Minimum-density anomaly and spatial ordering of softly repulsive particles in a narrow channel

We performed an extensive numerical investigation of a system of repulsive Gaussian particles confined in a thin cylindrical pore. In this setting, the fluid phase can be cooled down to very low temperatures, thus bypassing the freezing transition. Focusing on the thermal behavior of the average number density, we find a range of pressures within which, upon cooling, the system density first approaches a maximum that is then followed by a minimum at lower temperatures. As the width of the pore is reduced, the density minimum shifts to larger pressures, in line with what happens in the same model in one dimension. As far as the system structure is concerned, a pronounced layering is observed at the wall; moreover, when the pore radius is not too small, the relative fraction of solid-like (i.e., well coordinated) particles increases overall on cooling, in a somewhat larger amount when crossing the region bounded by the two density extrema. On account of this phenomenology, we surmise that the anomalous behavior of the system density stems from the smoothening of the density jump occurring at the three- dimensional freezing point. By analogy, our findings suggest that the essential driving mechanism leading to the volumetric anomaly exhibited by supercooled water confined in silica nanopores at ambient pressure is an effective soft repulsion between water molecules at short distances.