Collective motions of microorganisms in geometric confinement.

This article focuses on the collective motion of microswimmers, modeled as squirmers, in viscoelastic fluids confined within circular chambers at low Reynolds numbers, using a fictitious domain numerical method. It finds that both pusher-type and puller-type squirmers accumulate near the chamber walls, a behavior enhanced by fluid elasticity, which in turn reduces their swimming speeds. At higher concentrations, pullers exhibit competing behaviors of forming multilayer structures near the walls and clustering in the chamber center, while their orientations tend to incline toward the walls, further promoting accumulation. The study also shows that fluid elasticity suppresses fluid velocity fluctuations and swimmer speeds, with implications for understanding microorganism dynamics in confined complex fluids and for designing synthetic microswimmers in biomedical applications.