Anisotropic self-diffusion in nanofluidic structures

By means of equilibrium molecular dynamics simulations, we investigate self-diffusion in a simple fluid confined to nanoscopic slit-pores. The pore walls are decorated with wettable and nonwettable chemical stripes that alternate in the x direction and are assumed infinitely long in the y direction. We consider the impact of pore width as well as variations of the width of the wettable stripes d(wet). Depending on these model parameters and the thermodynamic conditions, the confined fluid may exist as one of three morphologically distinct phases: a thin fluid film adsorbed by the wettable stripes, a fluid bridge spanning the gap between the (aligned) stripes on the two opposite substrates, or a nanostructured liquid where molecules occupy the entire space between the substrate surfaces. By analyzing mean square displacements, velocity autocorrelation functions, and their power spectra, a detailed picture of mass transport and its relation to substrate decoration emerges. In particular, we find that the axial symmetry of the self-diffusion tensor D with respect to the z axis is approximately preserved in liquidlike phases, whereas no such symmetry exists as far as both film and bridge phases are concerned.