration have sustained flow rates of 150 ml per minute with a differential meniscus position between the first and last emitter of 5 microns. These same printheads have also achieved flow rates of up to 300 ml per minute. These higher flow rates enable for instance the flowing liquid  52  to help maintain thermal uniformity of the droplet emitter  40 . In particular, not only does the flowing liquid  52  itself have less opportunity to heat up due to excess heat generated during the acoustic emission process but because the flowing liquid  52  is in thermal contact with the substrate  42  the flowing liquid may also absorb excess heat generated in the substrate  42  during operation and prevent excess heating of the substrate  42  as well. In particular, printheads constructed as above tested at maximum emission rates with all emitters emitting at approximately 30 watts have shown a maximum instantaneous temperature differential between the first and last emitter of between approximately 2.9 degrees centigrade and 5 degrees centigrade. As can be readily appreciated, this is a large improvement over the performance of the prior art droplet emitter. 
     FIG. 5 shows a cross-sectional view of how the droplet emitter of FIGS. 3 and 4 can be assembled with fluid manifold  62  to provide the flowing liquid  52  to the droplet emitter. While unitary construction of the fluid manifold  62  may in some circumstances be desirable, in this implementation the fluid manifold  62  is divided into two portions, an upper manifold  98  and a lower manifold  92  with a flexible seal  84  therebetween. 
     The lower manifold  92 , which is in direct contact with the base substrate  42  and the liquid level control plate  56 , must be made from materials which have a thermal expansion coefficient relatively similar to the material the base substrate  42  is made from and preferably within a range of +/−0.5×10 −6  per degree centigrade. This is primarily because the base substrate  42  during the course of alignment to the lower manifold  92  and liquid level control plate  56  and subsequent bonding and curing steps may go through large temperature variations of up to 250 degrees centigrade and a differential thermal expansion of the parts of more than 5 microns can damage the assembly. The most common material for constructing the base substrate  42  is glass which has a thermal expansion coefficient of approximately 3.9×10 −6  per degree centigrade. Possible materials for constructing the lower manifold  92 , when the base substrate  42  is made from glass, include Alloy  42 , KOVAR, various ceramics and glass, which all have acceptable thermal expansion coefficients. However, as the length of the droplet emitter  40  increases, and hence the length of both the base substrate  42  and the liquid level control plate  56 , either the allowable variation in thermal expansion coefficients, or the maximum temperature variation, or both must be correspondingly decreased. 
     The lower manifold  92  has a liquid level control gap protrusion  94 . The liquid level control plate  56  is attached a liquid level control gap protrusion  94 . The liquid level control gap protrusion  94  is used to achieve a precise spacing between the base substrate  42  and the liquid level control plate  56  when the parts are assembled into the droplet emitter  40  and attached to the lower manifold  92 . 
     The assembly of the droplet emitter  40  and attachment to the fluid manifold  62  creates a liquid sheet flow chamber  90  starting at the manifold inlet  86 , proceeding through the gap between the base substrate  42  and the liquid level control plate  56  and ending at the manifold outlet  88 . Both the manifold inlet  86  and the manifold outlet  88  have a sheet flow partition  64  which creates and maintains a sheet flow of the liquid flowing through the liquid sheet flow chamber  90 . 
     It should be noted that in the embodiments shown in FIGS. 3,  4 , and  5 , the liquid sheet flow chamber  90  has no physical or structural obstructions in the path of the flow, particularly in the portion of the sheet flow chamber  90  between the base substrate  42  and the liquid level control plate  56 . This is the preferred embodiment as it ensures a uniform flow velocity for all the emitters across the entire length of the array. Furthermore, this decreases the possibility of trapped air-bubbles created during filling of the printhead or by perturbations in the liquid flow  52  and allows for the rapid removal of air bubbles that may get introduced into the system. However, it should be noted that as the length L of the droplet emitter gets larger, it may be desirable to provide additional support to the liquid level control plate  56 . Such liquid level control plate supports  48  may be placed within the liquid flow chamber  90  provided that have a minimal footprint and are placed a minimal distance of at least five times the channel height h from both the ends of the liquid flow channel  90  and each other as shown in FIG.  6 . Additionally, the supports must also be spaced at least a distance of five times the channel height h from the apertures  60 . Note that the liquid level control plate supports  48  are placed in the flow direction, effectively creating several large flow chambers  50  between the liquid level control plate supports  48  in the portion of the liquid sheet flow chamber  90  where they reside. 
     An additional part assembled with the lower manifold  92  and the droplet emitter stack  40  is a bridge plate  82 . The bridge plate  82  is used to mount a flex cable  100 . The flex cable  100  is used to provide connections for discrete circuit components  76  which are mounted on the flex cable  100  and are used to generate and control the focussed acoustic wave. Bond wires  96  provide electrical connections between the flex cable  100  and circuit chips  80  mounted on the base substrate  42 . Control circuitry for the droplet emitter has described for instance in U.S. Pat. No. 5,786,722 by Buhler et al. titled “Integrated RF Switching Cell Built In CMOS Technology And Utilizing A High Voltage Integrated Circuit Diode With A Charge Injecting Node” issued Jul. 28, 1998 or U.S. Pat. No. 5,389,956 by Hadimioglu et al. titled “Techniques For Improving Droplet Uniformity In Acoustic Ink Printing” issued Feb. 14, 1995, both incorporated by reference hereinabove. 
     FIG. 7 shows a perspective view of the cross section of the droplet emitter shown in FIG. 5 with additional thermally conductive components. Specifically, a heat conductive backplane is inserted in the gap between the flex cable  100  and the manifold  62 . Additionally, a thermally conductive connection  74  is made between the heat conductive back plane  72  and the upper manifold  98 . The thermal conduction between the heat conductive backplane  72  and the manifold  62  allows heat generated by the circuit chips  80  to be transferred to the flowing liquid  52  via the manifold  62 . It should be noted that the assembly is arranged such that the excess heat is transferred to the flowing liquid  52  on the exit portion of the device or after the flowing liquid  52  has passed through most of the liquid sheet flow chamber  90  and is ready to exit the manifold  62  through the manifold outlet tube  68 . This allows excess heat to be carried away from the droplet emitter  40  and helps to maintain thermal uniformity within the droplet emitter  40 . 
     Another feature shown in FIG. 7 is spring clip  78 . The spring clip  78  is used to secure the entire assembly but allows for some movement of upper manifold  98  relative to the lower manifold  92  due to the different thermal expansion coefficients of the upper manifold  98  and the lower manifold  92 . However, other fastening methods that would accomplish the same function are also known, For instance, the upper manifold  98  could be attached to the lower manifold  92  with an elastomer glue joint. An elastomer glue joint would fixedly attach the upper manifold  98  to the lower manifold  92  while also allowing for some movement of the upper manifold  98  relative to the lower manifold  92  due to the different thermal expansion coefficients. However, when spring clips  78  are used, their number and position should be such that the flexible seal is leak free and the seal compression is uniformly distributed along the length L of the array  54  of the droplet emitter  40  in order to minimize resultant gap nonuniformities between the base substrate  42  and the liquid level control plate  56 . In order to accomplish this, it should be noted that the two flexible seals  84 , in the embodiment shown in FIG. 7 are two elongated o-rings. The compliance or stiffness of this type of o-ring seal is fairly uniform along the length of the o-ring except for the ends of the o-ring. This type of o-ring is much stiffer at the ends than along the rest of the length of the o-ring. Therefore, in order to insure that the seal is under substantially uniform compression, more force is needed at the ends of the o-ring than along the rest of the length of the o-ring. One method of accomplishing this, is to do as shown in FIG. 8, and place the spring clips  78  over the stiffer ends of the o-rings. However, this is not the only method available, for instance, a full lengthwise spring clip with applies more clamping force above the ends of the o-ring than along the rest of the length of the o-ring could be used. Also, a series of small spring clips applying a small force could be placed along the length of the o-ring while using larger spring clips which apply a greater force at the ends of the o-ring. 
     FIGS. 8 and 9 show exploded views of the upper manifold  98  and the lower manifold  92  respectively. Again, while many manufacturing techniques are known, one method to make the upper manifold  98  is to divide the upper manifold into easily manufacturable components which can then be assembled into the upper manifold. The upper manifold is divided into an upper portion  98   a  and a lower portion  98   b  which are then assembled with a pair of baffles  102  which is inserted therebetween. The baffles  102  are to used aide in the conversion of the liquid flow into the upper manifold  98  in a sheet flow. The manifold inlet and outlet tubes  66 ,  68  can then be inserted into the upper portion  98   a  to complete assembly of the upper manifold  98 . 
     The lower manifold  92  can be assembled from a stack of parts in a similar manner along with the flex cable  72 , base substrate  42 , and the liquid level control plate  56 . The lower manifold  92  is manufactured in four sheet-like portions  92   a ,  92   b ,  92   c , and  92   d . This allows for easy manufacture of the lower manifold  92  because each portion can be easily and accurately stamped, chemically etched or laser cut out of a sheet material such as readily available sheet metal stock. The liquid sheet flow chamber is defined by the patterns removed out of each portion  92   a ,  92   b ,  92   c ,  92   d  when the portions are stacked and assembled together with the base substrate  42 , and the liquid level control plate  56 .