Flow structure around a microswimmer at fluid–fluid interface

Abstract

Living organisms such as bacteria and algae often form biofilms at air-liquid and/or liquid-liquid interfaces. Therefore, it is important to understand the hydrodynamic interaction between the fluid-fluid interface and microorganisms. In the present study, a squirmer model is employed to study the flow field structures around a symmetrically trapped spherical microswimmer at an interface separating two fluids with viscosity contrast. In these simulations, Reynolds (Re) and Capillary (Ca) numbers are very small, and hence the contribution of inertia and interface deformations are neglected. The squirmer model is validated against the analytical solution obtained by considering a source dipole. It is observed that the flow structure and vorticity distribution around the microswimmers are strongly influenced by the squirmer parameter (β) and viscosity contrast (λ). Furthermore, the interplay between force-dipole and source-dipole along with viscosity contrast leads to a range of flow structures such as symmetric four-lobe and asymmetric quadrupolar flow fields. This flow structure asymmetry around the swimmer is quantified with different steady state orientations (Φ). The effect of flow profile on the passive tracer particle transport is explored in terms of trajectories. Specifically, larger values of Φ result in more profound curly trajectories.