The Dynamics of a Circular Object Immersed In A Confined Active Suspension

Numerical simulations in two space dimensions are used to examine the dynamics, transport, and equilibrium behaviors of a free-floating circular object immersed in an active suspension within a closed container with fixed walls. The continuum model of Gao et al. (Phys. Rev. Fluids, 2017) is used to represent the suspension, which models non-interacting, immotile, extensor-type microscopic agents that have a direction and strength and an established quasi-nonequilibrium fourth-moment closure model. Such a suspension is well-known to be unstable above a threshold activity level that depends upon the length scale of the confinement. Introducing the free-floating object leads to additional phenomenology. Its presence can effectively confine nearby fluid, which can in turn suppress local suspension activity. However, as a rigid body, the object's motion also tends to correlate strains near its surface, which in turn correlates the activity and its net effect on induced stresses. For different activity levels and geometries, these mechanisms can lead to either an effective attraction toward or repulsion away from fixed no-slip wall boundaries. In addition, a curious propagating behavior is found for some conditions when the object is near a wall boundary, which provides a potential mechanism for long-range transport.