The stability and pull-in voltage of electrostatic parallel-plate actuators in liquid solutions

This paper deals with parallel-plate electrostatic actuators in liquids. We study the stability conditions of such actuators and show that the pull-in effect can be shifted beyond one-third of the gap, and can even be suppressed. We demonstrate that the insulating layers of the actuator plates, which are originally designed to avoid any current leakages or short-circuits, play a major role in this phenomenon. Experiments are performed on fabricated devices; silicon nitride layers are used to completely encapsulate the actuator plates. The voltages required to close the actuator gap are measured in various liquids and compared to the values obtained by analytical calculations. This study gives guidelines for the design of parallel-plate actuators featuring in liquids either a binary-state operation when the pull-in effect occurs, or a continuous displacement within the full gap.