Intrinsic Free Energy in Active Nematics

TL;DR

This paper shows that activity in active nematic systems can generate an effective free energy, promoting order and concentration variations, as demonstrated through analytical and numerical methods.

Contribution

It introduces the concept of activity-induced free energy in active nematics, linking flow fields to enhanced ordering and concentration changes.

Findings

Activity induces an effective free energy promoting order.

Flow fields enhance ordering fluctuations.

Activity can drive concentration variations.

Abstract

Basing our arguments on the theory of active liquid crystals, we demonstrate both analytically and numerically, that the activity can induce an effective free energy which enhances ordering in extensile systems of active rods and in contractile suspensions of active discs. We argue that this occurs because any ordering fluctuation is enhanced by the flow field it produces. A phase-diagram in the temperature-activity plane compares ordering due to a thermodynamic free energy to that resulting from the activity. We also demonstrate that activity can drive variations in concentration, but for a different physical reason that relies on the separation of hydrodynamic and diffusive timescales.