Heterogeneous colloidal assembly: Band formation in a mixture of suspended particles

Evanescent-wave visualizations of the near-wall region in microchannels have shown that colloidal (radius a = O(100 nm)) polystyrene particles in a dilute (bulk volume fractions < 4×10^-3) suspension assemble into “bands” that only exist within a few μm of the wall in Poiseuille and electroosmotic (EO) “counterflow.” The particles are concentrated in a liquid state into high aspect ratio structures with a cross-sectional dimension of a few μm and a length comparable to the channel length of O(1 cm). Recent results demonstrate that heterogeneous assembly is also possible with this approach. A dilute mixture of suspended a ≈ 250 nm particles with nearly a two-fold difference in ζ-potential (labeled with different fluorophores) form bands, and their composition—whether heterogeneous structures that contain both types of particles, or homogeneous structures consisting of only a single type of particle—can be controlled by the magnitude of the dc electric field that drives the EO flow at a given near-wall shear rate. The minimum electric field magnitude for heterogeneous assembly is less than that for either type of particle at its respective concentration in the mixture, suggesting that the different particles interact.