Boundaries Control Collective Dynamics of Inertial Self-Propelled Robots

Abstract

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.

Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

“…Regarding the first situation, one of the best realization of our model equations (22) for active Langevin dynamics can be found for active granulates [13][14][15][16][17][18][19][20][21][22] . Typically these are hoppers with a Janus-like body or with tilted legs.…”

“…These motions are only virtually damped. Another important realization are granulates made self-propelling on a vibrating plate [13][14][15][16][17][18][19][20][21][22] or equipped with an internal vibration motor 23 where it has been shown that the active Langevin model indeed describes their dynamics well [24][25][26] . Further examples for self-propelled particles with inertia range from mini-robots 27,28 to macroscopic swimmers like beetles flying at water interfaces 29 and whirling fruits self-propelling in air 30 .…”

Magnetotactic bacteria in a droplet self-assemble into a rotary motor

“…When the magnetic field is rapidly reversed, the MTB that are accumulated at the right of the NP and at the left of the SP can suddenly rotate to align along the new magnetic field, without any restriction from the droplet boundary. After this, they cross the droplet roughly along the y-direction, producing a transient convective circulation with positive values of Ω d (corresponding to CW direction) measured just after the magnetic field reversal (see Recently, considerable efforts have been undertaken to harness the microscopic activity of living or synthetic agents like bacteria [21,44], eukaryotic cells [45], Janus colloids [9] or micro-robots [46,47], in order to extract macroscopic work from microscopic mechanical structures. Here, we show a remarkable example of living biological entities self-assembling into a rotary motor actuated by a controlled, external aligning field.…”

Dynamics of a Self-Propelled Particle in a Harmonic Trap

“…As a matter of fact, a harmonic trap is not the only way to probe the self-alignment of the par-ticle: confining the particle with a hard wall of radius R w (in dimensionless units), an orbiting solution exists if τ n < R w , which slides along the wall at a velocity v = 1 − (τ n /R w ) 2 [18]. This suggests that the presence of such a coupling could be investigated in a num-ber of active systems, not only those using self-propelled particles similar to the present hexbugs [8,20], but also those in the colloidal realm, using for instance acoustic traps [6] or simply hard wall circular confinement.…”