Effect of fluid-solid interactions on symmetry breaking in closed nanoslits

Symmetry breaking in confined fluids

The recent progress in the theoretical investigation of the symmetry breaking (the existence of a stable state of a system, in which the symmetry is lower than the symmetry of the system itself) for classical and quantum fluids is reviewed. The emphasis is on the conditions which cause symmetry breaking in the density distribution for one component fluids and binary mixtures confined in a closed nanoslit between identical solid walls. The existing studies have revealed that two kinds of symmetry breaking can occur in such systems. First, a one-dimensional symmetry breaking occurs only in the direction normal to the walls as a fluid density profile asymmetric with respect of the middle of the slit and uniform in any direction parallel to the walls. Second, a two-dimensional symmetry breaking occurs in the fluid density distribution which is nonuniform in one of the directions parallel to the walls and asymmetrical in the direction normal to the walls. It manifests through liquid bumps and bridges in the fluid density distribution. For one component fluids, conditions of existence of symmetry breaking are provided in terms of the average fluid density, strength of fluid-solid interactions, distance at which the solid wall generates a hard core repulsion, and temperature. In the case of binary mixtures, the occurrence of symmetry breaking also depends on the composition of the confined mixtures.

Symmetry breaking of the density distribution of a quantum fluid in a nanoslit

The phenomenon of symmetry breaking (SB) of the fluid density distribution in a slit between parallel identical solid walls examined previously for a classical fluid (argon) [G. O. Berim and E. Ruckenstein, J. Chem. Phys. 126, 124503 (2007)] is examined for a quantum fluid ((4)He) on the basis of a nonlocal density functional theory. The Lennard-Jones potential was employed for the fluid-fluid and fluid-solid interactions. Regarding the latter interaction potential, it was supposed that each wall generates a hard core repulsion at some distance h(r) from the wall. In addition, the Chizmeshya-Cole-Zaremba (CCZ) potential was considered for the fluid-solid interactions. SB was found at all considered temperatures (0 K < or = T < or = 3.0 K) in ranges of average densities of the fluid which decreased, as for classical fluids, with increasing temperature. It was concluded that the existence of SB does not depend on the value of the parameter h(r), whereas, for classical fluids, SB did not occur when h(r) became smaller than a critical value, h(r,c). For the CCZ potential, the asymmetric (symmetry breaking) density profile can be metastable, whereas for the Lennard-Jones potential when an asymmetric density profile occurred it was always stable (had a smaller free energy than the symmetric profile). No effect of the (4)He transition from nonsuperfluid to superfluid state was detected.

Symmetry breaking in binary mixtures in closed nanoslits

The symmetry breaking (SB) of the fluid density distribution (FDD) in closed nanoslits between two identical parallel solid walls described by Berim and Ruckenstein [J. Chem. Phys. 128, 024704 (2008)] for a single component fluid is examined for binary mixtures on the basis of a nonlocal canonical ensemble density functional theory. As in Monte Carlo simulations, the periodicity of the FDD in one of the lateral (parallel to the wall surfaces) directions, denoted as the x direction, was assumed. In the other lateral direction, y direction, the FDD was considered to be uniform. The molecules of the two components have different diameters and their Lennard-Jones interaction potentials have different energy parameters. It was found that depending on the average fluid density in the slit and mixture composition, SB can occur for both or none of the components but never for only one of them. In the direction perpendicular to the walls (h direction), the FDDs of both components can be asymmetrical about the middle plane between walls. In the x direction, the SB occurs as bumps and bridges enriched in one of the components, whereas the composition of the mixture between them is enriched in the other component. The dependence of the SB states on the length Lx of the FDD period at fixed average densities of the two components was examined for Lx in the range from 10 to 120 molecular diameters of the smaller size component. It was shown that for large Lx, the stable state of the system corresponds to a bridge. Because the free energy of that state decreases monotonically with increasing Lx, one can conclude that the real period is very large (infinite) and that a single bridge exists in the slit.