Simulation of binary fluids exposed to selectively adsorbing walls: a method to estimate contact angles and line tensions

For an understanding of interfacial phenomena of fluids on the nanoscale a detailed knowledge of the excess free energies of fluids due to walls is required, as well as of the interfacial tension between coexisting fluid phases. A description of simulation approaches to solve this task is given for a suitable model binary (A + B) fluid. Sampling the order parameter distribution of the system without walls, the curvature dependent and flat interfacial tensions of coexisting 'bulk' phases is extracted. In a thin film geometry, the difference in wall free energies is found via a new thermodynamic integration method. Thus the contact angle of macroscopic droplets is estimated from Young's equation, for varying interactions between the fluid particles and the walls, which compares well with direct observations of inclined interfaces in ultrathin slit pores. Studying two-phase situations where a wall-attached droplet exists in the slit pore in thermal equilibrium, the excess free energy due to the droplet is found as a function of the droplet size, which is compatible with the classical theory for heterogeneous nucleation, if a line tension correction to the contact angle is made. For = 90 an alternative method for extracting the line tension is also presented.