Three-Dimensional Accumulation of Active Colloids Under AC Electric Field

Understanding how electrokinetically driven active particles migrate and self-organize in three dimensions is a key to controlling transport phenomenon in microfluidic colloidal system. In this study the three-dimensional accumulation behavior of density-matched Janus particles under AC electric fields was examined in a confined microfluidic chamber of 120 µm height. Each particle consisted of a polystyrene core with hemispherical coatings of 15 nm titanium and 15 nm gold. Electrokinetically driven active colloids are advantageous since the metallodielectric interface alignment with the electric field prevents out of plane motion and accumulation at the surface. Under applied AC fields, the particles exhibited asymmetric accumulation, with a pronounced preference toward the upper boundary layer of the chamber. Saturation was defined as the point at which the bulk particle concentration reached a low steady state, indicating near-complete depletion from the interior. The time required to reach saturation decreased with increasing field strength, suggesting that stronger electric fields enhance the kinetics of particle migration. The observed top-biased accumulation likely arises from the coupled influence of gravitational torque and orientation-dependent propulsion, which align the metallic hemisphere downward and promote upward migration. An open question remains regarding how these competing mechanisms field-driven alignment, hydrodynamic interactions, and gravitational torque collectively determine the onset and extent of this asymmetric accumulation.