Orientation, displacement, and accumulation of anisotropic microswimmers under surface gravity waves

A three-dimensional numerical model is developed to investigate the instantaneous and time-averaged dynamics of anisotropic microswimmers under surface gravity waves over a range of particle shapes (characterized by eccentricity ε ϵ [-1 +1]) and particle motilities (vs). The particles are idealized as finite-sized prolate/oblate spheroids with small Stokes numbers and particle Reynolds numbers. We find that aspect ratio-dependent preferential alignment of the swimmers closely follows that of an inertial passive particle. However, there is a symmetry break in preferential orientation depending on whether the initial orientation is beyond some critical limit, and this is particularly consequential for microswimmers. Under the influence of surface gravity waves, negatively buoyant swimmers’ dynamics unfold the existence of regime changes from the domain wherein swimmers settle down despite their upward motility to the parameter domain where swimmers have vertically upward displacement defeating its negative buoyancy. When multiple swimmers are allowed to evolve, they tend to accumulate, which is primarily influenced by the velocity shear in the wave field. This result reveals that the thin layers of phytoplankton, as ecological activity hotspots in coastal oceans, can also be the result of swimmer-wave interaction.