Durotaxis in fluids

Microorganisms and other active particles often swim through inhomogeneous environments prevalent in nature, such as gradients in chemical concentration, light, nutrients, temperature or salinity. The active particles often respond to these inhomogeneities by exhibiting directed motion (or taxis) along gradients. Well known types of taxis include chemotaxis in chemical or nutrient gradients, phototaxis in light gradients, and rheotaxis in fluid velocity gradients. In this talk I will discuss how the dynamics of active particles can be dramatically altered by the introduction inhomogeneity in the mechanical properties of fluids. Specifically, I will discuss the effects of spatial variations of fluid density, viscosity, and elasticity in the context of biological locomotion and how gradients in these mechanical properties can all lead to novel mechanisms for the directed motion of microswimmers.