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Structures of simple liquids in contact with nanosculptured surfaces

We present a density functional study of Lennard-Jones liquids in contact with a nano-corrugated wall. The corresponding substrate potential is taken to exhibit a repulsive hard core and a van der Waals attraction. The corrugation is modeled by a periodic array of square nano-pits. We have used the modified Rosenfeld density functional in order to study the interfacial structure of these liquids which with respect to their thermodynamic bulk state are considered to be deep inside their liquid phase. We find that already considerably below the packing fraction of bulk freezing of these liquids, inside the nanopits a three-dimensional-like density localization sets in. If the sizes of the pits are commensurate with the packing requirements, we observe high density spots separated from each other in all spatial directions by liquid of comparatively very low density. The number, shape, size, and density of these high density spots depend sensitively on the depth and width of the pits. Outside the pits, only layering is observed; above the pit openings these layers are distorted with the distortion reaching up to a few molecular diameters. We discuss quantitatively how this density localization is affected by the geometrical features of the pits and how it evolves upon increasing the bulk packing fraction. Our results are transferable to colloidal systems and pit dimensions corresponding to several diameters of the colloidal particles. For such systems the predicted unfolding of these structural changes can be studied experimentally on much larger length scales and more directly (e.g., optically) than for molecular fluids which typically call for sophisticated X-ray scattering.