Migration of active fluid droplets through constrictions

Active fuid droplets are an intriguing example of bio-inspired self-propelled objects whose motion is driven by an assembled active gel. Experimental realizations of active gels include actomyosin solutions and suspensions of microtubules and kinesin, which are soft fuids comprising force dipoles and exhibiting long range orientational order typical of liquid crystals. These droplets may be useful for designing artificial microswimmers of interest, for example, in material science and pharmaceutics. In this work, we study, by lattice Boltzmann simulations, the physics of an active fluid droplet migrating through an interstice, whose design is inspired to realistic physiological conditions. Droplet and pore of the interstice are modeled using a phase field approach, where the pillars of the interstice are fluid-free static fields incorporating adhesive effects. We report a striking variety of dynamic regimes whose properties depend on droplet speed and elasticity, degree of confinement and adhesiveness to the pore. Our results suggest that non-uniform adhesion forces between droplet and constriction are instrumental in enabling the crossing, in contrast to larger gaps where a high enough speed is suffcient to guarantee the transition.