  Active fluid droplets are a class of soft materials exhibiting autonomous
motion sustained by an energy supply. Such systems have been shown to capture
motility regimes typical of biological cells and are ideal candidates as
building-block for the fabrication of soft biomimetic materials of interest in
pharmacology, tissue engineering and lab on chip devices. While their behavior
is well established in unconstrained environments, much less is known about
their dynamics under strong confinement. Here, we numerically study the physics
of a droplet of active polar fluid migrating within a microchannel hosting a
constriction with adhesive properties, and report evidence of a striking
variety of dynamic regimes and morphological features, whose properties
crucially depend upon droplet speed and elasticity, degree of confinement
within the constriction and adhesiveness to the pore. Our results suggest that
non-uniform adhesion forces are instrumental in enabling the crossing through
narrow orifices, in contrast to larger gaps where a careful balance between
speed and elasticity is sufficient to guarantee the transition. These
observations may be useful for improving the design of artificial
micro-swimmers, of interest in material science and pharmaceutics, and
potentially for cell sorting in microfluidic devices.
