Capillary force actuators

Existing micro actuation technologies---electrostatic, electromagnetic, piezoelectric, and thermal---have significant limitations that impede their use in microdevices. Many applications demand greater force, larger stroke, lower power, or higher bandwidth than is possible with these technologies. Furthermore, some of these actuators are very difficult to integrate into microfabrication. This dissertation examines a novel micro actuation technology, Capillary Force Actuation (CFA), which takes advantage of microscale capillary bridges to produce 10 to 100 times greater force than the most commonly used micro actuators. A standard configuration for capillary force actuators is introduced and the electromechanical principles underlying the actuation principle are studied. Semi-analytical investigation of the capillary bridge profile with and without applied electric field is performed. Parameters characterizing the device configuration are introduced and design charts in terms of these parameters are developed. Design optimization and limits are studied using an analytical approximation of the actuator force produced. It is shown that if the actuation voltage is fixed, the maximum force achievable is independent of the surface tension of the liquid. Numerical methods for calculating the force are introduced for more complex configurations. The stability of the capillary bridge is also investigated and the implications this has for actuator design are examined. It is shown that for typical values of configuration geometry, the capillary bridge will remain stable during actuation. Alternative configurations of CFA are considered. The boundary condition problem governing the bridge shape for each of these cases is derived and solution methods are presented. Examples of the various alternative configurations are provided and a comparison of the force level to that of the standard configuration is made. It is shown that the standard configuration is more effective than configurations with pinned contact line or constant contact angle on one surface for maximum change in force. The rate of the force change however may be increased by employing a surface energy gradient on the surfaces. For a device with only one active surface, it is shown that pinning the passive side would be most effective for maximum change in force.