Line Tension of Nanodroplets on a Concave Surface

The contact angle of a nanodroplet on a surface may deviate from that of a macrodroplet on a surface. Even though there are many studies regarding line tension, it is not well understood. In this paper, molecular dynamics simulation is performed for nanodroplets on a concave solid wall. The Lennard-Jones (L-J) potential is directly used or modified to simulate the force interaction between argon atoms and between solid-liquid particles. The initial droplet radius is 4, 5, and 6 nm, respectively. The k coefficient is defined as the ratio of the initial droplet radius with respect to the curvature radius of concave walls, which is in the range of 0-0.9, in which k = 0 refers to a flat surface. We found that indeed the contact angle θ of a nanodroplet on a concave wall deviates from that of a macrodroplet on a flat surface. Contact angles display a two region distribution, in which θ increases with increasing k for k < 0.5 and decreases with increasing k for k > 0.5. The k coefficient influences the droplet morphology. With k in the range of 0-0.9, the vapor-liquid interface is switched from a convex shape to a flat shape and finally reaches a concave shape. The line tension generally behaves in an increasing trend with the increase of k but becomes constant when k is beyond 0.7. The liquid densities, radial distribution functions, and coordination numbers show that the liquid particles are more closely packed with each other with the increase of k. The line tension achieves a positive sign and on the magnitude of 10^-11 N, which has a linear increase with respect to the peak density of the first liquid layer.