Understanding particle-wall interactions through bulk mobility measurements of electric field-driven active colloids*

Active colloid experiments face significant challenges: theoretical models typically apply to isolated particles in bulk, such as metallodielectric Janus spheres, but sedimentation of the metallic coating leads to measurements near the substrate. This interation affects mobility through localized gradients, electrohydrodynamic flow, and Stokes drag. This paper addresses difficulties of sending electrokinetically driven active colloids in the international space station (ISS), where microgravity aids bulk mobility measurements. Key issues include mitigating bubble formation from dissolved gas and preventing particle adhesion to chamber walls that must remain stable for up to 10 days.

On Earth, bulk mobility measurements are achieved through density matching. Electrokinetically driven colloids align their metallodielectric interface with the electric field, preventing out-of-plane motion and surface accumulation. Mobility reduction is quantified based on preximity to channel walls across frequencies (500Hz - 1 MHz), relevent to both induced-charge electrophoresis(ICEP) and self-dielectrophoresis (sDEP). Surprisingly, wall effects can persist at multiple radii into the channel, contrary to theoretical modells. These findings provide new insights into ICEP and sDEP mechanisms, opening new directions for active colloids experiments and model validation.