Nonlinear, electrocatalytic swimming in the presence of salt

TL;DR

This paper models nonlinear electrocatalytic swimming of bimetallic particles in salt solutions, revealing how ion fluxes and surface layers influence propulsion speed, with results aligning with experimental observations.

Contribution

It introduces a coupled transport model incorporating a Stern layer and electrochemical reactions to predict swimming speeds of catalytic particles in salt solutions.

Findings

Swimming speed increases linearly with hydrogen peroxide concentration at low levels.

Ion diffusion effects can cause a concave speed versus concentration relationship.

A compact ion layer on the particle surface reduces reaction rate and swimming speed.

Abstract

A small, bimetallic particle in a hydrogen peroxide solution can propel itself by means of an electrocatalytic reaction. The swimming is driven by a flux of ions around the particle. We model this process for the presence of a monovalent salt, where reaction-driven proton currents induce salt ion currents. A theory for thin diffuse layers is employed, which yields nonlinear, coupled transport equations. The boundary conditions include a compact Stern layer of adsorbed ions. Electrochemical processes on the particle surface are modeled with a first order reaction of the Butler-Volmer type. The equations are solved numerically for the swimming speed. An analytical approximation is derived under the assumption that the decomposition of hydrogen peroxide occurs mainly without inducing an electric current. We find that the swimming speed increases linearly with hydrogen peroxide concentration for small concentrations. The influence of ion diffusion on the reaction rate can lead to a concave shape of the function of speed vs. hydrogen peroxide concentration. The compact layer of ions on the particle diminishes the reaction rate and consequently reduces the speed. Our results are consistent with published experimental data.