  We numerically study the dynamics of a passive fluid droplet confined within
a microchannel whose walls are covered with a thin layer of active gel. The
latter represents a fluid of extensile material modelling, for example, a
suspension of cytoskeletal filaments and molecular motors. Our results show
that the layer is capable of producing a spontaneous flow triggering a
rectilinear motion of the passive droplet. For a hybrid design (a single wall
covered by the active layer), at the steady state the droplet attains an
elliptical shape, resulting from an asymmetric saw-toothed structure of the
velocity field. On the contrary, if the active gel covers both walls, the
velocity field exhibits a fully symmetric pattern considerably mitigating
morphological deformations. We further show that the structure of the
spontaneous flow in the microchannel can be controlled by the anchoring
conditions of the active gel at the wall. These findings are also confirmed by
selected 3D simulations. Our results may stimulate further research addressed
to design novel microfludic devices whose functioning relies on the collective
properties of active gels.
