At small length-scales, capillary effects are significant, and thus the mechanics of soft material interfaces may be dominated by solid surface stresses and liquid surface tensions. The balance between surface and bulk properties is described by an elasto-capillary length-scale, in which equilibrium interfacial energies are constant. However, at small length-scales in biological materials, including living cells and tissues, interfacial energies are not constant, but are actively regulated and driven far from equilibrium. Thus, the balance between surface and bulk properties depends upon the distance from equilibrium, defining a novel material parameter, what we term "active" elasto-capillarity. Here, we model the adhesion and spreading (wetting) of living cell aggregates as 'active droplets', with a non-equilibrium surface energy that depends upon internal stress generated by the actomyosin cytoskeleton. Depend upon the extent of activity, the droplet may exhibit both surface stress and surface tension, and each may adapt to the mechanics of their surroundings. The impact of this activity-dependent adaptation challenges contemporary models of interfacial mechanics, including traditional and extensively used models of contact mechanics and wetting. Finally, we show the origin of adaptation is in the breaking of detailed balance at the molecular scale by stochastic binding in the actomyosin cytoskeleton.
