Title: Towards a continuum mechanics for (dry) active particles
Wen Yan, Simons Foundation

Abstract: Active particles swim by converting some energy into self-propulsion. Collectively, they form a category of material called active matter. Non-equilibrium thermodynamic models are successful but are not sufficient when the detailed dynamics, structure and deformation are of interest, especially when external perturbations are applied, for example, in sedimentation or micro-rheology problems. A continuum mechanical theory is proposed based on the minimal Active Brownian Particles (ABP) model. On the macroscopic length scale, the surface force is found to be the ‘swim stress’, and the body force includes the average ‘swim force’ as an internal contribution. On the microscopic scale, behaviors of active particles are analyzed as a micromechanical basis for the continuum mechanics. The continuum mechanical theory is shown to accurately describe the behaviors of chemically active Janus particles. Particle clustering is explained with a linear stability analysis, and the steady state is explained with a sedimentation-like mechanical force balance.