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

Microorganisms such as bacteria and algae navigate complex fluids, where their dynamics are vital for medical and industrial applications. However, the influence of the Reynolds number (Re) on the transport and rotational behavior of microswimmers in viscoelastic media remains poorly understood. Here, we investigate these effects for a model squirmer in flexible polymer solutions across a range of Re using Lattice Boltzmann simulations. The interaction between swimmer activity and polymer heterogeneity strongly affects behavior, with rotational enhancement up to 1400-fold and reduced self-propulsion and diffusivity for squirmers. These effects result from hydrodynamic and mechanical interactions: polymers wrap ahead of pushers and accumulate behind pullers, enhancing rotation while hindering translation through forces and torques from direct contacts or asymmetric flows. The influence of Re and squirmer-polymer boundary conditions (no-slip vs. repulsive) is also examined. Notably, no-slip conditions intensify effects above a critical Reynolds number ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>Re</mml:mi></mml:mrow> <mml:mrow><mml:mi>c</mml:mi></mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.2</mml:mn></mml:math> ). Below this value, stronger viscous drag minimizes differences. Our findings emphasize the crucial role of polymer-swimmer interactions in shaping microswimmer behavior in viscoelastic media, informing microrobotic design in complex environments.