Investigation of spreading characteristics of nano-droplet on solid substrate using MD simulations

Surface wettability and spreading of liquid droplets on solid surfaces are of significant fundamental and practical importance in various fields of science and technology, ranging from classical coating and spray systems to the modern micro and nanofluidic devices, and biotechnology. Due to the complexity of solid/liquid/vapor interfaces, there are still difficulties in accurately predicting the behavior of liquid droplets on solid surfaces. Droplet spreading on a surface represents a multi-scale phenomenon with scales ranging from continuum to molecular. In this thesis, molecular dynamics (MD) simulations have been employed to investigate wetting and spreading characteristics of a nanodroplet on solid surfaces. The work focuses on the development of correlations and scaling laws to examine the effect of droplet size on the spreading characters. The dynamic spreading of an initially spherical liquid droplet in contact with a solid surface and surrounded by ambient gas has been studied under forced and spontaneous spreading. An efficient algorithm has been developed to track the liquid-phase interface and the dynamic contact angle. The algorithm was demonstrated to reproduce the entire wetting regime by varying the solid-liquid interaction energy parameters.;Extensive simulations have been performed to investigate the effects of surface and liquid droplet characteristics on the wetting phenomena and contact angle. In the first set of the simulations, liquid droplet, solid surface and ambient gas were modeled using argon atoms. The next set of simulations was performed using more realistic potential functions to simulate water SVS-B molecules for liquid, silicon crystal for solid surface, and nitrogen molecules for the ambient gas. Results for the argon droplet indicate a highly complex relationship between the contact angle and surface and liquid parameters. The spreading behavior was analyzed in terms of the temporal evolution of the dynamic contact angle and spreading diameter and was validated through comparison with the previous data. Results indicate that for forced spreading the spreading dynamics is governed by inertial and surface forces, with negligible influence of viscous forces, whereas the spontaneous spreading process is governed by viscous and surface forces. The results were further analyzed to obtain scaling laws for the effect of droplet size on the spreading parameters. In addition, global kinetic energy and surface energy considerations were used to provide a physical basis for these correlations. Scaling relationship between contact line velocity and droplet size was also determined. The correlations were found to be generally consistent with the experimentally observed spreading behavior of macroscopic droplets. Results for water- silicon system exhibit the expected behavior for the forced spreading water droplet. The computed equilibrium contact angle is within the range of experimentally observed contact angles. This set of simulations was performed for three surfaces with different wettabilities by changing the interaction energy of the surface. The present study demonstrates that MD simulations can be effectively used for the fundamental investigations of the wetting and spreading phenomena, and for characterizing the effects of various parameters on spreading characteristics.