Molecular dynamics simulations of spontaneous spreading of a nanodroplet on solid surfaces

Abstract

The dynamics of spontaneous spreading of nano-sized droplets on solid surfaces were investigated using molecular dynamics simulations. The spreading behavior was analyzed in terms of the temporal evolution of instantaneous spreading diameter and contact angle for surfaces with different wetting characteristics. The computational model was validated through qualitative comparison with existing numerical and experimental data, including correlations for the variation of dynamic contact angle and spreading diameter. The results indicated that the spreading dynamics are mainly governed by surface and viscous forces. The spontaneous spreading process on a wettable surface can be described by three different stages, namely the initial, intermediate and final stages. The initial stage is characterized by the development of a precursor film, which moves ahead of the droplet, whereas the intermediate and final spreading stages are governed by a balance between surface and viscous forces. Simulations were used to develop correlations for the temporal variation of contact angle and spreading diameter for wettable, partially wettable and non-wettable surfaces. These correlations were found to be closer to those based on the molecular kinetic model than to those based on the hydrodynamic model. The results were further analyzed to obtain correlations for the effect of droplet size on the spreading parameters. These correlations indicated that the normalized spreading diameter and contact angle scale with non-dimensional drop diameter as Dm/D0 ∝ D0−0.6 ± 0.04 and θR ∝D00.67 ± 0.12 and the normalized spreading time scales as t ∝ D00.25 ± 0.05. Global surface energy and viscous dissipation energy considerations were used to provide a physical basis for these correlations. Significant differences were observed between the dynamics of spontaneous and forced spreading, especially with respect to the effect of droplet size on the spreading behavior.