469471 Influence of Electrolyte Concentration on the Aggregation Ofcolloidal Particles Near Electrodes in Oscillatory Fields

Thursday, November 17, 2016: 10:45 AM
Union Square 23 & 24 (Hilton San Francisco Union Square)
Sukhleen Saini, Scott C. Bukosky and William D. Ristenpart, Chemical Engineering, University of California Davis, Davis, CA

Micron-scale particles suspended in various aqueous electrolytes have been widely observed to aggregate near electrodes in response to oscillatory electric fields, a phenomenon believed to result from electrically induced flows around the particles. Previous work has focused on elucidating the effects of the applied field strength, frequency, and electrolyte type on the aggregation rate of particles, with less attention paid to the ionic strength. Here we demonstrate that an applied field causes micron-scale particles in aqueous NaCl to rapidly aggregate over a wide range of ionic strengths, but with significant differences in aggregation morphology. Optical microscopy observations reveal that at higher ionic strengths (∼1 mM) particles arrange as hexagonally closed-packed (HCP) crystals, but at lower ionic strengths (∼0.05 mM) the particles arrange in randomly closed-packed (RCP) structures. We interpret this behavior in terms of two complementary effects: an increased particle diffusivity at lower ionic strengths due to increased particle height over the electrode and the existence of a deep secondary minimum in the particle pair interaction potential at higher ionic strength that traps particles in close proximity to one another. The results suggest that electrically induced crystallization will readily occur only over a narrow range of ionic strengths.

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See more of this Session: Active Colloidal Systems
See more of this Group/Topical: Engineering Sciences and Fundamentals