Emergent non-reciprocal pair interactions in catalytically propelled active polyhedra

Abstract

Particle shape plays a fundamental role in the dynamics and pair interactions of active particles. Yet, experimental measurements of anisotropic active particles are largely lacking. Here, we introduce an experimental system that allows us to observe and measure the effective pair interactions of self-propelled active colloidal polyhedra. We realize such a system by 3D micro-printing quasi- 2D polyhedral colloids with edge number N= 3,4,5,6 and infinity and induce propulsion by partially coating them with an iridium catalytic cap and dispersing them in hydrazine. Despite their different geometries, all shapes exhibit similar single-particle positional dynamics. Stochastic Force Inference reveals predominantly radial, non-reciprocal pair interactions with similar attractive and repulsive regimes for all particle shapes at center-to center distances of about two particle diameters. A minimal plausible phoretic active-Brownian-particle model with a disc shape reproduces the experimentally observed behavior and identifies the physical origins: concentration-dependent slowing down sets their dominant angular symmetry, while the spatial geometry of the chemical coupling and phoretic attraction recover their relative magnitudes and angular dependence. We find that the shape anisotropy present in experiments only modulates the magnitudes and angular dependence of these interactions, and that these variations become more pronounced the stronger the anisotropy. Our results demonstrate the interplay of chemically mediated interactions and particle geometry-induced effects at intermediate interaction distances.