Microscopic origin of droplet line tension

The size dependence of the equilibrium contact angle of sessile droplets, commonly termed line tension, lies beyond classic Young's law. Here, we identify a fundamental contribution to line tension arising from body gravity effects and surface-pressure effects within an adsorption layer. This mechanism resolves the multiscale behavior of droplets from nanometric to millimetric sizes, for which the apparent line tension changes sign and spans several orders of magnitude, consistent with existing experiments and simulations. The sign and magnitude are governed by surface wettability, the surface composition in the adsorption layer, and droplet size. Our results provide a unified physical interpretation of the experimentally observed variability in both the sign and magnitude of line tensions.