Active particles crossing a sharp viscosity gradient

Swimming microorganisms and other active particles often move through and even exploit complex and inhomogeneous environments. These active particles tend to perform taxis in such environments by reorienting to move up or down gradients in (chemical or material) properties of their environments. Recent experiments show that the alga Chlamydomonas reinhardtii displays complex refraction and scattering behavior when encountering sharp viscosity gradients. Motivated by these experiments, we have modeled an active particle as a spherical squirmer and analyzed the motion of it near and across similar sharp viscosity gradients. We find these active particles will either cross the interface or scatter off it in accordance with the experiments. Scattering only happens when the initial orientation relative to the interface normal is large and the particle is crossing from low to high viscosity. Otherwise, the particle just crosses the interface by undergoing some reorientation. The law governing the reorientation of neutral swimmers resembles ray optics, while the reorientation process of the pusher/puller is qualitatively similar.