The tendency of colloidal particles to form oriented chains in oscillatory (AC) electric fields was noted nearly a century ago (2) and was recognized to result from field-induced dipolar interactions. More recently, AC fields also have been found to organize colloidal spheres into circulating chevron bands oblique to the field (3,7,5,4,6), glassy bands perpendicular to the field (8), and even highly ordered colloidal crystals (9,8,10). Still other patterns form when electric fields are used to drive charged colloid against electrodes' surfaces, with two-dimensional fractal aggregates appeared in monodisperse suspensions (11,12,14,13) and planar superlattices forming in binary colloids (15). More recently, a colloidal model system has been introduced whose phase diagram can be tuned with salt concentration and AC electric field (16).
Even constant (DC) electric fields give rise to complex dynamics. For example, charged spheres driven onto electrodes' surfaces by DC fields self-organize into epitaxial colloidal crystals (17,18,20,19). DC fields also have been observed to induce complex labyrinthine patterns in nonaqueous nanocolloids (21).
Most of these effects have been explained on the basis of electrostatic and hydrodynamic coupling among the spheres mediated by field-induced forces and fluxes of ions. These complex interdependent processes generally are said to result in electrokinetic or electrohydrodynamic forces acting on the particles, with the former typically referring to systems with intrinsic equilibrium surface charges and the latter to intrinsically neutral systems. This synthesis, however, has not provided explanations for phenomena such as the compression of colloidal fluids into three-dimensional crystals by oscillatory electroosmosis (9). Indeed, striking and surprising field-induced phenomena can arise in what might appear to be the simplest systems.
This paper reports the remarkably diverse set of patterns that charged colloidal particles can form as they sediment under gravity in a vertical DC electric field. Preliminary results have been published in Ref. (22). Beautiful, highly organized dynamic patterns with spatial periods between 20 and 200 micrometers form at biases just above the threshold for electrolysis. These are supplanted by macroscopic patterns with spatial periods extending to millimeters at higher biases. We describe how such patterns form through the interplay of electrohydrodynamic coupling driven by reaction-diffusion of ions and the uniform body force provided by gravity. We further show that the microscopic patterns result from many-particle interactions rather than an underlying convective instability of the electrolyte.