Nanoscale electrostatic actuators in liquid electrolytes: analysis and experiment

摘要

The objective of this dissertation is to analytically model a parallel plateelectrostatic actuator operating in a liquid electrolyte and experimentally verify theanalysis.The model assumes the system remains in thermodynamic equilibrium duringactuation, which enables the ion mass balance equations and Guass?? Law to be combinedinto the Poisson-Boltzmann equation. The governing equations also include the linearmomentum equation including the following forces: the electric force, the osmotic force,the spring force, the viscous damping force, and the van der Waals force. Equations arealso derived for the energy stored in the actuator. The analytical results emphasize thestored energy at mechanical equilibrium and the voltage versus electrode separationbehavior including the instability. The analytical results predict that the system may notbe a good actuator because the displacement has a very limited stable range, although theactuator would be suitable for bistable applications.The experiment consisted of a fixed flat gold electrode and a movable goldelectrode consisting of a gold sphere several micrometers in diameter mounted on the end of an Atomic Force Microscope (AFM) cantilever, which serves as the spring. Theelectrodes were separated by approximately 100nm of 1mM NaCl aqueous solution.The analytical results were not verified by the experiment. Relative to the analysis,the experiments did not show distinct critical points, and the experiments showed lesselectrode separation for a given applied electric potential. The experiments did showpoints at which the electrode separation versus electric potential rapidly changed slope,which may be instability points.It is suggested that this phenomenon may be due to coalesced gas bubbles onhydrophobic regions of the electrode surfaces, which are not included in the model.Although clean gold surfaces are hydrophilic, gold surfaces may become hydrophobicdue to impurities.