  Microswimmers in nature often experience spatial gradients of viscosity. In
this work we develop theoretical results for the dynamics of active particles,
biological or otherwise, swimming through viscosity gradients. We model the
active particles using the squirmer model, and show how viscosity gradients
lead to viscotaxis for squirmers, and how the effects of viscosity gradients
depend on the swimming gait of the microswimmers. We also show how such
gradients in viscosity can be used to control active particles and suggest a
mechanism to sort them based on their swimming style.
