Is chemical asymmetry necessary for directed motion in catalytic swimmers?

Abstract

In the context of directed motion or self-propelled catalytic swimmers, a central open question is whether Janus-type chemical asymmetry is essential for inducing self-propulsion. However, to tackle this unresolved puzzle, we demonstrate that anisotropic catalytic colloids, possessing chemically homogeneous surface activity but asymmetric geometry, exhibit sustained non-equilibrium propulsion, biasing Brownian dynamics toward directed motion. Self-propelled anisotropic colloids provide a versatile platform for investigating non-equilibrium transport and collective dynamics in active matter systems. Shape anisotropy introduces additional degrees of freedom that strongly influence propulsion mechanisms and self-assembly in bulk suspensions. Here, we report the synthesis of anisotropic platinum-coated polystyrene (PS–Pt) particles with an acorn-like geometry (1 μm) using a temperature-induced deformation approach. We examine the propulsion and collective behavior of these acorn-shaped particles in their monomeric, dimeric, and trimeric forms whose pronounced geometric asymmetry distinguishes them from Janus colloids. Self-propulsion is driven by the asymmetric catalytic decomposition of hydrogen peroxide on the platinum-coated region, resulting in sustained translational motion. The curvature anisotropy of the acorn geometry generates uneven solute gradients and induces motion. Statistical analyses based on Gaussian and non-Gaussian displacement distributions confirm the active nature of the observed transport and elucidate the flow behavior of both individual particles and self-assembled structures.