Dynamic electrical response of colloidal micro-spheres in compliant micro-channels from optical tweezersvelocimetry

Abstract

We report the dynamic response of colloidal silica in aqueous electrolytes to oscillatory electric fields at frequencies up to ∼ 50 kHz. Particles were optically trapped at various positions across the gap of straight and crossed parallel-plate micro-channels. Using back-focal-plane interferometry, we measured the apparent electrophoretic mobility in NaCl and CaCl₂ electrolytes over a wide range of salt concentrations. The mobility has a strikingly complex dependence on channel position and forcing frequency that cannot be understood on the basis of standard electrokinetic theory for rigid micro-channels. We ascribe the anomalous dynamics to coupling of electro-osmotic flow and elastic modes of the micro-channel and auxiliary hardware. By integrating into the classical theory a complex-valued channel-compliance parameter—that modulates the phase and amplitude of the dynamic electro-osmotic flow—theoretical interpretation of the frequency-dependent mobility furnishes robust measurements of the intrinsic particle electrophoretic mobility and the upper and lower channel-wall ζ-potentials. Together, the single-particle experiments and accompanying theoretical interpretation highlight—for the first time—how spatially and temporally resolved particle dynamics are exquisitely sensitive to channel compliance. Accordingly, specially designed compliant micro-fluidic channels and flexible tube connections might be tailored for dynamic electrical micro-fluidic diagnostic applications.