Unravel the rotational and translational behavior of a single squirmer in flexible polymer solutions at different Reynolds numbers

Microorganisms thrive in complex environments and their behavior in fluids holds significant importance for various medical and industrial applications. By conducting Lattice Boltzmann simulations, the transport and rotational properties of a generic squirmer are investigated in solutions embedded with flexible polymers at different Reynolds numbers. The interplay of activity and heterogeneously distributed polymers have profound influences on these properties. Remarkable enhancements of up to three orders of magnitude in the rotational motion, along with apparent decays in self-propelling velocities, are observed for squirmers with non-zero active stresses. These extraordinary phenomena stem from the squirmer-polymer mechanical and hydrodynamic interactions. Specifically, polymer wrapping occurs in front of a pusher, while numerous polymers are absorbed in the rear of a puller. Both mechanisms enhance the rotational motion and simultaneously impede translations through forces and torques arising from direct contacts or asymmetric local flows induced by polymers. The source dipole flow fields generated by a neutral swimmer rapidly advect polymers to the rear, leaving no apparent impacts on its rotational and transport properties. The influences of Reynolds number Re and squirmer-polymer boundary conditions (no-slip and repulsive) on the dynamics are addressed. In short, the no-slip boundary condition results in more profound effects on both rotational and translational properties at Re = 0.8. However, at Re = 0.04, the disparity between the two boundary conditions diminishes due to the heightened fluid viscous drag, which impedes direct contacts between squirmers and polymers. Our results reveal the relevance of system heterogeneity and highlight the essential role of squirmer-polymer mechanical and hydrodynamic interactions in shaping the behavior of swimmers in viscoelastic fluids.