Dennis C. Prieve1, Paul J. Sides, Jeffrey A. Fagan2, and James D. Hoggard3. (1) Dept. of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, (2) Polymers Division, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, MD 20899-8542, (3) Chemical Engineering, Carnegie Mellon University, 5000 Forbes Ave, 3120 Doherty Hall, Pittsburgh, PA 15213
In an effort to understand the mechanism of 2-D aggregation of colloidal particles observed on a/c electrodes, we have used video microscopy to observe x,y motion of isolated pairs of polystyrene latex particles and Total Internal Reflection Microscopy (TIRM) to monitor the elevation z of a single particle very near one of two parallel horizontal electrodes. In a/c electric fields, the direction of motion depends on which electrolyte composes the aqueous solution: isolated pairs of particles aggregate in solutions of NaHCO3 or KCl and move apart in solutions of KOH, NaOH, H2CO3 or NH4OH. TIRM measurements of elevation of single particles in a/c fields reveal a phase shift between the oscillating electric field E(t) and the oscillating elevation z(t): in solutions of NaHCO3 or KCl the phase angle is greater than 90º and in KOH, NaOH, H2CO3 or NH4OH the phase angle is equal to or smaller than 90º. We speculate that the electrolyte dependence is associated with kinetics of electrode reactions.