Phase coexistence in a monolayer of active particles induced by Marangoni flows

TL;DR

This paper models how active particles at a fluid interface generate Marangoni flows that lead to phase coexistence and stratified structures, revealing collective effects that influence observable spatial arrangements.

Contribution

It introduces a mean-field model showing how activity-induced Marangoni flows cause phase coexistence and stratification in active particle monolayers at fluid interfaces.

Findings

Development of onion-like density distributions.

Observation of phase transitions from liquid to hexatic and solid to hexatic.

Marangoni flows significantly influence spatial structures despite weak activity detection.

Abstract

Thermally or chemically active colloids generate thermodynamic gradients in the solution in which they are immersed and thereby induce hydrodynamic flows that affect their dynamical evolution. Here we study a mean-field model for the many-body dynamics of a monolayer of active particles located at a fluid-fluid interface. In this case, the activity of the particles creates long-ranged Marangoni flows due to the response of the interface, which compete with the direct interaction between the particles. For the most interesting case of a $r^{-3}$ soft repulsion that models the electrostatic or magnetic interparticle forces, we show that an "onion-like" density distribution will develop within the monolayer. For a sufficiently large average density, two-dimensional phase transitions (freezing from liquid to hexatic, and melting from solid to hexatic) should be observable in a radially stratified structure. Furthermore, the analysis allows us to conclude that, while the activity may be too weak to allow direct detection of such induced Marangoni flows, it is relevant as a collective effect in the emergence of the experimentally observable spatial structure of phase coexistences noted above. Finally, the relevance of these results for potential experimental realizations is critically discussed.