Collective effects in confined active Brownian particles.

Area of Science:

• Physics
• Soft Matter Physics
• Statistical Mechanics

Background:

• Active matter systems exhibit complex behaviors due to self-propulsion.
• Confining active particles can lead to emergent collective phenomena.
• Understanding particle interactions and system geometry is crucial for predicting emergent behavior.

Purpose of the Study:

• To investigate the collective motion of active particles in a confined annular domain.
• To identify the transition from disordered to ordered states in this system.
• To characterize the dynamics and correlations in both states.

Main Methods:

• Simulations of a 2D system of active particles in a narrow annulus.
• Introduction of velocity polarization as an order parameter.
• Measurement of spatial velocity correlation function and correlation length.

Main Results:

• A transition from a disordered, stuck state to an ordered state of global collective motion (clockwise/anticlockwise rotation) was observed.
• Velocity polarization effectively measures global alignment.
• In the rotating phase, velocity correlation decays algebraically, dependent on system size.
• In the stuck regime, correlation decays exponentially, dependent on persistence time.

Conclusions:

• Global collective motion arises from the interplay of finite-size, periodicity, and active forces.
• The phenomenon is size-dependent and disappears in infinite systems.
• This suggests the observed collective motion is not a traditional thermodynamic phase transition.