A Mechanical Origin of Cooperative Transport

Cooperative transport is a striking phenomenon where multiple agents join forces to transit a payload too heavy for the individual. While social animals such as ants are routinely observed to coordinate transport at scale [1–5], reproducing the effect in artificial swarms remains challenging, as it requires synchronization in a noisy many-body system [6–27]. Here we show that cooperative transport spontaneously emerges in swarms of stochastic self-propelled agents, without requiring any form of sensing, feedback, or control. We show that a minute modification to the mechanical design of the individual agent dramatically changes its alignment response to an external force. We experimentally demonstrate that with the proper design, a swarm of active particles spontaneously cooperates in the directional transport of larger objects. Surprisingly, transport increases with increasing payload size. A mechanical, coarse-grained description reveals that force-alignment is intrinsic and captured by a signed, charge-like parameter with units of curvature. Numerical simulations of swarms of active particles with a negative active charge corroborate the experimental findings. We analytically derive a geometrical criterion for cooperative transport which results from a bifurcation in a non-linear dynamical system. Our findings generalize existing models of active particles [28–37], offer new design rules for distributed robotic systems, and shed light on cooperative transport in natural swarms.