An Apparatus to Determine the Pull-Off Force of a Conducting Microparticle from a Charged Surface

Abstract

A unique experimental apparatus has been developed to determine the pull-off force of a conducting microparticle resting on a conducting surface in the presence of a DC electric field. The apparatus precisely measures the applied electric-field strength at which an individual microparticle of known diameter, which is measured in situ, is removed from the surface. This information is used to determine the adhesion pull-off force required to remove the microparticle from the surface to within an uncertainty of approximately 12% at 95% confidence. In the experiment, the electric field strength between a conducting-surface electrode and a grounded electrode is increased in time using a microcontroller-driven digital-to-analog converter. Both electrodes are semi-transparent, which permits the surface-resident microparticles to be viewed from underneath the surface using microphotography. The microphotography system is operated in conjunction with pixel-intensity, gradient-search software to determine the diameter of each surface-resident microparticle. The apparatus is designed to explore a wide range of operating conditions, including microparticles of diverse composition and sizes, surfaces of differing roughness, and environments of various relative humidity including vacuum. The theory of operation and instrument design are presented in detail. Preliminary results also are given as a proof of concept. © 2007 Taylor & Francis Group, LLC.