Nanodroplets on a solid plane: wetting and spreading in a Monte Carlo simulation

The wetting behavior and spreading dynamics of small polymer melt droplets in the course of transition from partial to complete wetting conditions on a flat structureless solid substrate have been studied by dynamic Monte Carlo simulation. From the density profiles of the drops we determine the contact angles at varying strength of the van der Wails surface forces in the whole interval of partial wetting. The validity of Young's equation is then tested whereby the surface tension of the melt/vapor interface is derived independently from interfacial fluctuation analysis, and the surface free energy of the melt at the substrate-from the anisotropy of the local pressure at the wall. The bending rigidity of the melt/vapor interface turns out negative, as recently predicted for short-range interactions. We carry out computer experiments which show that Tanner's law for the kinetics of drop spreading holds also on nanoscopic scales. The observed density profiles of spreading droplets confirm earlier predictions that the central cap-shaped region of the droplets shrinks at the expense of a transition region ("foot") surrounded by a precursor film which is roughly one monolayer thick. At later times the precursor film breaks into individual polymer chains and advances in typically diffusive manner as found in laboratory experiments. Eventually we investigate the impact of line tension on nanodroplets behavior at varying strength of adhesion and demonstrate that the Gretz equation which incorporates line tension into Young's rule holds even on nanoscale and predicts important properties of the drops subject to droplet size. (C) 2002 Elsevier Science B.V. All rights reserved.