Fluids confined by nanopatterned substrates of low symmetry

Abstract

By means of Monte Carlo simulations in the grand canonical ensemble (GCEMC) the phase behaviour of a ‘simple’ fluid (in which molecules possess only translational degrees of freedom) confined between planar nanopatterned substrates is investigated. The fluid—substrate interaction is purely repulsive except for circular regions of radius R attracting fluid molecules. Because the circular pattern is finite in size, system properties are not translationally invariant in any spatial dimension. Thus, the grand potential Ω is not a homogeneous function of degree 1 in any of its extensive variables, which precludes a ‘mechanical’ expression for Ω in terms of stresses and conjugate strains. Therefore, in order to determine the phase behaviour, Ω versus chemical potential μ is calculated by thermodynamic integration under isothermal conditions (T = const). Besides the more conventional gas and liquid phases, fluid bridges arise as a third type of thermodynamically stable phase depending on T and μ. In a bridge phase, molecules are preferentially adsorbed by attractive circular region, and span the gap between the opposite substrate surfaces.