Optical tweezers as a force sensor for separating dielectrophoresis and AC electroosmosis forces

Forces experienced by colloidal particles in an AC electric field such as dielectrophoresis (DEP) and AC electro-osmosis (ACEO) have been widely investigated for their application in microfluidic devices. In order to provide a more complete theoretical basis for such AC electrokinetic mechanisms, we propose a method to quantify the two forces upon one individual particle using optical tweezers as a force transducer and lock-in phase sensitive detection technique to allow high selectivity. Using this method, we isolated the ACEO force from the DEP force for charged polystyrene sphere in deionized (DI) water. ACEO free DEP crossover frequencies and a comprehensive 2D-mapping of the frequency dependent ACEO forces are presented in this paper.