Chiral active fluids: what can we learn from the total momentum?

TL;DR

This paper explores how chiral active fluids exhibit a fundamental difference between center-of-mass and total momentum due to microscopic rotation, impacting their rheological behavior and constraints on interactions.

Contribution

It demonstrates that total momentum, including spin effects, is essential for understanding chiral active fluids, contrasting with traditional equilibrium fluid descriptions.

Findings

Total momentum differs from CM momentum in chiral active fluids.

Total momentum constrains the form of stress and interactions.

The study relates odd viscosities to microscopic spin and interactions.

Abstract

Chiral active materials are those that break both time-reversal symmetry and parity microscopically, which results in average rotation of the material's complex molecules around their center-of-mass (CM). These materials are far from equilibrium due to their local non-vanishing spin angular momentum. In this paper we show that, unlike passive fluids, the non vanishing spin angular momentum brings about a difference between the CM momentum and the total momentum, which accounts for the momentum of all atoms that compose the complex rotating molecules. This is in stark contrast to equilibrium fluids where the CM stress and the total momentum are essentially equivalent. In fact, we find that generally the CM dynamics are insufficient to describe the dynamics of a chiral active material. The total momentum, other than being experimentally accessible in simple rheological experiments, also imposes another constraint -- its stress must be allowed to be written in a symmetric way. We find that the latter imposes a relation between possible central-force interactions and spin-spin interactions, and constraints the ammount of {\it odd} viscosities in the system to the well-known odd (Hall) viscosity and the odd pressure.