The role of fluid flow in the dynamics of active nematic defects

TL;DR

This paper develops a theoretical framework to understand how fluid flow influences the motion and interactions of topological defects in active nematic materials, revealing the roles of local and non-local hydrodynamics.

Contribution

It adapts the Halperin-Mazenko formalism to active nematics with fluid coupling, deriving evolution laws for defects and their density fields, including non-local hydrodynamic effects.

Findings

+1/2 defects are propelled by fluid flow and nematic orientation.

Non-local hydrodynamics are significant in incompressible flows.

Derived continuum equations for defect gas coupled to fluid flow.

Abstract

We adapt the Halperin-Mazenko formalism to analyze two-dimensional active nematics coupled to a generic fluid flow. The governing hydrodynamic equations lead to evolution laws for nematic topological defects and their corresponding density fields. We find that $\pm 1/2$ defects are propelled by the local fluid flow and by the nematic orientation coupled with the flow shear rate. In the overdamped and compressible limit, we recover the previously obtained active self-propulsion of the +1/2 defects. Non-local hydrodynamic effects are primarily significant for incompressible flows, for which it is not possible to eliminate the fluid velocity in favor of the local defect polarization alone. For the case of two defects with opposite charge, the non-local hydrodynamic interaction is mediated by non-reciprocal pressure-gradient forces. Finally, we derive continuum equations for a defect gas coupled to an arbitrary (compressible or incompressible) fluid flow.