The motion of a circular object in a confined active suspension with wall-constrained nematic ordering

The effect of wall-constrained nematic ordering on neutrally buoyant suspended object in an active nematic fluid within a closed circular container is studied with a simulation model. The active nematic fluid is described using the continnum model of Gao et al. (Phys. Rev. Fluids, 2017), which includes a slender-body strain response of rod-like agents, Maier-Saupe steric interaction, and a Bingham closure for fourth-moments of the orientation. Dirichlet boundary condition enforces orientation and alignment of the D field on both the object and container. This study considers apolar extensors. When both the object and the container have radial nematic ordering, above a critical activity, a stable limit-cycle solution is observed in which the object moves around the center in a circular trajectory. Below a critical activity level, the container center becomes a stable fixed point. When the nematic ordering of the container is changed to comet-type, which is similar to the director field of a +0.5 defect, the system shows two stable symmetric fixed points above a critical activity. For the activity just below the critical activity, two stable periodic trajectories appear and they are also symmetric to each other. As the activity decreases further, there exists only one symmetric periodic trajectory. In sum, these results show how wall-constrained nematic ordering causes the object into interesting behavior, potentially useful from the perspective of biological tasks.