Threebody correlations and conditional forces in suspensions of active hard disks
Abstract
Selfpropelled Brownian particles show rich outofequilibrium physics, for instance, the motilityinduced phase separation (MIPS). While decades of studying the structure of liquids have established a deep understanding of passive systems, not much is known about correlations in active suspensions. In this work we derive an approximate analytic theory for threebody correlations and forces in systems of active Brownian disks starting from the manybody Smoluchowski equation. We use our theory to predict the conditional forces that act on a tagged particle and their dependence on the propulsion speed of selfpropelled disks. We identify preferred directions of these forces in relation to the direction of propulsion and the positions of the surrounding particles. We further relate our theory to the effective swimming speed of the active disks, which is relevant for the physics of MIPS. To test and validate our theory, we additionally run particleresolved computer simulations, for which we explicitly calculate the threebody forces. In this context, we discuss the modeling of active Brownian swimmers with nearly hard interaction potentials. We find very good agreement between our simulations and numerical solutions of our theory, especially for the nonequilibrium pairdistribution function. For our analytical results, we carefully discuss their range of validity in the context of the different levels of approximation we applied. This discussion allows us to study the individual contribution of particles to threebody forces and to the emerging structure. Thus, our work sheds light on the collective behavior, provides the basis for further studies of correlations in active suspensions, and makes a step towards an emerging liquid state theory.
 Publication:

Physical Review E
 Pub Date:
 January 2018
 DOI:
 10.1103/PhysRevE.97.012606
 arXiv:
 arXiv:1708.01115
 Bibcode:
 2018PhRvE..97a2606H
 Keywords:

 Condensed Matter  Soft Condensed Matter
 EPrint:
 Phys. Rev. E 97, 012606 (2018)