Fake μs: A cautionary tail of shear-thinning locomotion

Propulsion through fluids is a key component in the life cycle of many microbes, be it in development, infection, or simply finding nutrients. In systems of biomedical relevance, this propulsion is often through polymer suspensions that endow the fluid with complex non-Newtonian properties, such as shear-thinning and viscoelastic behavior. Due to the complexity of three-dimensional (3D) non-Newtonian modeling, two-dimensional (2D) undulatory propulsion has recently been extensively studied as a means of garnering physical intuition for these systems. However, while streamlines, swimming speeds, and swimmer trajectories are strikingly similar in 2D and 3D Newtonian calculations, behavior in non-Newtonian fluids depends upon flow derivatives, such as the shear rate, which are radically different. Taking shear thinning as an example rheology, prevalent in biological fluids such as physiological mucus, this Rapid Communication demonstrates how failing to account for this difference can misguide our understanding of 3D non-Newtonian swimming.