Observations of the initial stages of colloidal band formation*

A number of studies have shown that particles suspended in a conducting fluid near a wall are subject to wall-normal repulsive ``lift'' forces, even in the absence of interparticle interactions, in a flowing suspension. Evanescent-wave visualizations have shown that colloidal particles in a dilute (volume fractions \textless 0.4{\%}) suspension are instead \textit{attracted} to the wall when the suspension is driven through $\sim 30\mbox{\thinspace }\mu \mbox{m}$ deep channels by a pressure gradient and an electric field when the resulting combined Poiseuille and electroosmotic (EO) flow are in opposite direction, $i.e.$, ``counterflow,'' although the particles and channel walls both have negative zeta-potentials. Above a minimum ``threshold'' electric field magnitude $\left| {E_{\mbox{min}} } \right|$, the particles assemble into dense ``bands'' with cross-sectional dimensions of a few $\mu \mbox{m}$ and length comparable to that of the channel ($i.e.$, a few cm). The results suggest that the threshold field $\left| {E_{\mbox{min}} } \right|$ is large enough so that there is a region of ``reverse'' flow, along the direction of the EO flow, near the wall. Visualization of a large segment of the channel (\textgreater 300 hydraulic diameters) at frame rates as great as 1 kHz is used to determine banding maps for a variety of dilute colloidal suspensions and to investigate the initial stages of band formation over a wide range of flow conditions.