Instabilities driven by diffusio-phoretic flow on catalytic surfaces

TL;DR

This paper investigates how diffusiophoretic flows cause instabilities on catalytic surfaces, revealing critical Peclet numbers for flow transitions and explaining chaotic particle motion observed experimentally.

Contribution

It provides a combined theoretical and numerical analysis of flow instabilities driven by diffusiophoresis, identifying key parameters and flow regimes, including non-linear effects and chaotic behavior.

Findings

Flow becomes unstable for Peclet number > 8pi.

Multiple concentration plumes form and merge at high Peclet numbers.

Chaotic motion of catalytic particles occurs beyond a critical Peclet number.

Abstract

We theoretically and numerically investigate the instabilities driven by diffusiophoretic flow, caused by a solutal concentration gradient along a reacting surface. The important control parameter is the Peclet number Pe, which quantifies the ratio of the solutal advection rate to the diffusion rate. First, we study the diffusiophoretic flow on a catalytic plane in two dimensions. From a linear stability analysis, we obtain that for Pe larger than 8pi, mass transport by convection overtakes that by diffusion, and a symmetry-breaking mode arises, which is consistent with numerical results. For even larger Pe, non-linear terms become important. For Pe larger than 16pi, multiple concentration plumes are emitted from the catalytic plane, which eventually merges into a single larger one. When Pe is even larger, there are continuous emissions and merging events of the concentration plumes. This newly-found flow state reflects the non-linear saturation of the system. The critical Peclet number for the transition to this state depends on Schmidt number Sc. In the second part of the paper, we conduct three-dimensional simulations for spherical catalytic particles, and beyond a critical Peclet number again find continuous plume emission and plume merging, now leading to chaotic motion of the phoretic particle. Our results thus help to understand the experimentally observed chaotic motion of catalytic particles in the high Pe regime.