Durotaxis in viscoelastic fluids

Active particles often navigate through inhomogeneous environments, where they display directed motion known as taxis by swimming up or down the gradients of inhomogeneities. Well-known types of taxis include chemotaxis in chemical or nutrient gradients, phototaxis in light gradients, rheotaxis in fluid velocity gradients, and viscotaxis in viscosity gradients. Another example is durotaxis, which is the movement of cells on a substrate in the direction of stiffness gradients. Here we report an analogue of durotaxis in fluids, demonstrating directed motion of active particles in the gradients of stiffness (or relaxation timescale) of a viscoelastic fluid. This work provides insights into the dynamics of active particles in inhomogeneous viscoelastic fluids, and the taxis identified here could be exploited to sort active particles by probing the inhomogeneities.