Swimming of active drops in confinements with external flows*

Biological microswimmers commonly navigate through liquid flows in confinements, e.g. locomotions of spermatozoa in the reproductive tract and bacteria in blood vessels. Artificial microswimmers designed for drug delivery also swim in confinements having flows. So far, there are theoretical works explaining the dynamics of microswimmers in Poiseuille flow. However, there is little quantitative experimental data regarding swimming of active matter in confined microflows. Here, we elucidate the swimming behaviour of active drops in Poiseuille flow. The Marangoni stress dominated `self-propulsion’ of these droplets serves as a model for studying the swimming hydrodynamics of microorganisms, specifically pushers like E. Coli. We quantitatively explain the upstream swimming and downstream tumbling motion of the active drops in externally imposed flows; we also provide a comparative analysis of the experimental data with the hydrodynamics based non-linear theory. We feel that understanding such swimming behaviour of active matter will make significant contribution towards better comprehension of certain important biological phenomena.