| Fluid motion and colloidal aggregation near electrodes are investigated both theoretically and experimentally. The main goal is to answer the question: why do particles near electrodes migrate towards one another to form aggregates?; Three key results are obtained. First, a scaling analysis is performed of the electrohydrodynamic (EHD) flow induced by a polarized particle near an electrode. The EHD velocity is predicted to increase as the square of the applied field strength and decrease inversely with frequency. A technique is developed for measuring the kinetics of aggregation via optical microscopy at low magnifications, and the experimental measurements obtained are shown to be in accord with the scaling predictions.; Next, an 'exact' analytical solution is derived for the EHD flow around a spherical particle with arbitrary surface conductivity in the limit of infinitesimally thin double layers. A major assumption of the theory is that the electric field strength on the electrode is constant. Numerical calculations confirm that this is a suitable approximation in the limit as the thickness of the polarization layer approaches zero. In steady fields, EHD flow is predicted to be similar in magnitude to electroosmotic flow. Experimental measurements of fluorescent tracer velocities in oscillatory fields corroborate the predicted toroidal streamlines around an individual spherical particle. Treating the surface conductivity as an adjustable parameter yields tracer velocities in excellent agreement with the theory.; Finally, the behavior of binary suspensions is investigated. Depending on the frequency and relative particle concentrations, either triangular or square-packed planar superlattices are assembled in oscillatory fields. Superlattices form at low and high frequencies, but not at intermediate frequencies. The low frequency behavior is explained in terms of a balance between attractive EHD flow and repulsive dipolar interactions, while the high frequency behavior is explained in terms of attractive dipolar interactions. Calculations of the particle interaction energy with a dipolar term are shown to be consistent with the experimental observations. |
