Lamellar to micellar phases and beyond: when tactic active systems admit free-energy functionals

TL;DR

This paper demonstrates that certain tactic active particle systems can be described by equilibrium-like fluctuating hydrodynamics, explaining phase behaviors such as lamellar, micellar, and chemotactic collapse phenomena.

Contribution

It establishes a mapping between active particle models and equilibrium colloid systems, revealing new insights into phase separation and pattern formation.

Findings

Active systems can be described by equilibrium-like hydrodynamics.

Taxis can induce lamellar and micellar phases in active matter.

Chemotactic interactions can cause liquid-gas phase separation.

Abstract

We consider microscopic models of active particles whose velocities, rotational diffusivities, and tumbling rates depend on the gradient of a local field, which is either externally imposed or depends on all particle positions. Despite the fundamental differences between active and passive dynamics at the microscopic scale, we show that a large class of such tactic active systems admit fluctuating hydrodynamics equivalent to those of interacting Brownian colloids in equilibrium. We exploit this mapping to show how taxis may lead to the lamellar and micellar phases observed for soft repulsive colloids. In the context of chemotaxis, we show how the competition between chemoattractant and chemorepellent may lead to a bona-fide equilibrium liquid-gas phase separation in which a loss of thermodynamic stability of the fluid signals the onset of a chemotactic collapse.