Swim pressure: stress generation in active matter

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Statistical Mechanics

Background:

• Active matter systems, unlike passive ones, exhibit unique behaviors due to self-propelled constituents.
• The concept of 'swim pressure' has been proposed as a key characteristic of these systems, stemming from the tendency of active elements to move and explore space.

Purpose of the Study:

• To elucidate the micromechanical underpinnings of swim pressure in active matter.
• To investigate the role of swim pressure in phenomena such as self-assembly and phase separation.
• To develop a new theoretical framework for understanding active soft matter behavior.

Main Methods:

• Theoretical analysis of swim stress at the micromechanical level.
• Development of a nonequilibrium equation of state for active matter.
• Brownian dynamics simulations to validate theoretical predictions.

Main Results:

• A novel contribution to the understanding of pressure (or stress) in suspensions of self-propelled bodies is presented.
• The swim pressure is identified as an entirely athermal phenomenon arising from confinement.
• Active matter exhibits pressure-volume phase diagrams analogous to equilibrium gas-liquid coexistence, characterized by a van der Waals loop.

Conclusions:

• The new perspective on swim stress provides a valuable tool for analyzing and manipulating diverse active soft matter systems.
• This framework applies to systems ranging from biological entities like swimming bacteria to engineered systems such as catalytic nanobots and molecular motors.
• The study deepens our comprehension of nonequilibrium physics and its implications for self-organization in active materials.