Phase behaviour of the confined lattice gas Lebwohl-Lasher model

The phase behaviour of the Lebwohl-Lasher lattice gas model (one of the simplest representations of a nematogenic fluid) confined in a slab is investigated by means of extensive Monte Carlo simulations. The model is known to yield a first order gas-liquid transition in both the 2D and 3D limits, that is coupled with an orientational order-disorder transition. This latter transition happens to be first order in the 3D limit and it shares some characteristic features with the continuous defect mediated Berezinskii-Kosterlitz-Thouless transition in 2D. In this work we will analyze in detail the behaviour of this system taking full advantage of the lattice nature of the model and the particular symmetry of the interaction potential, which allows for the use of efficient cluster algorithms.Comment: 6 pages, 5 figure

Disordered hyperuniformity in two-component non-additive hard disk plasmas

We study the behavior of a two-component plasma made up of non-additive hard disks with a logarithmic Coulomb interaction. Due to the Coulomb repulsion, long-wavelength total density fluctuations are suppressed and the system is globally hyperuniform. Short-range volume effects lead to phase separation or to hetero-coordination for positive or negative non-additivities, respectively. These effects compete with the hidden long-range order imposed by hyperuniformity. As a result, the critical behavior of the mixture is modified, with long-wavelength concentration fluctuations partially damped when the system is charged. It is also shown that the decrease of configurational entropy due to hyperuniformity originates from contributions beyond the two-particle level. Finally, despite global hyperuniformity, we show that in our system, the spatial configuration associated with each component separately is not hyperuniform, i.e., the system is not "multihyperuniform.

Phase behaviour of attractive and repulsive ramp fluids: integral equation and computer simulation studies

Using computer simulations and a thermodynamically self consistent integral equation we investigate the phase behaviour and thermodynamic anomalies of a fluid composed of spherical particles interacting via a two-scale ramp potential (a hard core plus a repulsive and an attractive ramp) and the corresponding purely repulsive model. Both simulation and integral equation results predict a liquid-liquid de-mixing when attractive forces are present, in addition to a gas-liquid transition. Furthermore, a fluid-solid transition emerges in the neighbourhood of the liquid-liquid transition region, leading to a phase diagram with a somewhat complicated topology. This solidification at moderate densities is also present in the repulsive ramp fluid, thus preventing fluid-fluid separation.Comment: 29 pages, 10 figure

Pattern formation in binary fluid mixtures induced by short-range competing interactions

Molecular dynamics simulations and integral equation calculations of a simple equimolar mixture of diatomic molecules and monomers interacting via attractive and repulsive short-range potentials show the existence of pattern formation (microheterogeneity), mostly due to depletion forces away from the demixing region. Effective site-site potentials extracted from the pair correlation functions using an inverse Monte Carlo approach and an integral equation inversion procedure exhibit the features characteristic of a short-range attractive and long-range repulsive potential. When charges are incorporated into the model, this becomes a coarse grained representation of a room temperature ionic liquid, and as expected, intermediate range order becomes more pronounced and stable

Phase behavior of the Confined Lebwohl-Lasher Model

The phase behavior of confined nematogens is studied using the Lebwohl-Lasher model. For three dimensional systems the model is known to exhibit a discontinuous nematic-isotropic phase transition, whereas the corresponding two dimensional systems apparently show a continuous Berezinskii-Kosterlitz-Thouless like transition. In this paper we study the phase transitions of the Lebwohl-Lasher model when confined between planar slits of different widths in order to establish the behavior of intermediate situations between the pure planar model and the three-dimensional system, and compare with previous estimates for the critical thickness, i.e. the slit width at which the transition switches from continuous to discontinuous.Comment: Submitted to Physical Review

An integral equation approach to orientational phase transitions in two and three dimensional disordered systems

The use of inhomogeneous Ornstein-Zernike equations to analyze phase transitions and ordered phases in magnetic systems is explored both in bulk three dimensional disordered Heisenberg systems and in a simple model for a two dimensional ferrofluid monolayer. In addition to closures like the Mean Spherical Approximation, Hypernetted Chain and Zerah-Hansen approximation, the inhomogeneous Ornstein-Zernike equation must be complemented by a one-body closure, for which the Born-Green equation has been used in this paper. The results obtained prove that the proposed approach can furnish accurate estimates for the paramagneticferromagnetic transition in the three dimensional Heisenberg spin fluid, reproducing reliably the structure of the isotropic and ordered phases. In two dimensions, the results are fairly accurate as well, both for the dipolar film alone and in the presence of external perpendicular fields. At high densities/dipole moments the equation seems to predict a transition to a phase in which the dipoles lie mostly in the plane and are aligned into vortex-like structures. Evidence of this new phase is found in the simulation at somewhat higher couplingsВикористання неоднорідних рівнянь Орнштейна-Церніке для вивчення фазових переходів і впорядкованих фаз в магнітних системах досліджується як у невпорядкованих гайзенбергівських системах так і в простій моделі для двовимірного ферофлюїдного моношару. Неоднорідне рівняння Орнштейна-Церніке, крім таких замикань як середньосферичне, гіперланцюгове і наближення Зера-Гансена, мусить бути доповнене одно-частинковим замиканням, для якого було використано в цій статті рівняння Борна-Гріна. Отримані результати доводять, що запропонований підхід може давати точні оцінки для переходу парамагнетик-феромагнетик в тривимірному гайзенбергівському спіновому флюїді, надійно відтворюючи структуру ізотропної і впорядкованої фаз. У двох вимірах, результати є, безумовно, точними як для дипольної плівки без поля, так і в присутності зовнішніх перпендикулярно направлених полів. При високих густинах/дипольних моментах рівняння передбачають перехід до фази, в якій диполі лежать в основному в площині і утворюють вихороподібні структури. Наявність цієї нової фази є знайдена при дещо сильніших параметрах при моделюванні

Temperature of maximum density and excess properties of short-chain alcohol aqueous solutions : a simplified model simulation study

We perform an extensive computational study of binary mixtures of water and short-chain alcohols resorting to two-scale potential models to account for the singularities of hydrogen bonded liquids. Water molecules are represented by a well studied core softened potential which is known to qualitatively account for a large number of water’s characteristic anomalies. Along the same lines, alcohol molecules are idealized by dimers in which the hydroxyl groups interact with each other and withwater with a core softened potential as well. Interactions involving non-polar groups are all deemed purely repulsive. We find that the qualitative behavior of excess properties (excess volume, enthalpy, and constant pressure heat capacity) agrees with that found experimentally for alcohols such as t-butanol in water. Moreover, we observe that our simple solute under certain conditions acts as a “structuremaker,” in the sense that the temperature of maximum density of the bulk water model increases as the solute is added, i.e., the anomalous behavior of the solvent is enhanced by the solute