Cooperation in a fluid swarm of fuel-free micro-swimmers

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.

Activity-induced interactions and cooperation of artificial microswimmers in one-dimensional environments

“…That is, they do not require cooperation, which typically relies on information exchange to enhance the efficiency of their behavior. Even the exciting recently observed corralling of passive particles by swarms of light-driven synthetic swimmers was explained purely by geometric arguments 42 . Other collective effects such as the formation of self-spinning microgears 43 and active colloidal molecules 44 required external fields for their assembly and/or propulsion.…”

Morphological computation and decentralized learning in a swarm of sterically interacting robots

“…Future physical models of a learning swarm should go beyond the mean-field approximation used here and account for the spatial density fluctuations [as is done for describing the motility-induced phase separation observed in active fluids; ( 57 , 83 )]. From that point of view, our work shows that, building upon the recent theoretical progress made in describing the population of active particles, one can envision a rapid development of a statistical mechanics description of smart active matter for future swarm engineering.…”

Light, Matter, Action: Shining Light on Active Matter

“…In the case of active droplets, the Marangoni effect is (self-) induced by the droplet itself or by surrounding droplets. Such gradients of surface energy can be produced optically, e.g., by illuminating the droplet surface and harnessing the thermal or photochemical effects of the light absorbed within the droplet. ,− For example, lipophilic droplets stabilized by photoresponsive surfactants can move in light gradients. Light irradiation induces the dissociation of photoresponsive surfactants combined with a rapid pH change in the surrounding aqueous phase, which results in fast movement of the droplet away from the light source due to a change in surface tension .…”

“…The efficiency of critical demixing varies by 2 orders of magnitude from ≈10 −16 to 10 −14 , which is likely related to the proximity of the initial experimental temperature to the critical demixing temperature. 18,64,167 Thermophoresis in bulk liquid yields comparably low efficiencies of ≈10 −16 to 10 −14 , 63,95,168 while similar active particles confined at liquid−liquid interfaces propelled via the Marangoni effect demonstrate a drastic increase in efficiency of several orders of magnitudes up to ≈10 −11 . 67,169 Even higher efficiencies are reported for particles propelled by self-electrophoresis (≈10 −9 to 10 −8 ), 61,170 or selfdiffusophoresis (≈10 −9 ).…”