  Active droplets are artificial microswimmers built from a liquid dispersion
by microfluidic tools and showing self-propelled motion. These systems hold
particular interest for mimicking biological phenomena, such as some aspects of
cell locomotion and collective behaviors of bacterial colonies, as well as for
the design of droplet-based biologically inspired materials, such as engineered
tissues. Growing evidence suggests that geometrical confinement crucially
affects their morphology and motility, but the driving physical mechanisms are
still poorly understood. Here we study the effect of activity on a droplet
containing a contractile polar fluid confined within microfluidic channels of
various sizes. We find a surprising wealth of shapes and dynamic regimes, whose
mechanics is regulated by a subtle interplay between contractile stress,
droplet elasticity and microchannel width. They range from worm-like and
cell-like shaped droplets displaying an oscillating behavior within wider
channels to bullet-shaped droplets exhibiting rectilinear motion in narrower
slits. Our findings support the view that geometrical confinement can provide a
viable strategy to control and predict the propulsion direction of active
droplets. It would be of interest to look for analogues of these motility modes
in biological cells or in synthetic active matter.
