Simultaneous Aggregation and Height Bifurcation ofColloidal Particles near Electrodes in Oscillatory Electric Fields.

Micrometer-scale particles suspendedin NaCl solutions aggregatelaterally near the electrode upon application of a low-frequency (∼100Hz) field, but the same particles suspended in NaOH solutions areinstead observed to separate laterally. The underlying mechanism forthe electrolyte dependence remains obscure. Recent work by Woehl etal. (PRX, 2015) revealed that, contrary to previous reports, particlessuspended in NaOH solutions indeed aggregate under some conditionswhile simultaneously exhibiting a distinct bifurcation in averageheight above the electrode. Here we elaborate on this observationby demonstrating the existence of a critical frequency (∼25Hz) below which particles in NaOH aggregate laterally and above whichthey separate. The results indicate that the current demarcation ofelectrolytes as either aggregating or separating is misleading andthat the key role of the electrolyte instead is to set the magnitudeof a critical frequency at which particles transition between thetwo behaviors. [ABSTRACT FROM AUTHOR]