Active Elasto-Capillarity Determines the Wetting Dynamics of Living Droplets*

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. 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. Dependent 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.