Reversible and dissipative macroscopic contributions to the stress tensor: active or passive?

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Non-equilibrium Thermodynamics

Background:

• Dynamic contributions to the macroscopic stress tensor are crucial in bio-inspired active systems.
• A key focus is the coupling between the stress tensor and the order parameter in active systems.

Purpose of the Study:

• To analyze reversible and irreversible dynamic contributions to the stress tensor in diverse passive and active macroscopic systems.
• To investigate the relationship between system activity and observed stress tensor dynamics.

Main Methods:

• Analysis of systems with various orders (tetrahedral/octupolar, polar, non-polar nematic/smectic).
• Examination of active fluids with dynamic preferred directions.
• Symmetry analysis of macroscopic variables and cross-coupling terms.

Main Results:

• Activity (active vs. passive) cannot be determined solely from symmetry or cross-coupling structure.
• Distinguishing active from passive systems depends on whether the variables driving cross-couplings are externally driven.
• Simplified active system models neglecting counter terms violate linear irreversible thermodynamics.

Conclusions:

• The distinction between active and passive systems is subtle and depends on the dynamics of underlying variables.
• Neglecting counter terms in active system models leads to unphysical entropy production.
• A rigorous thermodynamic framework is necessary for accurate modeling of active matter.