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

• Published in: Electrophoresis Vol. 42; no. 21-22; pp. 2215 - 2222
• Main Authors: Yee, Andrew J., Yoda, Minami
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.11.2021
• Subjects: Accumulation; Banded structure; Counterflow; electric field; Electric fields; Electroosmosis; electrophoresis; Fluid dynamics; Fluid flow; fluorescent dyes; Lift forces; Particle assembly; Particle electrophoresis; Particle manipulation; Reynolds number; Shear flow; Suspensions; Tracer particles; Particle assembly; Lift forces; Particle electrophoresis; Particle manipulation
• ISSN: 0173-0835; 1522-2683; 1522-2683
• DOI: 10.1002/elps.202100151

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.