Active spheres induce Marangoni flows that drive collective dynamics

TL;DR

This study demonstrates that chemically active, spherically symmetric particles at a fluid interface can induce collective Marangoni flows, leading to emergent collective dynamics even without individual self-propulsion.

Contribution

It provides experimental evidence that activity-induced Marangoni flows can drive collective behavior in passive-symmetry particles, supported by a theoretical mean field model.

Findings

Collective motion observed with surfactant addition.

Dynamics depend on particle coverage.

Marangoni flows arise from chemical activity.

Abstract

For monolayers of chemically active particles at a fluid interface, collective dynamics are predicted to arise owing to activity-induced Marangoni flow even if the particles are not self-propelled. Here we test this prediction by employing a monolayer of spherically symmetric active TiO_2 particles located at an oil-water interface with or without addition of a non-ionic surfactant. Due to the spherical symmetry, an individual particle does not self-propel. However, the gradients produced by the photochemical fuel degradation give rise to long-ranged Marangoni flows. For the case in which surfactant is added to the system, we indeed observe the emergence of collective motion, with dynamics dependent on the particle coverage of the monolayer. The experimental observations are discussed within the framework of a simple theoretical mean field model.