Electrostatic actuation of microscale liquid-metal droplets

This paper reports sliding of micro liquid-metal droplets by electrostatic actuation for MEMS applications, bi-stable switching in particular. Basic theory concerning droplets on a plane solid surface is exposed followed by experimental study. Being a major parameter in the modeling of sliding droplets, the contact angle has been characterized in the case of mercury on an oxidized silicon wafer. The method used involves both traditional optical microscope and confocal laser imaging. The contact angle is found to be around 137° with an associated standard deviation of 8°. The sample preparation is detailed. The droplets deposition method is based on selective condensation of mercury vapor on gold dots acting as preferred nucleation sites. This technique provides control of droplet dimensions and locations and is suitable for batch fabrication. Experimental study of electrostatic actuation coupled with finite-element method (FEM) analysis is described, leading to the determination of the sliding condition parameter, which represents a contact angle hysteresis of about 6°. Experimental results also confirm the proportionality between minimum driving force and droplet dimension. Finally, a design optimization methodology is proposed, based on the use of finite-element model simulations.