(620e) Simultaneous Aggregation and Height Bifurcation of Colloidal Particles Near Electrodes in Oscillatory Electric Fields

Micrometer-scale particles suspended in aqueous NaCl solutions aggregate laterally near the electrode upon application of a low-frequency (â?¼100 Hz) field, but the same particles suspended in NaOH solutions are instead observed to separate laterally. The underlying mechanism for the electrolyte dependence remains obscure. Recent work by Woehl et al. (PRX, 2015) revealed that, contrary to previous reports, particles suspended in NaOH solutions indeed aggregate under some conditions while simultaneously exhibiting a distinct bifurcation in average height above the electrode. Here we elaborate on this observation by demonstrating the existence of a critical frequency (â?¼25 Hz) below which particles in NaOH aggregate laterally and above which they separate. Furthermore, we establish that the height of the upper layer of particles scales as the inverse square root of the applied frequency (h~Ï?^-1/2), with particles reaching stable heights as high as 30 μm off the electrode at sufficiently low frequencies. The results indicate that the current demarcation of electrolytes as either aggregating or separating is misleading, and that the key role of the electrolyte instead is to set the magnitude of a critical frequency at which particles transition between the two behaviors.