Liquid-vapor interface of a polydisperse fluid

We report a Grand Canonical Monte Carlo simulation study of the liquid-vapor interface of a model fluid exhibiting polydispersity in terms of the particle size $\sigma$. The bulk density distribution, $\rho^0(\sigma)$, of the system is controlled by the imposed chemical potential distribution $\mu(\sigma)$. We choose the latter such that $\rho^0(\sigma)$ assumes a Schulz form with associated degree of polydispersity $\approx 14$. By introducing a smooth attractive wall, a planar liquid-vapor interface is formed for bulk state points within the region of liquid-vapor coexistence. Owing to fractionation, the pure liquid phase is enriched in large particles, with respect to the coexisting vapor. We investigate how the spatial non-uniformity of the density near the liquid-vapor interface affects the evolution of the local distribution of particle sizes between the limiting pure phase forms. We find (as previously predicted by density functional theory, Bellier-Castella {\em et al}, Phys. Rev. {\bf E65}, 021503 (2002)) a segregation of smaller particles to the interface. The magnitude of this effect is quantified for various $\sigma$ via measurements of the relative adsorption. Additionally, we consider the utility of various estimators for the interfacial width and highlight the difficulties of isolating the intrinsic contribution of polydispersity to this width.Comment: 9 pages, 10 Fig

Wetting transitions in polydisperse fluids

The properties of the coexisting bulk gas and liquid phases of a polydisperse fluid depend not only on the prevailing temperature, but also on the overall parent density. As a result, a polydisperse fluid near a wall will exhibit density-driven wetting transitions inside the coexistence region. We propose a likely topology for the wetting phase diagram, which we test using Monte Carlo simulations of a model polydisperse fluid at an attractive wall, tracing the wetting line inside the cloud curve and identifying the relationship to prewetting.Comment: 4 Pages, 4 figures. Accepted for publication in Physical Review Letter

Enthalpies of formation of lanthanide oxyapatite phases

A family of lanthanide silicates adopts an oxyapatite-like structure with structural formula Ln9.33∎0.67(SiO4)6O2 (Ln 4 La, Sm, Nd, Gd, ∎ = vacancy). The enthalpies of solution, DHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO·B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, ΔHs f-oxides (kJ/mol): La9.33∎0.67(SiO4)6O2 = −776.3 ± 17.9, Nd9.33∎0.67(SiO4)6O2 = −760.4 ± 31.9, Sm9.33∎0.67(SiO4)6O2 = −590.3 ± 18.6, and Gd9.33∎0.67(SiO4)6O2 = −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 f (kJ/mol): La9.33∎0.67(SiO4)6O2 = −14611.0 ± 19.4, Nd9.33∎0.67(SiO4)6O2 = −14661.5 ± 32.2, Sm9.33∎0.67(SiO4)6O2 = −14561.7 ± 20.8, and Gd9.33∎0.67(SiO4)6O2 = −14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases

Metastable liquid-liquid coexistence and density anomalies in a core-softened fluid

Linearly-sloped or `ramp' potentials belong to a class of core-softened models which possess a liquid-liquid critical point (LLCP) in addition to the usual liquid-gas critical point. Furthermore they exhibit thermodynamic anomalies in the density and compressibility, the nature of which may be akin to those occurring in water. Previous simulation studies of ramp potentials have focused on just one functional form, for which the LLCP is thermodynamically stable. In this work we construct a series of ramp potentials, which interpolate between this previously studied form and a ramp-based approximation to the Lennard-Jones (LJ) potential. By means of Monte Carlo simulation, we locate the LLCP, the first order high density liquid (HDL)-low density liquid (LDL) coexistence line, and the line of density maxima for a selection of potentials in the series. We observe that as the LJ limit is approached, the LLCP becomes metastable with respect to freezing into a hexagonal close packed crystalline solid. The qualitative nature of the phase behaviour in this regime shows a remarkable resemblance to that seen in simulation studies of accurate water models. Specifically, the density of the liquid phase exceeds that of the solid; the gradient of the metastable LDL-HDL line is negative in the pressure (p)-temperature (T) plane; while the line of density maxima in the p-T plane has a shape similar to that seen in water and extends well into the {\em stable} liquid region of the phase diagram. As such, our results lend weight to the `second critical point' hypothesis as an explanation for the anomalous behaviour of water.Comment: 7 pages, 8 figure

Reasoned action approach and compliance with recommended behaviours to prevent the transmission of the SARS‐CoV‐2 virus in the UK

Objectives To examine associations between demographics, people’s beliefs, and compliance with behaviours recommended by the UK government to prevent the transmission of the SARS‐CoV‐2 virus that causes COVID‐19. Design A two‐wave online survey conducted one week apart during the national lockdown (April, 2020). Measures A sample of 477 UK residents completed baseline measures from the reasoned action approach (experiential attitudes, instrumental attitudes, injunctive norms, descriptive norms, capacity, autonomy, and intention) and perceived susceptibility for each of the following recommended behaviours: limiting leaving home, keeping at least 2 m away from other people when outside and when inside shops, not visiting or meeting friends or other family members, and washing hands when returning home. Self‐reported compliance with each of the recommended behaviours was assessed one week later. Results Rates of full compliance with the recommended behaviours ranged from 31% (keeping at least 2 m away from other people when inside shops) to 68% (not visiting or meeting friends or other family members). Capacity was a significant predictor of compliance with each of the five recommended behaviours. Increasing age and intentions were also predictive of compliance with three of the behaviours. Conclusions Interventions to increase compliance with the recommended behaviours to prevent the transmission of the SARS‐CoV‐2 virus, especially those relating to social distancing, need to bolster people’s intentions and perceptions of capacity. This may be achieved through media‐based information campaigns as well as environmental changes to make compliance with such measures easier. Such interventions should particularly target younger adults

Modelling colloids with Baxter's adhesive hard sphere model

The structure of the Baxter adhesive hard sphere fluid is examined using computer simulation. The radial distribution function (which exhibits unusual discontinuities due to the particle adhesion) and static structure factor are calculated with high accuracy over a range of conditions and compared with the predictions of Percus--Yevick theory. We comment on rigidity in percolating clusters and discuss the role of the model in the context of experiments on colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in colloidal systems with short-range attractive forces", Messina, December 2003

Critical Point Field Mixing in an Asymmetric Lattice Gas Model

The field mixing that manifests broken particle-hole symmetry is studied for a 2-D asymmetric lattice gas model having tunable field mixing properties. Monte Carlo simulations within the grand canonical ensemble are used to obtain the critical density distribution for different degrees of particle-hole asymmetry. Except in the special case when this asymmetry vanishes, the density distributions exhibit an antisymmetric correction to the limiting scale-invariant form. The presence of this correction reflects the mixing of the critical energy density into the ordering operator. Its functional form is found to be in excellent agreement with that predicted by the mixed-field finite-size-scaling theory of Bruce and Wilding. A computational procedure for measuring the significant field mixing parameter is also described, and its accuracy gauged by comparing the results with exact values obtained analytically.Comment: 10 Pages, LaTeX + 8 figures available from author on request, To appear in Z. Phys.

Wetting of a symmetrical binary fluid mixture on a wall

We study the wetting behaviour of a symmetrical binary fluid below the demixing temperature at a non-selective attractive wall. Although it demixes in the bulk, a sufficiently thin liquid film remains mixed. On approaching liquid/vapour coexistence, however, the thickness of the liquid film increases and it may demix and then wet the substrate. We show that the wetting properties are determined by an interplay of the two length scales related to the density and the composition fluctuations. The problem is analysed within the framework of a generic two component Ginzburg-Landau functional (appropriate for systems with short-ranged interactions). This functional is minimized both numerically and analytically within a piecewise parabolic potential approximation. A number of novel surface transitions are found, including first order demixing and prewetting, continuous demixing, a tricritical point connecting the two regimes, or a critical end point beyond which the prewetting line separates a strongly and a weakly demixed film. Our results are supported by detailed Monte Carlo simulations of a symmetrical binary Lennard-Jones fluid at an attractive wall.Comment: submitted to Phys. Rev.

Interfacial tension of the isotropic--nematic interface in suspensions of soft spherocylinders

The isotropic to nematic transition in a system of soft spherocylinders is studied by means of grand canonical Monte Carlo simulations. The probability distribution of the particle density is used to determine the coexistence density of the isotropic and the nematic phases. The distributions are also used to compute the interfacial tension of the isotropic--nematic interface, including an analysis of finite size effects. Our results confirm that the Onsager limit is not recovered until for very large elongation, exceeding at least L/D=40, with L the spherocylinder length and D the diameter. For smaller elongation, we find that the interfacial tension increases with increasing L/D, in agreement with theoretical predictions.Comment: 8 pages, 7 figures, and also 1 tabl

Phase behaviour of a symmetrical binary fluid mixture

We have investigated the phase behaviour of a symmetrical binary fluid mixture for the situation where the chemical potentials $\mu_1$ and $\mu_2$ of the two species differ. Attention is focused on the set of interparticle interaction strengths for which, when $\mu_1=\mu_2$, the phase diagram exhibits both a liquid-vapor critical point and a tricritical point. The corresponding phase behaviour for the case $\mu_1\ne\mu_2$ is investigated via integral-equation theory calculations within the mean spherical approximation (MSA), and grand canonical Monte Carlo (GCMC) simulations. We find that two possible subtypes of phase behaviour can occur, these being distinguished by the relationship between the critical lines in the full phase diagram in the space of temperature, density, and concentration. We present the detailed form of the phase diagram for both subtypes and compare with the results from GCMC simulations, finding good overall agreement. The scenario via which one subtype evolves into the other, is also studied, revealing interesting features.Comment: 22 pages, 13 figure