Flow fields around active droplets squeezing through tight confinements

We explain how self-propelling active droplets can squeeze through increasingly narrow microchannels with shape and velocity field adaptations. Using microscopy techniques, we show that these droplet microswimmers transform from a spherical to a `stadium'-like shape and eventually into an elongated `capsule'-like shape as they traverse extreme confined spaces. During this shape evolution, their hydrodynamic signature changes from a symmetric, quadrupolar to an asymmetric velocity field. Using finite-element based numerical simulations, we explain the flow field evolution by considering the thin lubricating film between the microchannel wall and the active droplet interface.