Dynamics of Chemically Active Suspensions

Chemically active particles may swim by self- diffusiophoresisin a concentration gradient of chemical solutes they created themselves by patterned surface catalytic reactions. Those particles can also interact via normal diffusiophoresisin the same solute concentration field. The interaction can be attractive or repulsive. This 'field-driven' nature of the system makes its dynamics different from a thermodynamic systemand is analyzed with a new simulation method. Simulations show that attractive active particles exhibit coexistence of dense and dilute regions, but it is different from a liquid-gasphase equilibrium. To explain the behavior, a continuum mechanicstheory is developed based on the minimal Active Brownian Particles (ABP) model. In the continuum description, the surface forceis found to be the swim stress, which can be anisotropic. The body forceincludes the average swim force as an internal contribution and an 'activity-gradient' force contribution. Further, behaviors of active matterat the sub-continuum scale are also analyzed. The continuum mechanicstheory is shown to accurately describe the behaviors of chemically active particles. Particle clustering is explained with a linear stability analysis, and the steady state is explained with a sedimentation-like mechanical force balance.