Diffusiophoretic Self-Propulsion for Partially Catalytic Spherical Colloids

TL;DR

This paper presents a theoretical analysis of self-propulsion in partially coated colloidal spheres, revealing how surface interactions and catalytic coverage influence velocity and direction, with implications for experimental interpretation.

Contribution

It introduces a continuum multi-component model and derives a slip-layer approximation to analyze how surface interactions affect particle propulsion.

Findings

Velocity is highly sensitive to molecule-surface interactions.

Asymmetry in catalytic coating affects propulsion direction.

Velocity can reverse with varying catalytic coverage.

Abstract

Colloidal spheres with a partial platinum surface coating perform auto-phoretic motion when suspended in hydrogen peroxide solution. We present a theoretical analysis of the self-propulsion velocity of these particles using a continuum multi-component, self-diffusiophoretic model. With this model as a basis, we show how the slip-layer approximation can be derived and in which limits it holds. First, we consider the differences between the full multi-component model and the slip-layer approximation. Then the slip model is used to demonstrate and explore the sensitive nature of the particle's velocity on the details of the molecule-surface interaction. We find a strong asymmetry in the dependence of the colloid's velocity as a function of the level of catalytic coating, when there is a different interaction between the solute and solvent molecules and the inert and catalytic part of the colloid, respectively. The direction of motion can even be reversed by varying the level of the catalytic coating. Finally, we investigate the robustness of these results with respect to variations in the reaction rate near the edge between the catalytic and inert parts of the particle. Our results are of significant interest to the interpretation of experimental results on the motion of self-propelled particles.