Capillary Force Actuation for Achieving Large Deflections of an Elastomeric Membrane at Low Voltage

Barth, Christina, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Knospe, Carl, Department of Mechanical and Aerospace Engineering, University of Virginia

Elastomeric materials offer many benefits to MEMS (Micro Electro Mechanical Systems). These benefits include smaller device footprints, greater range of motion, and lower required actuation forces. However, a complementary actuation technology is needed, to implement the capabilities of elastomeric components. Capillary forces are a novel and promising mechanism to produce motion in the flexible components of micro devices – such as elastomeric membranes.

Capillary Force Actuators (CFAs) are a new type of MEMS microactuator. They are capable of delivering significantly greater forces and larger actuation strokes than current technology. In the Capillary Force Actuated Membrane, CFAM, a conducting liquid bridge extends between two electrodes, each covered by a thin dielectric layer to form an electrowetting substrate. When voltage is applied, the contact angle of the liquid changes, a process known as Electrowetting on Dielectric (EWOD). The change in contact angle results in a change in the curvature of the liquid bridge's profile. From Laplace's equation for the pressure change across an interface, this change in curvature results in a decrease in the capillary pressure of the liquid. If the liquid bridge is in contact with an elastomeric membrane the change in pressure will result in deflection of the membrane. The amount of deflection achievable thus depends on the decrease in contact angle obtained via electrowetting and the surface tension of the liquid.

This thesis focuses on the design, fabrication, and experimental demonstration of a new actuation method for the displacement of elastomeric membranes. Two CFAM prototypes are designed after a study of CFAM physics, with the development of a predictive model for membrane displacement. The two prototypes are representative of two common types of electrowetting configurations. We utilize an optimized non-photolithography microfabrication process to produce the prototypes. A side view of the capillary bridge profile is implemented - via florescent lighting – to the existing displacement test platforms. Thus, we are able to characterize the accuracy of the CFAM deflection model. The CFAM prototypes performance is shown to match the predicted model, providing confidence for optimizing future designs. Experimental results of low actuation voltage and high displacement suggest that CFAM may be a viable technology for hybrid microdevices and actuators.

MS (Master of Science)
actuation, electrowetting , capillary force, elastomeric micro devices, MEMs
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