Abstract:
A pump includes two opposing plates having a space there between to define a secondary fluid passage. A first aperture is formed through the first plate and a second aperture is formed in the second plate to define a primary fluid channel extending across the secondary fluid channel. A heater on the second plate moves primary fluid through the primary fluid channel due to the Marangoni type effect. A heater on the first plate causes a meniscus to enlarge to thereby form a drop of fluid ejected out of the primary fluid channel.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to pumping devices, and more particularly to a fluid pump and ink jet print head using a temperature gradient across a multiple fluid interface to generate fluid motion. 
     BACKGROUND OF THE INVENTION 
     Various pumps are used in printers to pump ink out of a nozzle and onto a print medium. For example in a bubble jet printer, the ink in a channel is heated to a boil to create a bubble until the pressure ejects a droplet of the ink out of a nozzle. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace the ink that was ejected from the nozzle. Such thermal technology requires a cooling period between ejecting successive droplets from a nozzle and thus has speed limitations. Also, such thermal technology cannot be used to pump fluids that are adversely affected by boiling. 
     Piezoelectric pumps, such as that disclosed in U.S. Pat. No. 5,224,843, have a piezoelectric crystal in the fluid channel that flexes when an electric current flows through it to force a drop of fluid out of a nozzle. Piezoelectric technology is faster and provides more control over the fluid movement as compared to thermal technology. Also, because the fluid to be pumped is not heated significantly, the fluid can be selected based on its relevant properties rather than its ability to withstand high temperatures. However, piezoelectric microscale pumps are complex and thus expensive to manufacture. 
     Further, fluid pumps are often required in various applications in which a high degree of control is required and high temperatures are to be avoided. For example, pumps can be used in biological heat-pipe type devices, devices which administer small doses of fluid into a larger stream of fluid, devices which pump various solutions that are unstable when boiled, devices which pump biological materials and other materials that must be maintained at a constant temperature, and other generic pumping applications. 
     It is well known to utilize the “Marangoni type effect” to pump fluids. The Marangoni type effect refers to a phenomenon that occurs at the interface of two immiscible fluids when the surface tension on the interface is not constant, i.e. has a gradient. In particular, a fluid flow is established along the fluid interface in the direction of increasing surface tension. Successive layers of the fluid below the interface are dragged along due to the viscosity of the fluid to establish a general current in the fluid in the direction of the Marangoni type flow. The surface tension gradient can be established by a temperature gradient along the interface because surface tension varies with temperature. 
     For example, U.S. Pat. No. 4,813,851 discloses a device for conveying fluids utilizing the Marangoni type effect. However, the device disclosed in U.S. Pat. No. 4,813,851 does not exhibit the high degree of control required for ink jet printers and other applications. Further, this device is not compatible with standard semiconductor fabrication techniques and thus is difficult to manufacture in small scale. 
     Accordingly, there is a need for a fluid pump, for use in printers or the like, that is simple in construction and capable of pumping fluid quickly and accurately without boiling the fluid. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to increase the control accuracy of fluid pumps and print heads utilizing the thermally induced Marangoni type effect. 
     Another object of the invention is to simplify the construction of fluid pumps and print heads. 
     Another object of the invention is to impart motion to fluid without the need for moving parts or boiling of the fluid. 
     Another object of the invention is to utilize standard semiconductor fabrication techniques to manufacture a fluid pump and print head. 
     Another object of the invention is to improve the performance of ink jet print heads. 
     The invention achieves these and other objects through a first aspect of the invention which is an ink jet print head comprising a first plate having first and second sides and a first aperture formed therethrough, a second plate having first and second sides and a second aperture formed therethrough, and a spacer coupled to the second side of the first plate and the first side of the second plate to define a secondary fluid passage between the first plate and the second plate. The first aperture and the second aperture are substantially aligned to define an ink passage extending across the secondary fluid passage to thereby define an interface between ink in the ink passage and a secondary fluid in the secondary fluid passage. A first heater is disposed on the first side of the first plate proximate the first aperture, and a second heater is disposed on the first side of the second plate proximate the second aperture. A controller is operatively coupled to the first heater and the second heater to control energization of the first heater and the second heater in a predetermined manner. The interface is heated to create a temperature gradient, and thus a surface tension gradient, to thereby move ink through the ink passage. 
     A second aspect of the invention is a fluid pump comprising, a first fluid supply mechanism for supplying a primary fluid, a second fluid supply mechanism for supplying a secondary fluid, a first plate having first and second sides and a first aperture formed therethrough, a second plate having first and second sides and a second aperture formed therethrough and a spacer coupled to the second side of the first plate and the first side of the second plate to define a secondary fluid passage between the first plate and the second plate. The first aperture and the second aperture are substantially aligned to define a primary fluid passage extending across the secondary fluid passage. The primary fluid passage is coupled to the primary fluid supply and the secondary fluid passage is coupled to the secondary fluid supply to thereby define an interface between primary fluid in the primary fluid passage and secondary fluid in the secondary fluid passage. A first heater is disposed on said first side of the first plate proximate the first aperture and a second heater is disposed on the first side of the second plate proximate the second aperture. A controller is operatively coupled to the first heater and the second heater to control energization of the first heater and the second heater in a predetermined manner. Fluid is moved through the primary fluid passage due to the Marangoni type effect. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments of the invention and the accompanying drawings, wherein: 
     FIG. 1 is a top view of a pump in accordance with a preferred embodiment the invention; 
     FIG. 2 is a sectional view of the head of the pump of FIG. 1 taken along line  2 — 2 ; 
     FIG. 3 is a sectional view taken along line  2 — 2  of the head of the pump of FIG. 1 with a first modification; 
     FIG. 4 is a sectional view, taken along line  2 — 2 , of the head of the pump of FIG. 1 used in a modified manner; and 
     FIG. 5 is a sectional view of the head of a pump in accordance with another preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate a first preferred embodiment of the invention in the form of a pump for an ink jet printer. The preferred embodiment is formed of a semiconductor material such as silicon using VLSI semiconductor fabrication techniques. However, the invention can be formed of various materials using various fabrication techniques. As illustrated in FIG. 1, pump  10  comprises head  20 , primary fluid supply  30  (a supply of ink in the preferred embodiment), and secondary fluid supply  40  (a supply of air or any other suitable gas or liquid that is immiscible with respect to the primary fluid). Note that pump  10  is illustrated schematically and not to scale for the sake of clarity. However, one of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of the preferred embodiment based on the disclosure herein. 
     As illustrated in FIG. 2, head  20  comprises first plate  12  having first aperture  14  formed therethrough and second plate  52  having second aperture  54  formed therethrough. Spacer  60  is disposed between first plate  12  and second plate  52  abutting a second side of first plate  12  and a first side of second plate  52 . A first side of first plate  12  opposes printing medium  70 , such as paper, and a second side of second plate  52  faces downward in FIG.  2 . First aperture  14  and second aperture  54  are substantially in alignment to define primary fluid passage  32  which is coupled to primary fluid supply  30  at the second side of second plate  52 . Spacer  60  defines secondary fluid passage  42  between first plate  12  and second plate  52 . Secondary fluid passage  42  is coupled to secondary fluid supply  40  through an unillustrated port formed in spacer  60 , first plate  12 , or second plate  52 . 
     Heater  16 , in substantially a ring shape, is formed on the first side of first plate  12  around first aperture  14 , preferably in a concentric manner Note that heater  16  is illustrated in FIG. 1 as being disposed radially away from an edge of first aperture  14  for clarity. However, heater  16  preferably is disposed close to an edge of first aperture  14  as illustrated in FIG.  2 . In the preferred embodiment, heater  16  comprises an electric resistive heating element and thus conductors  18  and pads  22  are formed on the first side of first plate  12  to permit electrical connection between controller  80  and heater  16 . Controller  80  can be merely a power supply or can comprise logic for controlling pump  10  in a desired manner. For example, controller  80  can be a programmable microprocessor based device. Similarly, heater  56 , in substantially a ring shape, is formed on the first side of second plate  52  around second aperture  54 , preferably in a concentric manner. In the preferred embodiment, heater  56  comprises an electric resistive heating element and thus conductors  58  and pads  52  are formed on the first side of second plate  52  to permit electrical connection between controller  80  and heater  56 . 
     In operation of pump  10 , a primary fluid (ink in the preferred embodiment) is supplied at a predetermined pressure from primary fluid supply  30 , through second aperture  54 , to primary fluid passage  32 . Also, a secondary fluid, that is immiscible with respect to the primary fluid, is supplied from secondary fluid supply  40  to secondary fluid channel  42 . The relative pressures of the primary fluid and the secondary fluid are adjusted, using pressure regulators or the like, to create meniscus  25   a  at first aperture  14  and meniscus  55  at the interface of the primary fluid and the secondary fluid. When heaters  16  and  56  are energized, by applying an electric potential across pads  22  and pads  62  respectively, the primary fluid will flow out of primary fluid passage  32  to create meniscus  25   b , and eventually meniscus  25   c , due to the Marangoni type effect caused by the temperature gradient, and thus the surface tension gradient, established along the interface between the primary fluid and the secondary fluid at meniscus  55 . Heaters  16  and  56  are then turned off at the appropriate time and the primary fluid continues to move due to inertia to create meniscus  25   c . Ultimately, drop D of primary fluid separates from the remaining primary fluid in primary fluid passage  32  leaving meniscus  25   d  that returns to the equilibrium shape of  25   a.    
     Heater  16  causes meniscus  25   a  to bulge into meniscus  25   b  and so on, while heater  56  causes flow of primary fluid through primary fluid passage  32  due to the Marangoni type affect across the interface between the primary fluid and the secondary fluid, i.e. meniscus  55 . This procedure is accomplished for each drop of primary fluid to be ejected from primary fluid passage  32 . For example, in the case of an ink jet printer, controller  80  controls the timing of energizing heaters  16  and  56 , and possibly the pressure of the primary fluid and the secondary fluid, to eject drops D for forming a desired image on print medium  70 . 
     FIG. 3 illustrates a modification of head  10  of the preferred embodiment. Specifically, head  10  further includes heater  66  formed on a second side of first plate  12  concentrically around first aperture  14 , Heater  66  which permits flow of primary fluid through primary fluid passage  32  to be reversed. Heaters  16  and  56  are energized in the manner described above to begin to form drop D. However, when meniscus  25   a-c  is sufficiently extended, heaters  16  and  56  are turned off and heater  66  is energized by controller  80  to reverse flow of the primary fluid, due to the Marangoni type effect, and more reliably separate drop D from the remaining primary fluid in primary fluid passage  32 . 
     FIG. 4 illustrates another modification of the preferred embodiment. In FIG. 4, head  10  is operated to cause meniscus  25   c  to contact recording medium  70  and separate from the remaining primary fluid due to wetting of recording medium  70 . The modification of FIG. 4 can be achieved merely by placing recording medium closer to head  10  or by adjusting the operating parameters of head  10 , such as the dimensions of head  10 , the pressures of the primary and secondary fluids, the operation of the heaters, and the like. The modification of FIG. 4 can be achieved with the structure of head  10  illustrated in FIG. 2 or FIG.  3 . 
     FIG. 5 illustrates another preferred embodiment having multiple stages for creating the thermally driven Marangoni type effect. Specifically, heat can be applied to meniscuses  55   a  and  55   b  by heating elements  56   a  and  56   b , as a second heater, to propel fluid through fluid passage  32 . Otherwise operation of this embodiment is similar to that of the embodiment of FIG.  1 . Similar elements in FIG. 5 are labeled with like reference numerals as compared to FIG.  1 . However, the suffixes “a” and “b” are used to differentiate between Marangoni type effect stages. 
     The primary fluid can be any fluid that is to be pumped, such as a liquid or gas. The secondary fluid can be any fluid that is immiscible with respect to the primary fluid and presents an interface with the primary fluid having the desired surface tension and other properties. The secondary fluid can be selected based on the primary fluid, the pump structure, and other considerations of each application. 
     The pump can be constructed using standard semiconductor fabrication techniques. The pump can be formed using silicon substrates as the plates or using any other material. The heaters, pads, and conductors can be formed and patterned through vapor deposition and lithography techniques. The pump can be of any size and the components thereof can have various relative dimensions. Accordingly, the pump can be a microscale pump or a larger or smaller device. The heating elements can be any type of energy delivery device, such as resistive heaters, radiation heaters, convection heaters, chemical reaction heaters (endothermic or exothermic), nuclear reaction heaters, or the like. The pump can be controlled in any appropriate manner. The controller can be of any type, such as with a microprocessor based device having a predetermined program. The heating elements can be energized to provide a desired temperature gradient in any manner and with any scheme of time coordination. For example, the heating elements can be controlled by adjusting the current therethrough or by intermittent activation in a predetermined manner. Each heater can include one heating element or plural heating elements. The pump can be applied to pumping of various fluids, such as ink in a print head, biological materials, medicaments, or any other fluids. Any number of Marangoni type effect stages can be used in seriatim or in a parallel configuration. 
     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents. 
     PARTS LIST 
       10  Pump 
       12  First Mate 
       14  First Aperture 
       16  First Heater 
       18  Conductor 
       20  Head 
       22  Pads 
       25  and Meniscus 
       30  Primary Fluid Supply 
       32  Primary Fluid Passage 
       40  Secondary Fluid Supply 
       42  Secondary Fluid Passage 
       52  Second Plate 
       54  Second Aperture 
       55  Meniscus 
       56  Second Heater 
       58  Conductor 
       60  Spacer 
       62  Pad 
       70  Print Medium