Limiting propulsion of ionic microswimmers

Abstract

Catalytic Janus swimmers self-propel in electrolyte solutions, thanks to inhomogeneous release of active ions from their surface. Here, we consider the experimentally relevant cases of particles, which eject only one type of ions (referred to as swimmers of type I) or equal fluxes of cations and anions (termed swimmers of type II). Our main focus is on the impact of passive ions of the added salt on the self-propulsion. In the limit of a thin electrostatic diffuse layer, we derive a nonlinear outer solution for the electric field and concentrations of active and passive ionic species. We show that both the maximum ion flux and propulsion velocity are constrained for swimmers of type I, but they remain unaffected for particles of type II.