Hydrodynamic interactions between two microswimmers in viscoelastic fluids

We numerically investigate the hydrodynamic interactions between two microswimmers in viscoelastic fluids at low Reynolds number regimes using the fictitious domain method. In the Newtonian fluid, after contact, pushers rotate toward each other and remain in a "trapped" state for a long time, induced by a counterclockwise viscous torque. We believe it is related to the asymmetrical surrounding vortexes. As the pushers approach, two positive vortexes merge into one, disrupting the vortex balance and inducing a net counterclockwise torque on the swimmers. But in the Giesekus viscoelastic fluids, an clockwise elastic torque modifies the pushers' rotations during early contact. Subsequently, two negative vortexes around the pushers merge, generating a clockwise torque that causes the swimmers to separate. Additionally, a huge elastic stretching effect is observed at the rear of the pullers, which restricts their swimming behavior through elastic force. Post-collision, the pullers rapidly separate with a large scattering angle in both Newtonian and viscoelastic fluids. However, neutral swimmers separate with unchanged orientations, and their trajectories remain consistently aligned across various Weissenberg numbers. Furthermore, the elastic force impedes the relative motions of the swimmers and alters the pressure and viscosity force.