How to define temperature in active systems?

Area of Science:

• Statistical Mechanics
• Soft Matter Physics
• Non-Equilibrium Thermodynamics

Background:

• In equilibrium thermodynamics, different temperature definitions are equivalent.
• Active matter systems are inherently out of equilibrium, posing challenges for temperature measurement.
• Existing methods for defining temperature in active systems yield varying results.

Purpose of the Study:

• To systematically compare various temperature definitions for active systems.
• To identify conditions under which different temperature definitions yield consistent values.
• To discuss the practical implications for measuring temperature in active matter.

Main Methods:

• Theoretical analysis of inertial active Brownian particles.
• Computational simulations of active systems.
• Comparison of kinetic, configurational, effective, and virial-based temperatures.

Main Results:

• Two distinct classes of temperature definitions emerge.
• The first class (kinetic, configurational, velocity moments) shows strong agreement across a wide parameter range.
• The second class (effective, virial, Stokes-Einstein, confinement) also shows internal agreement but differs significantly from the first class.

Conclusions:

• Despite being out of equilibrium, certain temperature definitions in active systems exhibit remarkable convergence.
• The findings provide a framework for selecting appropriate temperature measures in active matter research.
• Understanding these temperature classes is crucial for accurate characterization of active systems.