Fluid handling – Flow affected by fluid contact – energy field or coanda effect – Means to regulate or vary operation of device
Reexamination Certificate
1998-12-23
2002-03-26
Chambers, A. Michael (Department: 3753)
Fluid handling
Flow affected by fluid contact, energy field or coanda effect
Means to regulate or vary operation of device
C137S803000, C137S833000, C385S017000, C385S018000
Reexamination Certificate
active
06360775
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to movement of a bubble of one fluid in another fluid within a capillary, and more particularly to switches for optical signals, to fluid valves, to fluid pumps, and to liquid degassers.
BACKGROUND
Bubbles in capillaries are useful in several respects. In optical devices they possess a different index of refraction than a surrounding liquid and so can reflect or refract optical signals. In addition they can be moved about within capillaries by exploiting temperature-dependent surface tension effects which tend to force bubbles from colder regions to warmer regions, by exploiting geometric effects which tend to squeeze bubbles out of small capillaries into larger capillaries, and by exploiting wetting effects which tend to favor the presence of a first fluid which easily wets a capillary wall over a bubble of a second fluid which does not wet that same wall as well.
Capillary devices that exploit the change in surface tension of a bubble with temperature are said to work by thermocapillarity, also known as the Marangoni effect (see, for example, L. E. Scriven and C. V. Sterling, “The Marangoni Effects,” Nature, V 187, p 186 (1960)). An example of recent work published on an optical switching device that exploits thermocapillarity is Makoto Sato, et al, “Waveguide Optical Switch for 8:1 Standby System of Optical Line Terminals,” paper WM16, Technical Digest, OFC '98 Optical Fiber Communication Conference and Exhibit, Feb. 22-27, 1998, San Jose Convention Center, San Jose, Calif., pp 194,195. The Marangoni effect is also exploited to control bubble movement for optical switching in the invention “TOTAL INTERNAL REFLECTION OPTICAL SWITCHES EMPLOYING THERMAL ACTIVATION,” U.S. Pat. No. 5,699,462. In that patent, for example, the embodiment showed in FIG. 29 therein, in a capillary
304
with a tapered width, a bubble is generated at the resistor
308
, and is held in place at that resistor by the Marangoni effect which counteracts the geometry-induced force on the bubble due to the tapered width of the capillary. When heating of the resistor is terminated, the bubble moves from resistor
308
toward the top left of the figure along tapered capillary
304
, where movement is generated by geometric forces. Thus the operation of the device in FIG. 29 of that patent depends on an opposing balance between Marangoni forces and geometry-induced forces.
It will be understood from the description of the present invention that forces due to geometric effects can be much larger than forces due to the Marangoni effect. Because of the relative feebleness of the Marangoni effect, the geometric effect designed into the device of FIG. 29 of U.S. Pat. No. 5,699,462 to balance the Marangoni effect must also be feeble and the operation of that device is relatively slow and susceptible to interference from mechanical shock. There still exists a need for a microfluidic device which uses large-magnitude forces to provide rapid and stable optical switching, and the present invention meets that need.
Bubbles are also usefull as valve elements to control fluid flow in capillaries. A bubble can be made to reside at a given position in a capillary by some combination of temperature effects and geometric effects which creates a local energy potential minimum for the bubble, and by blocking or nearly blocking the capillary can then impede the flow of a surrounding fluid. See, for example, John Evans, et al, “Planar Laminar Mixer,” Proceedings of the Tenth annual International Workshop on Micro Electro Mechanical Systems, Nagoya, Japan, Jan. 26-30, 1997, IEEE Catalog Number 97CH36021, pp 96-101. However, the Evans et al. paper teaches no technique for removing the bubble from its local energy potential minimum. Devices that do not provide adequate bubble removal can suffer from “vapor lock” if the bubble is composed of gas liberated from dissolved gas in the liquid, because such a bubble will fail to disappear when the heating resistor which creates it is turned off. There still exists a need for a reliable technique for confining a bubble in a channel securely, and when desired, for releasing and removing the bubble from the channel, in a repeatable and efficient manner. As will be seen in detail below, the present invention teaches such a technique.
Bubbles are also useful as pumping elements in capillaries. An expanding bubble in a capillary or chamber can act as a piston, displacing the surrounding fluid and causing it to move in a direction dictated by the capillary geometry. The same work by John Evans, et al, “Planar Laminar Mixer,” referenced above, employs an alternately expanding and contracting bubble in a chamber as a piston element. Like the valve described in the same work, the piston bubble can suffer from “vapor lock.” There still exists a need for a pump employing a bubble piston which removes that bubble from the pumping chamber when desired, and the present invention teaches such a pump.
SUMMARY
The present invention uses the difference in energy potential associated with a bubble in different regions of a capillary to trap the bubble within a region of the capillary when such trapping is desired, and does so while providing resistance to perturbations, which could lead to undesired changes in state. The difference in energy potential can be geometry-dependent and/or materials-dependent. The present invention also uses difference in energy potential to remove the bubble from the trap when desired, and does so more rapidly than prior art devices. The invention is useful for optical switching, for fluid valving, for fluid pumping, and for liquid degassing.
Operation of the device is as follows. In a capillary containing a first fluid, a wall-confined bubble of a second fluid is introduced into a designed-in trap in the capillary by some means, for example by boiling a liquid constituting the first fluid using a heating resistor to create a bubble of a vapor which constitutes the second fluid. Bubbles can also be introduced using electrical, chemical, electrolytic, pneumatic, hydraulic, optical, inertial, ultrasonic, and microfluidic techniques, including injecting bubbles from a source (e.g., a gas bubble from a gas source). Introduction of the bubble accomplishes a first desired event, such as switching of an optical beam or blocking of a fluid channel. The bubble is trapped because it sits at a local energy potential minimum within the capillary, and moving it would require an input of energy.
Next, the energy of the bubble is increased by, for example, increasing the power input to a heating resistor to introduce more gas, thereby increasing the size of the bubble. As the bubble grows its energy increases, and as it grows it encounters a designed-in spatial asymmetry (geometrical and/or material) in the energy potential of the capillary adjacent to the trap. It tends to grow in the direction of least energy potential. Growth of the bubble can accomplish a second desired event, such as pumping of the volume of the first fluid displaced by the bubble during its growth.
Then, the growing bubble reaches a metastable energy maximum and it encounters a designed-in region of low spatial energy potential within the capillary. Further growth of the bubble causes it to intrude into the region of low spatial energy potential, and the bubble becomes positionally unstable. In a manner analogous to the siphoning of water through a hose from a hillside pond to a lower pond due to gravitational energy potentials, the bubble moves from the trap and flows rapidly into the region of low spatial energy potential. As it moves, it accomplishes a third desired event, such as switching of an optical beam or unblocking of a fluid channel.
In some embodiments, as the bubble leaves the trap in a direction defined as downstream, it pushes downstream some of the first fluid ahead of it and sucks a volume of the first fluid from a direction defined as upstream to refill the volume of the trap.
Because the invention employs designed-in asymmetries in energy poten
Barth Phillip W.
Donald David K.
Field Leslie A.
Agilent Technologie,s Inc.
Chambers A. Michael
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