Self-regulation in Self-Propelled Nematic Fluids

TL;DR

This paper develops a hydrodynamic theory for self-propelled nematic fluids, revealing how active currents and curvature effects lead to instabilities and local polar order, explaining phenomena observed in simulations.

Contribution

It introduces a novel hydrodynamic framework for self-propelled nematic fluids, highlighting unique self-regulation mechanisms and the emergence of local polar order.

Findings

Active currents destabilize the uniform nematic state.

Curvature-driven currents cause instabilities deep in nematic phase.

Nematic order induces local polar order, promoting density fluctuations.

Abstract

We consider the hydrodynamic theory of an active fluid of self-propelled particles with nematic aligning interactions. This class of materials has polar symmetry at the microscopic level, but forms macrostates of nematic symmetry. We highlight three key features of the dynamics. First, as in polar active fluids, the control parameter for the order-disorder transition, namely the density, is dynamically convected by active currents, resulting in a generic, model independent dynamical self-regulation that destabilizes the uniform nematic state near the mean-field transition. Secondly, curvature driven currents render the system unstable deep in the nematic state, as found previously. Finally, and unique to self-propelled nematics, nematic order induces local polar order that in turn leads to the growth of density fluctuations. We propose this as a possible mechanism for the smectic order of polar clusters seen in numerical simulations.