Abstract:
A liquid electrical switch is disclosed that uses a plurality of droplets of conducting liquid to form an electrical path. In a first embodiment, at least a first voltage differential is used to create a separation distance between two droplets. The droplets are illustratively contained within a housing and surrounded by an immiscible, insulating liquid. In this embodiment, the at least a first voltage differential draws at least a portion of at least one of the droplets away from a second droplet, thus preventing electrical current from flowing from the at least one droplet to the second droplet. In another embodiment, the at least a first voltage differential is changed in a way such that at least one liquid droplet is made to come into contact with a second droplet, thus creating an electrical path between the two droplets.

Description:
FIELD OF THE INVENTION 
   The present invention relates to electrical switches and, more particularly, to liquid electrical microswitches. 
   BACKGROUND 
   Small relays and switches, known as microswitches, are used in a wide variety of applications, such as automotive, test equipment, switchboard and telecommunications applications, for a wide variety of purposes. Due to the general trend of miniaturization of electronics, there is a growing need for ever-smaller relays and switches. Thus, a great deal of research has been conducted in the miniaturization of traditional electromechanical switches. However, traditional electromechanical switches, such as that switch shown in  FIG. 1 , tended to be inherently unreliable due to their mechanical nature—at some point the moving parts and/or the physical electrical contacts within the switches failed. 
   Referring to  FIG. 1 , illustrative prior art electromechanical switch  100  has load contacts  101   a  and  102   a  connected to arms  101  and  102 , respectively. When an input voltage is applied to coil mechanism  111 , the input voltage magnetizes the core  110 , which pulls arm  106  and contact  108  toward contact  109  of core  110  in direction  107 . This action causes, in turn, contact  103  on arm  105  to move upward in direction  104 , thus physically pushing arm  102  upward in a way such that the load contacts  101   a  and  102   a  come into contact with each other, thereby closing the load circuit. When the input voltage is removed from coil  111 , the spring lever  113  functions to push the contacts  101   a  and  102   a  away from each other, thus breaking the load circuit connection. 
   As discussed above, switch  100  must inherently make mechanical contacts in order to switch a load. At the point of these contacts, oxidation breakdown occurs over time at which point the switch will require maintenance or replacement. When switch  100  is activated, there is a window of time during which the load circuit flickers between an open and a closed state. This phenomenon, referred to as “bounce”, creates a delay between the time current is applied and the time the circuit is switched, thus creating a condition which may need to be considered in load design. 
   More recently, research efforts have focused on the development of miniature Microelectromechanical Systems (MEMS) switches which are, for example, fabricated using integrated circuit manufacturing processes, typically resulting in a lower production cost. However such switches are still somewhat unreliable because MEMS switches, similar to the electromechanical switch  100  of  FIG. 1 , typically are characterized by solid contacts that must be brought into contact with each other to close the switch. Accordingly, while MEMS switches are less costly than larger scale electromechanical switches, the aforementioned problems with electromechanical switches remain. Specifically, the solid-to-solid contact surfaces required in such switches suffer from the same oxidation/degradation during normal cycling. At relatively smaller scales, this can result in increased contact resistance, stiction, or microwelding at high throughput power. 
   In order to prevent the aforementioned problems associated with small electromechnical switches, other efforts have focused on solid state relays (SSRs).  FIG. 2  shows a typical prior art Metal Oxide Semiconductor Field-Effect Transistor (MOSFET)-based SSR  200 . Specifically, an input current is applied to light-emitting diode  201 , which is illustratively a Gallium Arsenide (GaAs) infrared LED, via leads  202  and  203 . The resulting emitted light  204  is reflected within an optical dome  205  onto a series of photo diodes  206  in photodiode array  206   a . The photodiodes generate a voltage which is passed to driver circuitry  207  which, as is well known, is used to control the gates of two MOSFETs  208  and, accordingly, to switch a load  209 . 
   All of the components of SSR  200  are, for example, fabricated out of semiconductor material and as a result, the solid state relay combines many operational characteristics not found in other types of devices. Because there are no moving parts, in contrast to the aformentioned electromechanical switches, solid state relays are characterized by relatively long switching lives and exhibit bounce-free operation. Additionally, the input LEDs require low signal levels, thus making such SSR switches attractive in low power applications. 
   While SSRs are, for the above reasons, advantageous in many applications, they are also limited in their usefulness in certain respects. Specifically, such devices may experience a relatively high degree of current leakage and, thus, may be relatively inefficient. Also, SSRs are typically limited in the power they can carry and, accordingly, are likewise limited in the ability to switch higher loads. Finally, SSRs are relatively expensive to manufacture compared to electromechanical and/or MEMs-based devices, thus increasing the cost of devices that use SSRs. 
   In order to address some of the above limitations, recent attempts have focused on developing liquid based microswitches. Such attempts are described in, for example, J. Kim et al., “A micromechanical switch with electrostatically driven liquid-metal droplet,”, SENSOR ACTUAT A-PHYS 97–8: 672–679 Apr. 1, 2002; L. Latorre et al., “Electrostatic actuation of microscale liquid-metal droplets,” JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 4, AUGUST 2002 and J. Simon, S. Saffer, and C.-J. Kim, “A Liquid-Filled Microrelay with a Moving Mercury Micro-Drop”, J. Microelectro-mechanical Systems, Vol. 6, No. 3, September 1997, pp. 208–216, which are hereby incorporated by reference herein in their entirety. As described in these references, typical prior liquid switches use a movable liquid droplet that either comes into contact or separates from the solid electrodes, thus connecting or disconnecting electrical circuit. Such an approach is limited in that the wetting and electrochemical phenomena at the liquid-solid contact interface can be relatively unreliable. 
   SUMMARY OF THE INVENTION 
   The present inventors have realized that, for the foregoing reasons, there remains a need for an efficient, long-life electrical microswitch that is relatively low cost to manufacture. Therefore, the present inventors have invented a microswitch that essentially eliminates the problems associated with prior devices, as discussed above. Specifically, the present inventors have invented a liquid electrical switch that uses a plurality of droplets of conducting liquid to form an electrical path. In a first embodiment, at least a first voltage differential is used to create a separation distance between two droplets. The droplets are illustratively contained within a housing and surrounded by an immiscible, insulating liquid. In this embodiment, the at least a first voltage differential draws at least a portion of at least one of the droplets away from a second droplet, thus preventing electrical current from flowing from the at least one droplet to the second droplet. 
   In another embodiment, the at least a first voltage differential is changed in a way such that at least one liquid droplet is made to come into contact with a second droplet, thus creating an electrical path between the two droplets. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows an illustrative prior art electromechanical electrical switch; 
       FIG. 2  shows an illustrative prior art solid state relay; 
       FIG. 3  shows one illustrative embodiment of a present invention showing a cathenoid electrical microswitch in a open position; and 
       FIG. 4  shows the illustrative microswitch of  FIG. 3  wherein the microswitch is in an closed position. 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows one embodiment in accordance with the principles of the present invention of a liquid electrical microswitch, specifically a catenoid structured liquid electrical switch. More specifically,  FIG. 3  represents a cross sectional view of a liquid electrical switch  301  having outer housing  302  that is, illustratively, cylindrical in shape. Illustratively, having a diameter between 10 and 500 micrometers. One skilled in the art will recognize that many suitable shapes and sizes may be chosen to achieve the teachings of the principles of the present invention. Illustrative electrodes  318  and  319  are disposed at the ends of the illustrative cylinder. Within housing  302 , droplets  303  and  304  are positioned on the opposing sides of two conductive plates,  305  and  306  respectively, which are electrically connected to the switching terminals  307  and  308 , also respectively that are in turn connected to an electrical load  309  for which switching is desired. Droplets  303  and  304  may be selected from a wide variety of desirable conductive liquids, such as, illustratively, ionic liquids, aqueous salt solutions, liquid metals, etc. One skilled in the art will recognize that many different liquids may be chosen advantageously for the droplet in accordance with the principles of the present invention. 
   Conductive droplets  303  and  304  are, illustratively, surrounded by a dielectric confinement fluid  310  which is, for example, immiscible and density-matched with the liquid chosen for droplets  303  and  304 . Such a confinement liquid is used, for example, in order to prevent evaporation of the droplets  303  and  304 , as well as to ensure gravity-independent, vibration and shock-resistant operation of the switch  300 . A number of common fluids can serve as a confinement fluid, including silicone oils, alkanes, and alcohols. Once gain, one skilled in the art will fully appreciate in light of the teachings herein that any number of suitable liquids may be chosen to achieve the principles described herein. 
   Liquid droplets  303  and  304  are, illustratively, allowed to penetrate partially into the chamber  311  between plates  305  and  306  via holes  312  and  313 , respectively. In operation, in order to maintain a separation between the two droplets  303  and  304  (i.e., maintain an open switch), a voltage differential is applied between electrode  318  and droplet  313 , as well as between electrode  319  and droplet  304 . For example, in the representative example of  FIG. 3 , voltage V 1  is applied to lead  307 , and hence to conducting plate  305  and droplet  303 . Similarly, a voltage V 0  is applied to lead  308  and, hence, to conducting plate  306  and droplet  304 . A voltage differential is, therefore, created between the conducting plates and droplets by, illustratively, applying a voltage of V 1 +·V to electrode  318  and a voltage of V 0 +·V to electrode  319 . The result is to draw the top portion  314  of droplet  303  in direction  316  toward electrode  318  and to draw the top portion  315  of droplet  304  in direction  317  toward electrode  319 , thus pulling the bottom portions  320  and  321  of droplets  303  and  304 , respectively, away from each other. Thus, an open electrical switch is created whereby substantially no electrical charge is conducted between the two droplets  303  and  304 . 
     FIG. 4  shows the electrical switch of  FIG. 3  that is configured in the closed, or on, position. Referring to  FIG. 4 , in order to establish electrical connection between leads  307  and  308 , the liquid is expelled from both upper chamber  402  and lower chamber  403  until the droplets  303  and  304  come into contact with each other so that an electrically conductive liquid bridge  401  is formed between conducting plates  305  and  306 . This expelling of liquid from chambers  402  and  403  is, for example, caused by removing the aforementioned voltage differential between the droplets  303  and  304  and the conducting plates  305  and  306 , respectively. Once these differentials are removed, some of the liquid in the upper portions  316  and  317  of droplets  303  and  304 , respectively, is pushed back into chamber  311  by the pressure exerted on the droplets by the immiscible liquid surrounding the droplets in the upper and lower chambers  402  and  403 . Once the droplets  303  and  304  are allowed to penetrate into chamber  311 , they contact each other thus forming, illustratively, a catenoid-shaped liquid bridge  401  between the conducting droplets  303  and  304 . Electricity is thus permitted to flow between the two droplets to supply power to load  309 . 
   To open the switch once again (i.e., turn the switch to the off position), it is only necessary to reapply a voltage potential between the droplets  303  and  304  and the electrodes  305  and  306 . This will serve to draw the liquid droplets closer to the respective electrodes and, accordingly, as the liquid is withdrawn back to the upper and lower chambers  402  and  403 , the liquid bridge  401  collapses and the electrical connection between leads  307  and  308  is terminated. 
   The embodiments of  FIG. 3  and  FIG. 4  show that the amount of the liquid in each chamber can be changed by electrostatically adjusting the curvature of the surface of the liquid that fills the chamber. One skilled in the art will be able to devise, in light of the teachings herein, alternate methods of altering the amount of liquid in the chambers. For example, this can be achieved by changing the pressure in the upper an dlower chambers, utilizing electrocapillary mechanisms or, alternatively, electrowetting mechanisms. One skilled in the art will recognize that these methods have been shown to be effective at controlling the geometry of the liquid microdroplets. For example, if pressure changes were relied upon, increasing the pressure within the upper and lower chambers could be used to expel a portion of one or both of the liquid droplets from the chambers so that the droplets contacted each other. A reduction of pressure within the chambers would have the opposite effect and would draw more of the droplet into the chambers, thus breaking the contact between the droplets. Such pressure changes could be achieved, for example, through the use of thermal energy by heating or cooling the liquid in the upper and/or lower chambers. One skilled in the art will recognize that, by heating the liquid, the droplet will be expelled from the chamber and by cooling the liquid, the droplet will be drawn back into the chamber. 
   If electrocapillary actuation were used, an illustrative electrolite (such as aqueous solution of potassium chloride (KCl) is used as a secondary liquid filling the chamber and an illustrative liquid metal (such as mercury) is used for the droplet liquid. A charged double layer is formed between the droplet and surrounding liquid that prevents the current from flowing through the liquid-to-liquid interface and thus provide electrical isloation between the two liquids. By applying a voltage differential between the droplet and surrounding electrolite the charge density of the double layer is changed and, as a result, the surface tension of the liquid-to-liquid interface is changed. Since the surface tension of this interface determines the curvature of the droplet, the amount of the liquid in the chamber will be changed as well. Electrocapillarity is a well known effect and is described in detail in many publications such as, for example, the book A. Adamson, Physical Chemistry of Surfaces, John Wieley &amp; Sons, Inc., 1990. 
   Similarly, if electrowetting were used, the contact angle of the conducting droplet could be made to change, thus altering the amount of liquid is in the chamber. Electrowetting as applied to changing the contact of a liquid microlens with a substrate is described generally in U.S. patent application Ser. No. 10/139,124, titled “Method And Apparatus For Calibrating A Tunable Microlens,” and is hereby incorporated by reference herein in its entirety. 
   One skilled in the art will recognize that the liquid microswitches thus described herein can be easily integrated in high-density two-dimensional arrays, thus opening the way to creating very large switching fabrics. In this case self-assembly techniques can be utilized to allow the liquid microdroplets to arrange themselves in the appropriate geometrical pattern without the need for special assembly. Such techniques are described in U.S. patent application Ser. No. 10/403,159, titled “Method and Apparatus for Variably Controlling the Movemenet of A Liquid on a Nanostructured Surface,” and filed Mar. 31, 2003, which is hereby incorporated by reference herein in its entirety. One skilled in the art will realize that, in addition to the methods discussed in the &#39;159 application, such self assembly can be achieved by selectively patterning the plates with the alternating pattern of the high and low surface energy coatings, thus attracting liquids to desired locations on the aforementioned switching fabric. 
   The advanced liquid-based microswitches described herein are advantageous in many regards. In addition to a relatively low-cost manufacturing process, for example, switches in accordance with the principles of the present invention are characterized in part by having no moving parts, other than the liquid droplets. Electrical connections are established and broken by actively changing the shape of the liquid droplets that remain stationary with respect to the rest of the device. As a result, mechanical wear, one of the leading causes of failure in traditional switches, is substantially eliminated with switches in accordance with the principles of the present invention. Additionally, the switches described herein have no physical contact-to-contact interface in the flow path of electrical current. As described above, electrical current is always conducted through the continuous illustrative catenoid body of the liquid (liquid bridge), which is formed between the neighboring droplets during each switching cycle. This eliminates chemical and electrochemical degradation of the contacts—another important failure mechanism common in traditional, prior switches. Finally, the proposed liquid-to-liquid contact design also provides for zero OFF state current (high isolation), very low ON state resistance, and high-speed, bounce-free operation. Thus, the liquid switches described herein seamlessly combine exceptional reliability, with high performance, small size, and low-cost. 
   The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof.