Aggregate Suppression by Active Reorientation in a Benchtop Active Granular Experiment Powered by Toy Vibrobots

Motility-induced phase separation (MIPS) describes the tendency for self-propelled particles to form coexisting dense aggregates and diffuse, gas-like regions at sufficiently high density. Here we show, by way of a benchtop active granular system and numerical simulations, that this behavior can be suppressed with the introduction of an internal rotational degree of freedom. Our grains are circular aluminum cups made active when placed upside down over toy vibrobots. Because the vibrobot is not constrained to a particular orientation inside the cup, when one of these active particles collides with an obstacle (another particle, for example), the robot actively reorients until the particle can move away. This mechanism stands in contrast to an active brownian system in which the particle’s escape depends on rotational diffusion, which is much slower. Thus, no aggregates form in our system, even as the density approaches the maximum packing fraction for disks in two dimensions. We explore the suppression of MIPS and other emergent dynamics in our benchtop experiment as well as in numerical simulations, and draw parallels to real macroscopic systems of motile agents capable of active reorientation.