Electrokinetically driven reversible self-assembly of colloidal particle bands near the wall

Recent studies in microchannels have shown that the near-wall dynamics of neutrally buoyant dielectric colloidal (radii $a <$ 1 $\mu $m) suspended particles are affected by an electric field of magnitude $E$ applied parallel to the wall. Evanescent-wave particle velocimetry was used to study $a = 245$ nm fluorescent polystyrene particles suspended at volume fractions of $O(10^{-4})$ in combined electroosmotic (EO) and Poiseuille flow of an aqueous electrolyte solution, which is effectively the superposition of simple shear and uniform flows within 0.5 $\mu $m of the wall. In ``counterflow,'' where the EO opposes the shear flow through fused-silica microchannels, at a large enough value of $E$ so that flow reversal occurs in the near-wall region, the particles self-assemble into concentrated bright ``stripes'' along the streamwise direction alternating with dark stripes containing almost no particles with a consistent cross-stream spatial frequency. These stripes are only observed within $\sim$ 1 $\mu $m of the wall, and disappear in the absence of an electric field. These observations suggest the existence of a novel electrokinetic instability, and could lead to new methods for controlled self-assembly of particles into anisotropic colloidal crystals.