Observations of the near-wall accumulation of suspended particles due to shear and electroosmotic flow in opposite directions

On the basis of previous studies, the particles in a dilute (volume fractions φ_∞ < 4 × 10^–3) suspension in combined Poiseuille and electroosmotic “counterflow” at flow Reynolds numbers Re ≤ 1 accumulate, then assemble into structures called “bands,” within ∼6 μm of the channel wall. The experimental studies presented here use a small fraction of tracer particles labeled with a different fluorophore from the majority “bulk” particles to visualize the dynamics of individual particles in a φ_∞ = 1.7 × 10^–3 suspension. The results at two different near-wall shear rates and three electric field magnitudes E show that the near-wall particles are concentrated about 150-fold when the bands start to form, and are then concentrated about 200-fold to a maximum near-wall volume fraction of ∼0.34. The growth in the near-wall particles during this accumulation stage appears to be exponential. This near-wall particle accumulation is presumably driven by a wall-normal “lift” force. The observations of how the particles accumulate near the wall are compared with recent analyses that predict that suspended particles subject to shear flow and a dc electric field at small particle Reynolds numbers experience such a lift force. A simple model that assumes that the particles are subject to this lift force and Stokes drag suggests that the force driving particles toward the wall, of O(10^–17 N), is consistent with the time scales for particle accumulation observed in the experiments.