The motion of catalytically active colloids approaching a surface

TL;DR

This study investigates how catalytically active Janus microspheres move near surfaces and in bulk, revealing the influence of hydrodynamics, salt concentration, and electrokinetic effects on their propulsion mechanisms.

Contribution

The paper introduces a method using acoustic tweezers to lift microswimmers from surfaces, enabling the study of their bulk and near-wall motion independently of wall effects.

Findings

Diffusion constants match Faxén's hydrodynamic predictions.

Swimmer speed decreases with increasing salt concentration.

Speed reduction in salt aligns with electrokinetic theory, with some deviations in hydrogen peroxide.

Abstract

Catalytic microswimmers typically swim close to walls due to hydrodynamic and/or phoretic effects. The walls in turn are known to affect their propulsion, making it difficult to single out the contributions that stem from particle-based catalytic propulsion only, thereby preventing an understanding of the propulsion mechanism. Here, we use acoustic tweezers to lift catalytically active Janus spheres away from the wall to study their motion in bulk and when approaching a wall. Mean-squared displacement analysis shows that diffusion constants at different heights match with Fax\'en's prediction for the near-wall hydrodynamic mobility. Both particles close to a substrate and in bulk show a decrease in velocity with increasing salt concentration, suggesting that the dominant factor for the decrease in speed is a reduction of the swimmer-based propulsion. The velocity-height profile follows a hydrodynamic scaling relation as well, implying a coupling between the wall and the swimming speed. The observed speed reduction upon addition of salt matches expectations from a electrokinetic theory, except for experiments in 0.1 wt% hydrogen peroxide in bulk, which could indicate contributions from a different propulsion mechanism. Our results help with the understanding of ionic effects on microswimmers in 3D and point to a coupling between the wall and the particle that affects its self-propulsion speed.