Interface-mediated spontaneous symmetry breaking and mutual communication between drops containing chemically active particles

Abstract

Symmetry breaking and the emergence of self-organized patterns is the hallmark of complexity. Here, we demonstrate that a sessile drop, containing titania powder particles with negligible self-propulsion, exhibits a transition to collective motion leading to self-organized flow patterns. This phenomenology emerges through a novel mechanism involving the interplay between the chemical activity of the photocatalytic particles, which induces Marangoni stresses at the liquid-liquid interface, and the geometrical confinement provided by the drop. The response of the interface to the chemical activity of the particles is the source of a significantly amplified hydrodynamic flow within the drop, which moves the particles. Furthermore, in ensembles of such active drops long-ranged ordering of the flow patterns within the drops is observed. We show that the ordering is dictated by a chemical communication between drops, i.e., an alignment of the flow patterns is induced by the gradients of the chemicals emanating from the active particles, rather than by hydrodynamic interactions.

Titania-Based Micro/Nanomotors: Design Principles, Biomimetic Collective Behavior, and Applications

“…Localized chemical gradients arising from photochemical reactions on TiO 2 particles confined in a droplet can give rise to considerable Marangoni stresses and flows at the liquid-liquid interface. In addition, in collections of such TiO 2loaded droplets, long-ranged ordering of the flow patterns within the drops was observed [65].…”

The Encoding of Light‐Driven Micro/Nanorobots: from Single to Swarming Systems

“…Recently, Singh et al reported a light‐driven symmetry breaking and the emergence of self‐organized patterns with chemically active particles that exhibit negligible motility by themselves. [ 121 ] As shown in Figure 9d, titania powder particles with negligible self‐propulsion were confined in a sessile water droplet containing hydrogen peroxide, which was further immersed within a thick film of silicone oil. Under the illumination of UV light, the uniformly distributed particles exhibited a transition to collective motion, leading to self‐organized flow patterns.…”

“…The scale bars correspond to 150 μm. Reproduced with permission [121]. Copyright 2020, Nature Publishing Group.…”

Towards an analytical description of active microswimmers in clean and in surfactant-covered drops

“…For instance, a dense aqueous suspension of Bacillus subtilis confined inside a viscous drop self-organizes into a stable spiral vortex surrounded by a counter-rotating boundary layer of motile cells [107,110]. In addition, a sessile drop containing photocatalytic particles exhibits a transition to a collective behavior leading to self-organized flow patterns [111]. Under the effect of an external magnetic field, swimming magnetotactic bacteria confined into water-in-oil drops can self-assemble into a rotary motor that exerts a net torque on the surrounding oil phase [112,113].…”