Hydrodynamic boundary conditions and correlation functions of confined fluids

A phenomenological description of the dynamical correlations in a fluid confined between two parallel solid walls, using a generalization of Onsager's theory of fluctuations, is presented. Besides the bulk viscosity, this description involves parameters characterizing the boundary conditions. Correlations functions obtained in equilibrium molecular dynamics simulations are very well described within this framework. Featureless walls yield perfect slip boundary conditions, while corrugated walls in most cases correspond to stick boundary conditions. Only for very small corrugation amplitudes is it necessary to introduce a finite slipping length.