Effect of temperature on nanoscale wettability: an atomistic study

Understanding line tension effects on wettability in nanofluidics is crucial for nanoscale thermal management, nanofabrication, and interfacial fluid control. Wettability is characterized by the contact angle (CA) between the liquid-vapor interface and the solid surface described by Young's equation (YE). In nanodroplets, CA deviates significantly from predicted values using YE. It has been attributed to line tension effects which magnify as the droplet size decreases. A modified YE has been proposed (Boruvka and Newmann J. Chem. Phys.1977) including line tension effects however the fundamental relation between nanoscale wettability, line tension and CA remains an open question. It is not clear how local temperature and surface characteristics impact line tension magnitude and nanoscale CA. Here, we study how temperature in a sessile nanodroplet affects CA and line tension. To this end, we employ all-atom Molecular Dynamics simulations consisting of water droplets on a copper substrate. Species are described using the TIP4P/2005 water model, and a copper model that robustly reproduces thermal lattice vibrations, surface roughness and thermal transport. Different size Semi-spherical and semi-cylindrical nanodroplets are considered, the latter directly reproducing the macroscopic CA. Our results show the importance of considering equilibrium temperature in the modified YE to predict water CA.