Theory of Soft Active Wetting

We must address active matter in the context of soft boundaries in order to bridge the gap between our understanding of active matter and the dynamics of the biological systems (that the active matter represent) under natural conditions. However, the physics of such active drops (matter) in contact with a soft and deformable surface has remained elusive. In this letter, we attempt to fill this gap and develop a theory for soft, active wetting. Our theory, which accounts for the various free energies for passive substrate and active drops as well as the active stresses, provides an equilibrium description of (active) particle orientation inside the drop and an equilibrium shape of the drop-soft-solid system. We obtain the equation relating the activity to the internal pressure of an active drop. The equilibrium calculation yields an ordered state of the polarization field inside the drop. The presence of the extensile (contractile) activity is found to make the drop press into the soft surface (come out of the soft surface), while increasing (decreasing) the wetting radius as compared to non-active drops. Finally, the three-phase contact line undergoes a rotation that depends on the strength of activity. These findings shed new light on the manner in which the active stresses interact with surface tension and elasticity at the fundamental level.