Slipping and fluidisation in active crystalline rotors

Abstract

In equilibrium, commensurability between sliding surfaces underpins our understanding of nanoscale friction and yielding in crystals. However, these concepts have only recently begun to be imported into the realm of active matter. Here, we develop an experimental platform and a theoretical description for active colloids confined and self-assembled into small crystals. In our experiments with Quincke Rollers, these crystallites form due to cohesive interactions between the particles. We apply a circular confinement which controls the population of the system. We focus on perfect hexagonal crystallites of 61 particles whose dimensions are compatible with the confinement. Competition between solidity and self-propulsion leads to self-shearing and complex flow-inversion behaviour, along with self-sliding states and activity-induced melting. We discover active stick-slip dynamics, which periodically switch between a commensurate static state and an incommensurate self-sliding state characterised by a train of localised defects and describe the steady-state behaviour using a discretised model of active hydrodynamics. We then investigate the intermittent stick-slip dynamics using an extension of the Frenkel–Kontorova (FK) model, a fundamental workhorse of slipping and flow in crystals appropriate to our geometry. Our findings in a colloidal model system point to a wealth of phenomena in active solids as design principles for both assembly and robotics down to the nanoscale.