3D active particles in viscosity gradients

Viscosity gradients encountered in the marine ecosystem and gastrointestinal tract can affect the swimming and aggregation of microorganisms. Previous theoretical and numerical studies investigating this phenomenon have considered the effects of spatially differentiated viscous drag and/or propulsion, usually in isolation and using models of swimming with very simplified geometries. However, real active particles combine both effects, often with complex geometries, such as bacteria that swim by rotating flagella. The relative importance of drag- and propulsion-based effects for such active propulsion and the net effect of viscosity gradients on motility is currently unknown. To address these issues, we implement a regular perturbation scheme to numerically investigate the effect of slow varying viscosity fields on self-propelled swimmers with varying geometries and compare how viscosity gradients affect both drag and propulsion.