Gyrotactic tapping of spheroidal motile micro-swimmers

Several meters below the coastal ocean surface, thin layers of concentrated motile phytoplankton are hotspots of ecological activities. Gyrotactic trapping has been proposed as a potential mechanism for layer formation of bottom-heavy swimming cells. Prior research has often simplified these active particles as spheres. To what extent does the shape affect the trapping? In this study, we propose a model that considers the effects of spheroidal shape on the competence between the gravitational torque and the viscous torque. Based on that, we numerically investigate gyrotactic trapping in several flow field environments, like shear flow, Stokes drift, and wave. The orientation and distribution of swimmers are examined. Our findings reveal the importance of spheroidal shape for motile phytoplankton in flow.