Locomotion Through Complex Fluids: An Experimental View

Abstract

Recently, there has been renewed interest in the swimming of microorganisms for applications that include artificial swimmers, novel materials, drug delivery, and micro-robotics. Due to small length scales, the fluid mechanics of swimming of microorganisms are governed by low Reynolds number hydrodynamics. In such a regime, linear viscous forces dominate over nonlinear inertial forces. While our current understanding of locomotion at low Reynolds numbers is derived mainly from investigations in simple, Newtonian fluids (e.g., water), many of the fluids in which locomotion occurs contain solids and/or (biological) polymers that are instead not Newtonian. Examples include wet soils, human mucus, and fluids in the cervix and female reproductive track. A major challenge is to understand the propulsion mechanisms in fluids that display complex rheological behavior such as viscoelasticity and shear-thinning viscosity. Here, we will briefly review a few notable swimming experiments in Newtonian fluids and then discuss the latest experimental results on swimming in complex fluids, focusing on viscoelastic fluids.