  Cooperation is vital for the survival of a swarm$^1$. Large scale cooperation
allows murmuring starlings to outmaneuver preying falcons$^2$, shoaling
sardines to outsmart sea lions$^3$, and homo sapiens to outlive their
Pleistocene peers$^4$. On the micron-scale, bacterial colonies show excellent
resilience thanks to the individuals' ability to cooperate even when densely
packed, mitigating their internal flow pattern to mix nutrients, fence the
immune system, and resist antibiotics$^{5-14}$. Production of an artificial
swarm on the micro-scale faces a serious challenge $\frac{\;\;}{\;\;}$ while an
individual bacterium has an evolutionary-forged internal machinery to produce
propulsion, until now, artificial micro-swimmers relied on the precise chemical
composition of their environment to directly fuel their drive$^{14-23}$. When
crowded, artificial micro-swimmers compete locally for a finite fuel supply,
quenching each other's activity at their greatest propensity for cooperation.
Here we introduce an artificial micro-swimmer that consumes no chemical fuel
and is driven solely by light. We couple a light absorbing particle to a fluid
droplet, forming a colloidal chimera that transforms light energy into
propulsive thermo-capillary action. The swimmers' internal drive allows them to
operate and remain active for a long duration (days) and their effective
repulsive interaction allows for a high density fluid phase. We find that above
a critical concentration, swimmers form a long lived crowded state that
displays internal dynamics. When passive particles are introduced, the dense
swimmer phase can re-arrange and spontaneously corral the passive particles. We
derive a geometrical, depletion-like condition for corralling by identifying
the role the passive particles play in controlling the effective concentration
of the micro-swimmers.
