Microscopic swimming in biological fluids

Fluid interactions are ubiquitous in the natural world; all organisms must find strategies to generate, utilise or resist flow in order to be successful. A process fundamental to all life on earth is reproduction, which in many cases entails the swimming of sperm cells. Cell swimming arises from coupled interactions between physical and biological processes. We will focus on the effects of changing fluid rheology on microscopic swimmers, with a particular application to the study of internal mammalian fertilisation. To reach the egg, mammalian sperm must navigate the convoluted geometry of the female reproductive tract, actively bending their flagella in order to propel themselves through cervical mucus: a suspension of polymer chains that twist, tangle and align with flow, giving it complex properties. Whilst recent work has examined the effects of fluid viscoelasticity on sperm-like swimmers, relatively less attention has been given to the shear-thinning property. We develop a new finite element technique to simulate free swimmers with prescribed beat kinematics in shear-thinning fluids with nonlinear governing equations. This technique is then applied to three qualitatively different viscous swimmers in order to examine the different phenomena that arise from swimmer interactions with of shear-thinning fluid