Colloidal band assembly from different suspended particles

Particle visualizations, mainly based on evanescent-wave illumination, have shown that sulfate- and carboxylate-terminated polystyrene particles in a dilute suspension flowing through a microchannel assemble into near-wall bands when the flow is driven by a dc electric field and a pressure gradient along the channel axis applied in opposite directions. In these bands, the colloidal particles (of radius $a\approx 250-500$ nm) are concentrated in a liquid state in regions with a cross-sectional dimension of a few $\mu \mbox{m}$ and an axial extent comparable to the channel length of $O$(1 cm). In many cases, the particles first assemble into many closely spaced, fairly unstable bands before achieving a fairly stable ``steady-state'' configuration with fewer bands. Results at a given channel location for the timescales of particle assembly as well as the characteristics of the bands are presented for a range of particle and suspension properties including the particle volume fraction $\varphi_{\infty } ,\,\;a\mbox{,\thinspace }$ and particle zeta-potential $\zeta_{\mbox{p}} $ and flow properties such as the electric field magnitude $\left| E \right|$ and near-wall shear rate $\dot{{\gamma }}$. The band characteristics appear to scale with the electric field ``offset,'' or $\left| E \right|-\left| {E_{\mbox{min}} } \right|$ where $\left| {E_{\mbox{min}} } \right|$ is the minimum electric field magnitude at a given $\dot{{\gamma }}$ required for band formation.