(531b) Electrokinetic Manipulation of Colloids in Low Frequency Oscillatory Electric Field and pH Gradients

Micron sized colloidal particles are known to assemble into close packed colloidal crystals near charged electrodes in low frequency (< 1 kHz) oscillatory electric fields. This behavior is widely interpreted in terms of contractile electrohydrodynamic (EHD) fluid flows surrounding each colloid. EHD flows are particularly sensitive to colloid surface charge and solution pH: increasing surface charge and/or the presence of cations and anions with disparate mobilities (e.g. NaOH) diminishes EHD flows and colloidal assembly. Prior reports have shown strong experimental correlations between aggregation rate of colloids, solution pH, and particle zeta potential, but the exact mechanism for these trends remains unclear. It is currently thought that particle zeta potential mediates the strength and direction of the dipolar electric field surrounding each colloid, which modulates the strength and direction of the EHD fluid flow.

Here we utilize electrochemical reactions and electroactive molecules to establish pH gradients near electrodes to manipulate the assembly state and levitate colloids in low frequency oscillatory electric fields. An electroactive molecule that releases or consumes protons upon reduction and oxidation, para-benzoquinone (p-BQ), is used to actively modulate the solution pH near each electrode surface. Numerical reaction-diffusion simulations of the electrochemical reduction and oxidation of p-BQ show that pH gradient extend several tens of microns away from the electrode. Our experiments show that micron scale colloids assembled into colloidal crystals near a planar electrode in low frequency oscillatory fields (~500 Hz) burst apart in response to applied DC potentials of ~300 mV that increase the solution pH near the electrode (Figure 1a). Following disassembly, low density polystyrene colloids are observed to move phoretically to the top electrode, while higher density silica particles levitate several microns above the electrode surface. In the absence of p-BQ, colloidal crystals are unaffected by ~300 mV DC potentials (Figure 1b). We interpret disassembly in terms of diminished EHD fluid flow magnitude at high pH and repulsive DC electroosmotic flows between particles (Figure 1c). Levitation is interpreted in terms of a balance between gravitational forces and DC electrophoresis. We found that a pH gradient, and thus a gradient in the electrophoretic force on the colloids, is necessary to establish a stable saddle point in the vertical forces on the colloids. We demonstrate density-based separation of colloids and assembly of bilayer colloidal crystals based on the observed levitation phenomenon. Fundamentally, these experiments provide further experimental support to the hypothesis that pH-dependent zeta potential mediates EHD fluid flow surrounding colloids in low frequency oscillatory electric fields.