Wall accumulation of confined active Janus colloids due to effective active diffusivity

TL;DR

This study investigates how electrokinetically-driven Janus colloids confined between walls accumulate at boundaries due to an effective diffusivity, revealing a confinement-controlled timescale and a top-biased accumulation mechanism.

Contribution

It demonstrates a new boundary-accumulation mechanism driven by effective cross-channel diffusivity, distinct from traditional active particle accumulation, supported by experimental measurements and modeling.

Findings

Wall population grows exponentially while bulk depletes.

Data collapse onto a single curve when rescaled by the relaxation rate.

Persistent stochastic turning causes effective cross-channel drift and diffusion.

Abstract

Electrokinetically-driven Janus colloids, e.g., with one metallic and one dielectric hemisphere, confined between parallel walls exhibit a boundary-accumulation mechanism enabled by an effective cross-channel diffusivity which is distinct from wall accumulation of active Brownian or run-andtumble particles. Using density-matched suspensions and three-dimensional confocal imaging, we directly measure the full time-dependent redistribution of particles across the channel under an applied AC electric field. The wall population grows exponentially while the bulk depletes, and data obtained over multiple field strengths collapse onto a single curve when rescaled by the measured relaxation rate, revealing one dominant, confinement-controlled timescale. Propulsion follows the expected induced-charge electrophoretic scaling, with a mean orientation angle lying between 2 degrees and 10 degrees above horizontal, leading to a top-biased accumulation. Comparison with an overdamped Ornstein-Uhlenbeck turning model suggests that persistent stochastic turning about a small out-ofplane angle results in a cross-channel effective drift and diffusion. The drift governs the dominant timescale and the diffusion is strong enough to provide significant accumulation on the bottom wall despite a mean upward orientational bias.