Solid-liquid surface free energy of Lennard-Jones liquid on smooth and rough surfaces computed by molecular dynamics using the phantom-wall method.

Different model Lennard-Jones solid-liquid interfaces have been considered. In the systems, either the interaction strength between solid and liquid was varied, or the topography of the solid surface was modified. In all situations, the solid-liquid interfacial free energy variations with respect to a reference solid-liquid interface were quantified by means of a thermodynamic integration method [F. Leroy et al., Macromol. Rapid Commun. 30, 864 (2009)], referred to as the phantom-wall method. Additionally, the liquid-vapor surface free energy was determined. This result was combined with Young's equation for contact angle calculations of cylindrical liquid droplets. It allowed us to show that the change in contact angle of a droplet placed on smooth solid surfaces with respect to solid-liquid interaction strength could be obtained by neglecting the solid-vapor surface free energy contribution when the solid-liquid interaction was weak. We also showed that the implementation of roughness by means of parallel grooves whose the density was varied could yield either higher or lower solid-liquid surface free energy, depending on the solid-liquid surface free energy of the smooth interface. Roughness led to lower surface free energy when the smooth surface had favorable interaction with the liquid, while it led to a higher surface free energy when the smooth surface had loose interactions with the liquid, though the effect was found to be weak. The consistency of the whole set of results, as well as agreement with the existing results on similar systems, shows the ability of the thermodynamic integration method employed here to capture the variation of interfacial thermodynamic quantities when modifying either the chemical nature or the topography of a solid surface in contact with a given liquid phase.