Abstract
We study the contact angle of polymer melt sessile droplets on a flat structureless substrate subject to long-range van der Waals forces when the droplet size is diminished. For this purpose, droplets containing 4096 to 512 monomers for chains of length N = 32 are carefully equilibrated at 80% of the Θ-temperature using a coarse-grained bead spring model of flexible polymers in a Monte Carlo computer experiment. The spherically averaged density profile of these droplets, both in the z-direction perpendicular to the substrate surface and in the radial direction, is obtained. At weak adhesion we find that the contact angle grows steadily with the decrease of drop size so that below a critical size these drops dewet the surface. This suggests a strong contribution of a positive line tension κ in the determination of the equilibrium drop shape. At much stronger adhesion, however, the contact angle does not change with drop size, indicating κ ≈ 0. In general, our simulational results outline the limits of validity of Young's relation for the contact angle of sessile nanodroplets and support the more general Gretz rule which takes into account the role of line tension in the balance of surface forces.