Colloidal band assembly: Effect of streamwise position*

Various studies have shown that colloidal particles in a flowing dilute suspension (volume fractions < 0.4%) are subject to wall-normal “lift” forces, which are usually repulsive.  In combined Poiseuille and electroosmotic “counterflow,” however, the particles experience an attractive lift force instead.  In this flow, the particles become concentrated near the wall and assemble into structures—“bands”—above a minimum electric field magnitude |E_min|.  Recently, we have observed “partial banding,” where these structures only exist near the downstream end of the channel, suggesting that |E_min| depends on the streamwise location x in the channel.  Revised “banding maps” that give |E_min| as a function of x for different near-wall (essentially constant) shear rates are presented.  These observations also suggest that banding occurs once the near-wall particle concentration exceeds a minimum value—and the particle concentration increases with x because the particles are subject to the attractive lift force over a longer time.  Evanescent-wave visualization is used to estimate the near-wall particle concentration as a function of time at different channel positions x at electric fields below |E_min| and above |E_min| before the bands form.