PNNL

Abstract

Non-reciprocal interactions are common among living organisms, where the symmetry of Newton’s third law of action-reaction appears to be broken. Despite their ubiquity in the self-organization of living matter, the combination of reciprocal and non-reciprocal interactions between synthetic building blocks remains largely unexplored. Here, we form far from equilibrium assemblies of motile active particles and non-motile passive particles by overlapping bulk interactions with directed self-propulsion. We use a dispersion containing isotropic passive and metal-patched active particles as model systems to experimentally interrogate the assembly and reconfiguration processes. Application of an external alternating current (ac) electric field generates concurrent dipolar and induced-charge electrophoretic forces between the particles which we evaluate using high-speed microscopy. Direct measurement of the interaction forces in isolated particle pairs allows us to determine the corresponding degree of reciprocity which is tunable by designing the active particle and its trajectory. While linearly-propelled active particles evade assembly with passive particles, helically propelled active particles form active-passive clusters with dynamic reconfiguration and long-lived metastability. Large active-passive clusters display both programmable stability and tunable reconfigurability by controlling the fraction of active components. The study establishes principles of combining reciprocal and non-reciprocal interactions in active matter as a tool for guided assembly of structures with long-term metastability and non-equilibrium characteristics.