Boundaries Control Collective Dynamics of Inertial Self-Propelled Robots

Simple ingredients, such as well-defined interactions and couplings for the velocity and orientation of

self-propelled objects, are sufficient to produce complex collective behavior in assemblies of such entities.

Here, we use assemblies of rodlike robots made motile through self-vibration. When confined in circular

arenas, dilute assemblies of these rods act as a gas. Increasing the surface fraction leads to a collective

behavior near the boundaries: polar clusters emerge while, in the bulk, gaslike behavior is retained. The

coexistence between a gas and surface clusters is a direct consequence of inertial effects as shown by our

simulations. A theoretical model, based on surface mediated transport accounts for this coexistence and

illustrates the exact role of the boundaries. Our study paves the way towards the control of collective

behavior: By using deformable but free to move arenas, we demonstrate that the surface induced clusters

can lead to directed motion, while the topology of the surface states can be controlled by biasing the

motility of the particles.

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