Abstract
While motile bacteria display rich dynamics in dense colonies, the phoretic nature of artificial micro-swimmers restricts their activity when crowded. Here we introduce a new class of synthetic micro-swimmers that are driven solely by light. By coupling a light absorbing particle to a fluid droplet we produce a colloidal chimera that transforms optical power into propulsive thermo-capillary action. The swimmers’ internal drive allows them to operate for a long duration (days) and remain active when crowded, forming 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 to 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.
Highlights
While motile bacteria display rich dynamics in dense colonies, the phoretic nature of artificial micro-swimmers restricts their activity when crowded
We find drastically different collective dynamics of the active particles when their area fraction, φA, is increased, transitioning from quickly dispersing when diluting; to forming transient, aligned aggregates at the intermediate concentration (0.14 < φA < 0.28, Fig. 1d–g, and Supplementary Video 4); at high concentrations, swimmers crowd in a dense, active colony with internal flows (0.48 ≤ φA, Fig. 1h, i, and Supplementary Video 5)
When the initial concentration is less than dense packing, we identify three characteristic behaviours: active particles quickly disperse at low particle area fraction; a long-lived dense active state is formed at higher concentrations; at intermediate particle concentrations, the active particles corral the passive particles
Summary
While motile bacteria display rich dynamics in dense colonies, the phoretic nature of artificial micro-swimmers restricts their activity when crowded. The swimmers’ internal drive allows them to operate for a long duration (days) and remain active when crowded, forming a high density fluid phase. We find that above a critical concentration, swimmers form a long lived crowded state that displays internal dynamics. Production of an artificial swarm on the micron-scale faces a serious challenge— while an individual bacterium has an evolutionary-forged internal machinery to produce propulsion, until now, artificial microswimmers relied on the precise chemical composition of their environment to directly fuel their drive[14,15,16,17,18,19,20,21,22,23]. Artificial micro-swimmers compete locally for a finite fuel supply, quenching each other’s activity at their greatest propensity for cooperation
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