Aggregation and Segregation of Confined Active Particles

Abstract: We simulate a model of self-propelled disks with soft repulsive interactions confined to a box in two dimensions. For small rotational diffusion rates, monodisperse disks spontaneously accumulate at the walls. At low densities, interaction forces between particles are strongly inhomogeneous, and a simple model predicts how these inhomogeneities alter the equation of state. At higher densities, collective effects become important. We observe signatures of a jamming transition at a packing fraction $\phi \sim 0.88$, which is also the jamming point for non-active athermal monodisperse disks. At this $\phi$, the system develops a critical finite active speed necessary for wall aggregation. At packing fractions above $\phi \sim 0.6$, the pressure decreases with increasing density, suggesting that strong interactions between particles are affecting the equation of state well below the jamming transition. A mixture of bidisperse disks segregates in the absence of any adhesion, identifying a new mechanism that could contribute to cell sorting in embryonic development.

Aggregation and Segregation of Confined Active Particles

The paper "Aggregation and Segregation of Confined Active Particles" focuses on the behavior of self-propelled particles confined in a two-dimensional box. Active particles, such as biological cells and synthetic microswimmers, possess the ability to inject energy and exhibit complex dynamic behaviors. This study provides insights into how such active particles aggregate and segregate under confinement.

The authors simulate a model of self-propelled disks, analyzing their interactions and movements. A crucial parameter in these simulations is the rotational diffusion rate, which influences how particles accumulate at boundaries. The paper reveals that for small rotational diffusion rates, monodisperse particles (disks of the same size) spontaneously accumulate at the container's walls. Interestingly, pressures within the system decrease with increasing density at certain conditions, suggesting a unique pressure-density relationship distinct from classical systems, which typically show a monotonic increase in pressure with density.

Key numerical observations include the identification of a jamming transition at a packing fraction $\phi \sim 0.88$, similar to the jamming point observed for non-active athermal disks. At this transition, a finite active speed must be reached for wall aggregation to occur. Furthermore, pressure anomalies appear well below the jamming transition, notably above $\phi \sim 0.6$. The study highlights the role of strong particle interactions in affecting the equation of state. These findings point to the complex interplay between particle activity and crowding effects, suggesting potential applications in understanding and designing active matter systems.

Another novel aspect of the study is the behavior of mixtures of bidisperse disks. Segregation occurs without any adhesive interaction, providing a model potentially relevant to understanding cell sorting in embryonic contexts. In these scenarios, the system's geometry and activity appear to drive segregation rather than classical adhesive mechanisms.

The results have notable implications for active matter theories and could guide practical applications where confinement is critical, such as in microfluidic devices or controlled environments mimicking biological systems. On the theoretical front, understanding the dynamics of active particles could elaborate on how pressure and density dictate phase behaviors under non-thermal conditions. Future investigations might leverage these insights to explore more complex geometries and particle shapes, furthering advancements in the study of active materials.

Overall, this study adds significant knowledge to the field of non-equilibrium active matter by demonstrating aggregation patterns, jamming transitions, and segregation mechanisms intrinsic to active particle dynamics.