Collective behavior of microswimmers in non-Newtonian fluids

Abstract

This review examines how biological and synthetic microswimmers move through non-Newtonian media, such as reproductive fluids, mucus, biofilms, polymer solutions, and gels. The mechanics of an isolated swimmer in such a medium has received considerable attention, whereas the consequences for interacting swimmers and suspensions are less well established. Here, locomotion at low Reynolds number is treated as a coupled problem involving the swimmer’s length and time scales, the fluid microstructure, and collective motion. Swimmer-generated flow deforms and rearranges the surrounding material; the resulting shear-dependent viscosity, normal stresses, elastic memory, and yielding in turn modify propulsion and hydrodynamic interactions. We evaluate recent breakthroughs across biological and synthetic actuation, from prescribed surface motion and flagellar waves to rotating helices and deformable magnetic swimmers. Finally, we critique theoretical frameworks ranging from reduced particle models to fully coupled fluid-structure interactions (FSI), highlighting how the medium’s constitutive response changes pair interactions, orientational order, clustering, coherent flow, and the rheology of an active suspension.