(96g) 2D Assembly of Colloid Particles On An Electrode within the Ideally Polarizable Region

A low frequency (<1kHz) AC electric field induces particle assembly on an electrode in the plane perpendicular to the electric field's direction. The dispersing electrolyte has been found to have a profound effect on the direction of motion: particles aggregate in solutions of NaHCO₃ or KCl and move apart in solutions of KOH, NaOH, H₂CO₃ or NH₄OH. The direction of lateral motion between pairs correlates with the sign of cosΘ measured on single particles, where Θ is the phase angle between the oscillations in the elevation of the particle and the oscillations in the electric field driving it. Previously, we speculated that an electrochemical reaction (producing faradaic current) occurring at low-frequencies was the reason for the apparent electrolyte dependence. Using a new tool, called the Electrochemical Total Internal Reflection Microscope (ETIRM), we identify regions where the working electrode is ideally polarizable (no possibility of faradaic current). We present data from single and multi-particle experiments with and without the possibility of faradaic current that suggest faradaically coupled electroosmosis plays little to no role in the electrolyte dependent assembly of particles in low frequency AC fields. An insight into the dynamics of particle motion at low frequencies leads to a new understanding of how Θ arises.