Aggregation and segregation of confined active particles

Area of Science:

• Physics
• Soft Matter Physics
• Computational Physics

Background:

• Self-propelled particles exhibit complex behaviors not seen in equilibrium systems.
• Understanding particle interactions and collective effects is crucial for predicting system dynamics.
• Active matter models are relevant to biological systems, such as cell motility and sorting.

Purpose of the Study:

• To investigate the collective behavior and phase transitions of self-propelled disks with soft repulsive interactions in a 2D confined system.
• To explore the influence of particle density, activity, and size distribution on system properties like the equation of state and jamming.
• To identify novel mechanisms for particle segregation and their potential biological relevance.

Main Methods:

• Two-dimensional simulations of self-propelled disks with soft repulsive interactions.
• Analysis of particle accumulation at walls, interaction forces, and equation of state.
• Investigation of jamming transition and critical active speed requirements.
• Simulation of bidisperse disk mixtures to study segregation phenomena.

Main Results:

• Monodisperse disks accumulate at walls at low rotational diffusion rates.
• Strongly inhomogeneous interaction forces at low densities alter the equation of state.
• Signatures of a jamming transition observed at packing fraction ϕ ∼ 0.88, consistent with non-active disks.
• A critical active speed is required for wall aggregation at the jamming point.
• Pressure decreases with increasing density above ϕ ∼ 0.6, indicating deviations from ideal gas behavior.
• Bidisperse disk mixtures segregate spontaneously, even without adhesion.

Conclusions:

• The jamming transition in active disk systems shares similarities with passive systems, but requires a critical activity level for wall aggregation.
• Particle interactions significantly influence the equation of state well below the jamming density.
• Spontaneous segregation of bidisperse disks presents a new mechanism relevant to biological self-organization, such as cell sorting in embryonic development.