Nonreciprocity and odd viscosity in chiral active fluids.

Area of Science:

• Condensed matter physics
• Active matter physics
• Fluid dynamics

Background:

• Odd viscosity couples stress and strain rate dissipationlessly.
• Previously studied in plasmas, superfluids, and quantum-Hall fluids, often obeying Onsager reciprocal relations.
• Its behavior in active materials, particularly chiral active matter, remains less understood.

Purpose of the Study:

• To investigate whether Onsager reciprocal relations hold for odd viscosity in chiral active matter.
• To explore the consequences of breaking these relations on material dynamics.
• To identify conditions for the propagation of odd mechanical waves and mechanical instabilities.

Main Methods:

• Direct coarse-graining of kinetic energy.
• Application of the Poisson-bracket formalism.
• Kinetic theory derivation.

Main Results:

• Nonvanishing angular momentum density in chiral active matter breaks Onsager reciprocal relations.
• This results in a non-Hermitian dynamical matrix and odd viscosity.
• Identified regions for 3D odd mechanical wave propagation and mechanical instability, separated by exceptional points.

Conclusions:

• Angular momentum density is key to breaking Onsager relations in active matter.
• Odd viscosity and non-Hermitian dynamics arise from this breaking.
• The findings suggest a possible nonreciprocal phase transition in chiral active materials.