S. Amokrane

Structure of highly asymmetric hard-sphere mixtures: An efficient closure of the Ornstein-Zernike equations

A simple modification of the reference hypernetted chain (RHNC) closure of the multicomponent Ornstein-Zernike equations with bridge functions taken from Rosenfelds hard-sphere bridge functional is proposed. Its main effect is to remedy the major limitation of the RHNC closure in the case of highly asymmetric mixtures--the wide domain of packing fractions in which it has no solution. The modified closure is also much faster, while being of similar complexity. This is achieved with a limited loss of accuracy, mainly for the contact value of the big sphere correlation functions. Comparison with simulation shows that inside the RHNC no-solution domain, it provides a good description of the structure, while being clearly superior to all the other closures used so far to study highly asymmetric mixtures. The generic nature of this closure and its good accuracy combined with a reduced no-solution domain open up the possibility to study the phase diagram of complex fluids beyond the hard-sphere model.

On some empirical expressions for the contact values of the pair distribution functions and fluid-fluid phase separation in hard sphere mixtures

The fluid-fluid binodal of asymmetric hard sphere mixtures obtained from approximate expressions of the virial pressure is investigated. Also the behaviour of the Gibbs free energy following from particular combinations of standard expressions of the contact values of the pair distributions functions is examined. A recently proposed parametrization of the latter in the colloidal limit is then discussed and compared with existing simulation data for the binodal of the effective fluid.

Controlling the composition of a confined fluid by an electric field

Starting from a generic model of a pore/bulk mixture equilibrium, we propose a novel method for modulating the composition of the confined fluid without having to modify the bulk state. To achieve this, two basic mechanisms-sensitivity of the pore filling to the bulk thermodynamic state and electric field effect-are combined. We show by Monte Carlo simulation that the composition can be controlled both in a continuous and in a jumpwise way. Near the bulk demixing instability, we demonstrate a field induced population inversion in the pore. The conditions for the realization of this method should be best met with colloids, but being based on robust and generic mechanisms, it should also be applicable to some molecular fluids.

Structure of highly confined fluids: Mixture of polar and nonpolar macroparticles in an external field

In this paper we study the structure of highly confined mixtures of polar and nonpolar macroparticles in an external field by Monte Carlo simulation in the canonical ensemble. Without attempting a systematic investigation of the model, several effects including confinement, polarization, and solvation forces are considered. In particular, we show that layering at different length scales can be obtained in mixtures of differently sized particles subject to an external electric field.

Demixing and field-induced population inversion in a mixture of neutral and dipolar-hard spheres confined in a slit pore

We study the influence of an external field on demixing and its interplay with the field-induced population inversion for a binary mixture of neutral and dipolar non-additive hard-spheres confined in a slit pore. Demixing lines are determined by Gibbs Ensemble Monte Carlo simulations and population inversion paths by grand-canonical/canonical simulations of the pore/bulk equilibrium. Besides the demixing line in the bulk, results are given for two different pore widths and in parallel and normal fields. Similar to the effect of geometrical confinement, a normal field is found to favour the mixed state so that the population inversion does not interfere with demixing. A parallel field leads to more complex scenarios.

Monte Carlo simulation of confined fluids of polarizable particles: an efficient iterative treatment of the local field in slab geometry using Ewald summation

We propose a method for treating in Monte Carlo simulations the problem of the induced dipoles for polarizable particle fluids confined in slab geometry and subject to an external field. In order to compute the local field in a reasonable time, a partial update of the induced dipole moments is performed by introducing a cut-off distance, as in bulk systems. This strategy is then combined with a slab adapted 3D-Ewald summation for treating the long-range interactions between the induced dipoles. The method is illustrated by simulations of confined binary mixtures in the canonical and grand canonical ensembles.

Effect of an external field on the structure and the phase transitions of a confined mixture of neutral and dipolar hard spheres

We study by Monte Carlo simulation the model of a binary mixture of neutral and dipolar hard spheres confined between two widely separated planar walls and subjected to a uniform external field. The goal is to investigate the structural response and the phase transitions of a fluid of hard-sphere-like colloids dispersed in a low-permittivity solvent under the combined effect of geometrical confinement and applied field. In a wide slab, the direction of the field, either normal or perpendicular to the walls, remains one of the most important factors that govern the response of the mixture: in normal field, a wide variety of structural effects are evidenced, including partial wetting or drying of the wall; in parallel field, phase separation is favoured with a specific population of the region close to the wall and a clear separation of the two species. These results suggest possible means to modulate the response of the confined fluid for specific needs.

Comment on the contact values in asymmetric hard-sphere mixtures (Mol. Phys. 107, 685 (2009); Mol. Phys. 106, 607 (2008))

We comment on the equation of state of highly asymmetric hard-sphere mixtures obtained from the virial equation using parameterized contact values of the radial distributions functions

Ornstein–Zernike equations for highly asymmetric mixtures: confronting the no-solution challenge

The question of the absence of a solution to the Ornstein–Zernike integral equations for the pair structure of asymmetric binary mixtures is discussed. The behaviour of a recently proposed closure with a reduced no-solution domain in the case of highly asymmetric hard sphere mixtures is considered. The modifications that are required to treat non-hard-sphere interactions are examined. A possible implementation of the same strategy is illustrated for the potential of the mean force for solvophilic macroparticles in a Lennard–Jones fluid.

Contribution of non-hard core triplet correlations to the bridge function of dense fluids

A possible extension of Rosenfelds bridge functional to non-hard core interactions is considered. The hard-sphere bridge functional is supplemented with a contribution related to the non-hard core part of the triplet direct correlation function. The latter is estimated from a generalization of the factorization ansatz of Barrat, Hansen and Pastore. For the Lennard-Jones potential, this additional term is found to give a significant contribution to the tail of the bridge function. The relevance of this method to the case of highly asymmetric mixtures is underlined.