A molecular dynamics study of the force between planar substrates due to capillary bridges

Molecular dynamics simulations are used to study capillary liquid bridges between two planar substrates and the origin, strength and range of the resulting force between them. Pairwise interactions are described by the Lennard-Jones potential. Surface wettability is tuned by varying the fluid-substrate well depth interaction parameter. The force between the substrates due to a bridge of liquid is estimated by different methods including non-equilibrium simulations of moving substrates connected by liquid bridges and macroscopic balance of forces. The latter involves knowledge of liquid-vapor interfacial free energy, curvature radii, radius of wetted area and contact angle at the triple-phase contact line. All these physical quantities are estimated from equilibrium simulations. The force is attractive when the substrates are solvophilic or moderately solvophobic; and thus for cavities surrounded by the same liquid the force is attractive even when the substrates are moderately solvophilic. Two threshold values for the fluid-substrate potential interaction parameter can be identified; one for which the effective interaction between substrates due to liquid bridges changes from repulsive to attractive and another for which the capillary bridge becomes mechanically unstable and breaks into droplets.