Cluster phases and bubbly phase separation in active fluids: Reversal of the Ostwald process

TL;DR

This paper demonstrates that active matter systems can naturally exhibit microphase separation and phase behaviors that differ from classical Ostwald processes, through a coarse-grained field theory incorporating local active currents.

Contribution

It extends the $$ field theory to include local active currents, predicting transitions between bulk and microphase separation in active fluids without system-specific assumptions.

Findings

Active matter can exhibit microphase separation and bubble states.

Active currents can reverse the classical Ostwald process.

Transitions between phase regimes are predicted by the extended theory.

Abstract

It is known that purely repulsive self-propelled colloids can undergo bulk liquid-vapor phase separation. In experiments and large scale simulations, however, more complex steady states are also seen, comprising a dynamic population of dense clusters in a sea of vapor, or dilute bubbles in a liquid. Here we show that these microphase-separated states should emerge generically in active matter, without any need to invoke system-specific details. We give a coarse-grained description of them, and predict transitions between regimes of bulk phase separation and microphase separation. We achieve these results by extending the $\phi^4$ field theory of passive phase separation to allow for all local currents that break detailed balance at leading order in the gradient expansion. These local active currents, whose form we show to emerge from coarse-graining of microscopic models, include a mixture of irrotational and rotational contributions, and can be viewed as arising from an effective nonlocal chemical potential. Such contributions influence, and in some parameter ranges reverse, the classical Ostwald process that would normally drive bulk phase separation to completion.