Drops of Active Matter on Soft Surfaces

Active physics has grown to describe flocking behavior of self-driven systems such as bacterial colonies, cell growth in tissues, self-propelling colloids, protein transport in cells, and many more at the macroscopic level. This has sparked interest in the field of interfacial science and many models have been developed to capture the physics of Active Drops on Solid Surfaces, such as capillarity, evaporation, traction and motion of these drops. However, these models are limitingly tied to solid surface assumption, which is a very rare environment for active systems, and hence cannot capture the dynamics that occur due to elastocapillary-driven interactions on soft substrates. We develop a framework based on gradients in energy functionals of combined solid-liquid-vapor phases that can accurately capture the elasto-capillarity in the presence of active matter. We observe that the contributions from active free energies and active stress tensor aid in altering the overall surface tension and hence, contact angle condition at the three-phase interface. We believe this is the first step in unraveling many new dynamics pertaining to the active systems that more closely resemble their natural habitat.