**Contact angles from Young's equation in molecular dynamics simulations**

H Jiang and F Muller-Plathe and AZ Panagiotopoulos, JOURNAL OF CHEMICAL PHYSICS, 147, 084708 (2017).

DOI: 10.1063/1.4994088

We propose a method to calculate the equilibrium contact angle of
heterogeneous 3-phase solid/fluid/fluid systems using molecular dynamics
simulations. The proposed method, which combines the phantom-wall method
**F. Leroy and F. Muller-Plathe, J. Chem. Phys. 133, 044110 (2010)** and
Bennett's acceptance ratio approach **C. H. Bennett, J. Comput. Phys. 22,
245 ( 1976)**, is able to calculate the solid/fluid surface tension
relative to the solid surface energy. The calculated relative surface
tensions can then be used in Young's equation to estimate the
equilibrium contact angle. A fluid droplet is not needed for the
proposed method, in contrast to the situation for direct simulations of
contact angles. In addition, while prior free-energy based methods for
contact angles mainly focused on the wetting of fluids in coexistence
with their vapor on solid surfaces, the proposed approach was designed
to study the contact angles of fluid mixtures on solid surfaces above
the fluid saturation pressures. Using the proposed approach, the contact
angles of binary Lennard-Jones fluid mixtures on a non-polar solid
substrate were calculated at various interaction parameters and the
contact angle of water in equilibrium with CO2 on a hydrophilic polar
silica surface was obtained. For both non-polar and polar systems, the
calculated contact angles from the proposed method were in agreement
with those obtained from the geometry of a cylindrical droplet. The
computational cost of the proposed method was found to be comparable to
that of simulations that use fluid droplets, but the new method provides
a way to calculate the contact angle directly from Young's equation
without ambiguity. Published by AIP Publishing.

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