1,720,967 research outputs found
Fluid-fluid and fluid-solid phase separation in nonadditive asymmetric binary hard-sphere mixtures
No full textVery asymmetric mixtures of hard spheres naturally arise in the modellization of colloidal dispersions. Effective potentials have emerged as a powerful tool for describing these systems and have often been employed to extract the phase diagram in both the additive and nonadditive cases. However, most theoretical investigations have been carried out by means of mean-field-like approaches, so their quantitative accuracy remains to be assessed. Here we employ previously determined effective potentials for nonadditive hard-sphere mixtures to study the fluid-fluid phase transition by the hierarchical reference theory (HRT), which is designed to take realistically into account the effects of long-range fluctuations on phase separation. Fluid-solid equilibrium is addressed by supplementing HRT with thermodynamic perturbation theory for the solid phase. We apply this approach both to a potential with adjustable nonadditivity parameter (Louis et al 2000 Phys. Rev. E 61 R1028) and to the Asakura-Oosawa (AO) potential, which represents an extreme case of nonadditivity. Our results for the phase diagram, including modified hypernetted chain (MHNC) calculations, are compared to those of other liquid-state theories and are found to agree nicely with available simulation data. Unlike commonly adopted liquid-state theories, HRT is capable both of getting arbitrarily close to the fluid-fluid critical point, and of giving nontrivial critical exponents. In particular, the fluid-fluid coexistence curve is much flatter than that obtained via perturbation theory, in agreement with a recent finite-size scaling Monte Carlo analysis of the AO model
Star polymers: study of fluid–fluid transitions in a system with a repulsive ultrasoft-core
No full textWe study a model for star polymers in solution which, in addition to the ultrasoft repulsive interaction of entropic origin, has an attractive interpolymer interaction at longer range. This attraction can arise from a suitable tuning of the solvent and solute properties. For this model we study the phase diagram using mean-field theory and two fluid-state theories, the modified hypernetted chain (MHNC) integral equation and the hierarchical reference theory, and we explore star polymers with a different number of arms f ( f = 12, 24, 32, 40). All three theories give the same topology for the phase diagram in the presence of attraction. When the strength of the interaction is strong enough a fluid–fluid phase transition appears but the coexistence curve in the density–temperature (strength of attraction) bifurcates at a triple point into two lines of coexistence terminating at two critical points. This peculiar phase behaviour is related to the unusual form of the repulsive contribution Vrep(r): at low density and in a semidilute regime the soft-core Yukawa-like part of Vrep(r) is relevant, at higher densities the logarithmic, ultrasoft part of Vrep(r) is the relevant one. During our study we verify that the MHNC equation also gives a very accurate description of correlations for systems with an ultrasoft-core potential. 1
Effects of attractions on the dynamical arrest of soft colloids
No full textWe consider various scenarios leading to molecular control of the rheological state of concentrated solutions of soft colloids, based on the star polymer prototype system. At high values of star functionality, where a dynamically arrested state exists, we discuss the mechanisms that bring about a restoration of ergodicity upon addition of polymer chains, and the theoretical evidence for this effect which is supported by previously published experimental results. For intermediate functionalities, a systematic shift of the glass transition to higher star densities upon addition of additive-induced attractions is established. Finally, for low functionalities, we put forward evidence for a novel phenomenon that is specular to that appearing at high functionalities: a glassy state, which is absent for the pure star system, and that can be induced by the introduction of suitable additives. The latter phenomenon takes place when the effective star–star interaction is modified to display short-range attractions and longer-range repulsions
Star polymers with tunable interactions : cluster formation, phase separation, reentrant crystallization
No full textCritical behavior in colloid-polymer mixtures : theory and simulation
No full textWe extensively investigated the critical behavior of mixtures of colloids and polymers via the two-component Asakura-Oosawa model and its reduction to a one-component colloidal fluid using accurate theoretical and simulation techniques. In particular the theoretical approach, hierarchical reference theory [A. Parola and L. Reatto, Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of long-range fluctuations on phase separation giving exponents which differ strongly from their mean-field values, and are in good agreement with those of the three-dimensional Ising model. Computer simulations combined with finite-size scaling analysis confirm the Ising universality and the accuracy of the theory, although some discrepancy in the location of the critical point between one-component and full-mixture description remains. To assess the limit of the pair-interaction description, we compare one-component and two-component results
Star polymers: A study of the structural arrest in the presence of attractive interactions
No full textSimulations and mode-coupling theory calculations, for a large range of the arm number f and packing fraction eta have shown that the structural arrest and the dynamics of star polymers in a good solvent are extremely rich: the systems show a reentrant melting of the disordered glass nested between two stable fluid phases that strongly resemble the equilibrium phase diagram. Starting from a simple model potential we investigate the effect of the interplay between attractive interactions of different range and ultrasoft core repulsion, on the dynamics and on the occurrence of the ideal glass transition line. In the two cases considered so far, we observed some significant differences with respect to the purely repulsive pair interaction. We also discuss the interplay between equilibrium and nonequilibrium phase behavior. The accuracy of the theoretical tools we utilized in our investigation has been checked by comparing the results with molecular dynamics simulations
An investigation of critical and non critical correlations in model colloidal fluids
No full textWe present a study of the correlation length ξ of a hard-core fluid with three different tail interactions, namely, the Asakura–Oosawa potential, an attractive Yukawa tail, and a double Yukawa tail which is attractive at short distance and repulsive at long distance. The correlation length is obtained along the critical isochore in the whole range of supercritical temperatures by determining the leading pole of the structure factor S(k) in the complex plane, where S(k) is obtained from the hierarchical reference theory. We focus on how ξ changes from its Ornstein–Zernike form in the critical region to its high-temperature limit, and find instances of both the Fisher–Widom and Kirkwood scenarios for the monotonic to oscillatory crossover of the decay of correlations at long distance
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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