Active matter shepherding and clustering in inhomogeneous environments.

Area of Science:

• Physics of complex systems
• Statistical mechanics
• Active matter physics

Background:

• Run-and-tumble particles are a key model for active matter.
• Disordered environments significantly impact particle dynamics.
• Phase separation is a fundamental phenomenon in multi-component systems.

Purpose of the Study:

• To investigate the collective dynamics of mixed active and passive run-and-tumble disks in an inhomogeneous environment with quenched disorder.
• To identify and characterize distinct dynamical phases based on motor force and pinning density.
• To map these phases in a dynamic phase diagram.

Main Methods:

• Simulating a system of active and passive run-and-tumble disks.
• Introducing quenched disorder (pinning) in half of the simulation domain.
• Varying motor force and pinning density to observe phase transitions.
• Analyzing density gradients, species separation, and clustering behavior.

Main Results:

• At high motor forces and pinning densities, a two-step process leads to species phase separation and a more uniform overall density.
• At high pinning densities and low motor forces, a strong density gradient is maintained due to slow dynamics.
• At weaker pinning and large motor forces, a floating clustered state emerges with a uniform time-averaged density.

Conclusions:

• The interplay between active forces and quenched disorder dictates the emergent collective behavior of disk mixtures.
• Distinct dynamical phases, including phase separation and clustering, are observed and can be predicted by motor force and pinning density.
• A dynamic phase diagram effectively illustrates the transitions between these observed behaviors.