Shape-dependent direction reversal in anisotropic catalytic microswimmers

Abstract

The propulsion direction of active particles determines their non-equilibrium behavior and has been shown to depend on the propulsion mechanism and environmental conditions. However, whether and how an anisotropic particle shape can influence the propulsion direction is experimentally unexplored. Here, using 3D micro-printed catalytically active particles, we demonstrate that active discs, tori, and bent rods reverse their direction - moving with their inert side forward at low and with their catalytic side forward at high concentrations of hydrogen peroxide- while spheres and side-propelling straight rods do not. Direction reversal robustly occurs for different material compositions and sizes, only changing the hydrogen peroxide concentration at which reversal appears. We find that the propulsion direction originates from an interplay of pH-dependent zeta potential ratio and particle geometry, and is possibly further enhanced by substrate-induced solute confinement. Our findings demonstrate that particle shape affects the motion of catalytically active particles, important for understanding their interactions and collective effects in out-of-equilibrium model systems. The propulsion direction of active particles determines their non-equilibrium behavior, yet the impact of anisotropic shape remains largely unexplored experimentally. Here, the authors demonstrate that shape plays a crucial role in catalytically active particles as it can cause a reversal of the propulsion direction upon small changes in the fuel concentration.