Gyrotactic mechanism induced by fluid inertial torque for settling elongated microswimmers

Abstract

Marine plankton are usually modeled as settling elongated microswimmers. We consider the torque induced by fluid inertia on such swimmers, and we discover that they spontaneously swim in the direction opposite to gravity. We analyze the equilibrium orientation of swimmers in quiescent fluid and the mean orientation in turbulent flows using direct numerical simulations. Similar to well-known gyrotaxis mechanisms, the effect of fluid inertial torque can be quantified by an effective reorientation timescale. We show that the orientation of swimmers strongly depends on the reorientation timescale, and swimmers exhibit strong preferential alignment in an upward direction when the timescale is of the same order of the Kolmogorov timescale. Our findings suggest that the fluid inertial torque is a different mechanism of gyrotaxis that stabilizes the upward orientation of microswimmers such as plankton.