Morphodynamics of active drops

Many biological systems exhibit similarities to active liquid drops–drops containing units that acquire orientational order and exert stresses that induce spontaneous flows. Tissues, the mitotic spindle, and small mammalian organisms are notable examples in which cells, microtubules, and actin fibers can display nematic order and generate flow.

While the shape and flow inside slender drops resting on boundaries and thin films have been extensively studied using lubrication theory, less is known about drops of any aspect ratio where slender theory is not applicable. To address this gap in knowledge, we perform full time-dependent simulations of an active drop on a solid substrate, which exhibits nematic order due to anchoring at both the solid boundary and the drop's interface. We consider purely viscous flow and either extensile or contractile nematic stress, in which the orientational field undergoes pure relaxational behavior towards a minimum free energy state.We find that the drop reaches a stable steady state in which viscous, surface tension, and active forces are in balance. The type of active stress (extensile or contractile), the anchoring angle, and the active Capillary number (which compares the strength of active stresses with capillary pressure) play crucial roles in determining the distinct complex flows and shapes of the drop. These flows and shapes cannot be predicted by lubrication theory, which only gives a unique shape for a given Capillary number. Moreover, when symmetry is broken due to the anchoring, we find that the velocity and mixing inside the drop is maximized.

Overall, our work provides a deeper understanding of active drops, which could be relevant to various biological processes involving tissue morphodynamics and cellular flows, and could inspire new experiments on active materials.