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We present the first systematic application of the integral equation implementation of the replica method to the study of arrested states in fluids with microscopic competing interactions (short-range attractive and long-range repulsive, SALR), as exemplified by the prototype Lennard-Jones–Yukawa model. Using a wide set of potential parameters, we provide as many as 11 different phase diagrams on the density (ρ)–temperature (T) plane, embodying both the cluster-phase boundary, TC(ρ), and the locus below which arrest takes place, TD(ρ). We describe how the interplay between TC and TD—with the former falling on top of the other, or the other way around, depending on thermodynamic conditions and potential parameters—gives rise to a rich variety of non-ergodic states interspersed with ergodic ones, of which both the building blocks are clusters or single particles. In a few cases, we find that the TD locus does not extend all over the density range subtended by the TC envelope; under these conditions, the λ-line is within reach of the cluster fluid, with the ensuing possibility to develop ordered microphases. Whenever a comparison is possible, our predictions favorably agree with previous numerical results. Thereby, we demonstrate the reliability and effectiveness of our scheme to provide a unified theoretical framework for the study of arrested states in SALR fluids, irrespective of their nature. [ABSTRACT FROM AUTHOR]

In a binary mixture, stripes refer to a one-dimensional periodicity of the composition, namely, a regular alternation of layers filled with particles of mostly one species. We have recently introduced [Munaò et al., Phys. Chem. Chem. Phys. 25, 16227 (2023)] a model that possibly provides the simplest binary mixture endowed with stripe order. The model consists of two species of identical hard spheres with equal concentration, which mutually interact through a square-well potential. In that paper, we have numerically shown that stripes are present in both liquid and solid phases when the attraction range is rather long. Here, we study the phase behavior of the model in terms of a density functional theory capable to account for the existence of stripes in the dense mixture. Our theory is accurate in reproducing the phases of the model, at least insofar as the composition inhomogeneities occur on length scales quite larger than the particle size. Then, using Monte Carlo simulations, we prove the existence of solid stripes even when the square well is much thinner than the particle diameter, making our model more similar to a real colloidal mixture. Finally, when the width of the attractive well is equal to the particle diameter, we observe a different and more complex form of compositional order in the solid, where each species of particle forms a regular porous matrix holding in its holes the other species, witnessing a surprising variety of emergent behaviors for a very basic model of interaction. [ABSTRACT FROM AUTHOR]

We study theoretically a prototype hard-sphere two-Yukawa model with competing interactions, under thermodynamic conditions associated with the formation of clusters. We adopt the analytically solvable random phase approximation and show that this theory predicts reasonably well the structure of the fluid—in comparison with exact Monte Carlo results—within a unique parameterization of the direct correlation function inside the hard core of particles. In particular, the theory follows correctly the development, in the structure factor, of a local peak at low wavevectors, as peculiarly associated with the onset of aggregation. We then model the direct correlation function in the same wavevector regime by a Gaussian function, so as to systematically investigate, in a "reverse" scheme, how varying the properties of the local peak modifies the original underlying competing interaction. We show that large variations in the height of the peak are generally associated with comparatively smaller variations in the height of the microscopic repulsive barrier; moreover, the shrinking and shifting towards lower wavevectors of the peak may be interpreted in terms of the displacement of the barrier, producing a substantial enlargement of the range of both the attractive and repulsive contributions to the interaction potential. Finally, we document the way the repulsive barrier tends to vanish as the two-Yukawa fluid approaches a "simple fluid" behavior, heralding the onset of a liquid-vapor phase separation. [ABSTRACT FROM AUTHOR]

The emergence of supramolecular aggregates from simple microscopic interaction rules is a fascinating feature of complex fluids which, besides its fundamental interest, has potential applications in many areas, from biological self-assembly to smart material design. We here investigate by Monte Carlo simulation the equilibrium structure of a two-dimensional mixture of asymmetric dimers and spheres (disks). Dimers and disks are hard particles, with an additional short-range attraction between a disk and the smaller monomer of a dimer. The model parameters and thermodynamic conditions probed are typical of colloidal fluid mixtures. In spite of the minimalistic character of the interaction, we observe--upon varying the relative concentration and size of the two colloidal species--a rich inventory of mesoscale structures at low temperature, such as clusters, lamellæ (i.e., polymer-like chains), and gel-like networks. For colloidal species of similar size and near equimolar concentrations, a dilute fluid of clusters gives way to floating lamellæ upon cooling; at higher densities, the lamellæ percolate through the simulation box, giving rise to an extended network. A crystal-vapour phase-separation may occur for a mixture of dimers and much larger disks. Finally, when the fluid is brought in contact with a planar wall, further structures are obtained at the interface, from layers to branched patterns, depending on the nature of wall-particle interactions. [ABSTRACT FROM AUTHOR]

Extensive molecular dynamics simulations show that a short range central potential, suited to model C(sub 60), undergoes a high temperature transition to a glassy phase characterized by the positional disorder of the constituent particle. The effective hard-core exclusion plays an important role in a short role potential well suited to model various fullerenes and mode coupling theory calculations for the onset of the glass transition in a short range model of a globular protein solutions leads to low value of decay potential parameters.

Specialized Monte Carlo methods are nowadays routinely employed, in combination with thermodynamic integration (TI), to locate phase boundaries of classical many-particle systems. This is especially useful for the fluid-solid transition, where a critical point does not exist and both phases may notoriously go deeply metastable. Using the Lennard-Jones model for demonstration, we hereby investigate on the alternate possibility of tracing reasonably accurate transition lines directly by integrating the pressure equation of state computed in a canonical-ensemble simulation with local moves. The recourse to this method would become a necessity when the stable crystal structure is not known. We show that, rather counterintuitively, metastability problems can be alleviated by reducing (rather than increasing) the size of the system. In particular, the location of liquid-vapor coexistence can exactly be predicted by just TI. On the contrary, TI badly fails in the solid-liquid region, where a better assessment (to within 10% accuracy) of the coexistence pressure can be made by following the expansion, until melting, of the defective solid which has previously emerged from the decay of the metastable liquid. [ABSTRACT FROM AUTHOR]

We study the transient behavior of a model fluid composed by soft repulsive spheres subjected to a planar uniform shear. To this aim, we use a dynamical non-equilibrium molecular dynamics method originally developed by Ciccotti and Jacucci [Phys. Rev. Lett. 35, 789 (1975)] and recently applied to the study of the transient regimes in various fluid systems. We show that the dynamical method allows one to study the transient behavior of the viscous time-dependent response over a wide range of applied shear rates, provided that a temperature control is enforced on the system. In this study, we adopt in particular the configurational thermostat of Braga and Travis [J. Chem. Phys. 123, 134101 (2005)]. The initial behavior of the dynamical response to a θ-like perturbation is characterized by a rapid increase, culminating in a pronounced peak, later relaxing to a plateau value. The latter positively reproduces the values of the viscosity observed in standard steady-state non-equilibrium molecular dynamics. [ABSTRACT FROM AUTHOR]

The formation of aggregates is commonly observed in soft matter such as globular protein solutions and colloidal suspensions. A lively debated issue concerns the possibility to discriminate between a generic intermediate-range order taking place in the fluid, as contrasted with the more specific presence of a clustered state. Recently, we have predicted by Monte Carlo simulations of a standard colloidal model — spherical particles interacting via a short-range attraction followed by a screened electrostatic repulsion at larger distances — the existence of a tiny structural change occurring in the pair structure. This change consists in a reversal of trend affecting a portion of the local density as the attractive strength increases, that is shown to take place precisely at the clustering threshold. Here, we address the same issue by refined thermodynamically self-consistent integral equation theories of the liquid state. We document how such theoretical schemes positively account for the observed phenomenology, highlighting their accuracy to finely describe the aggregation processes in model fluids with microscopic competing interactions. [ABSTRACT FROM AUTHOR]

An accurate thermodynamically self-consistent integral equation theory of the liquid state is used to investigate model fluids with competing attractive interaction at short distances and long-range repulsion, focusing on thermodynamic states where the formation of clusters is expected to occur. We find a remarkable accuracy of theoretical predictions, through a detailed assessment against results of Monte Carlo simulations. The behavior of theoretical radial distribution functions and structure factors faithfully follows the onset and growth of cluster aggregates in the homogeneous dense-vapor phase. The thermodynamic properties of the system sensitively depend on the ratio between the repulsive barrier and the attraction strength. We elucidate the role of accurate theoretical tools to investigate the properties of fluids with complex phase behaviors. [ABSTRACT FROM AUTHOR]

Standard Monte Carlo simulations are carried out to assess the accuracy of theoretical predictions for the structural properties of a model fluid interacting through a hard-core two-Yukawa potential composed of a short-range attractive well next to a hard repulsive core, followed by a smooth, long-range repulsive tail. Theoretical calculations are performed in the framework provided by the Ornstein-Zernike equation, solved either analytically with the mean spherical approximation (MSA) or iteratively with the hypernetted-chain (HNC) closure. Our analysis shows that both theories are generally accurate in a thermodynamic region corresponding to a dense vapor phase around the critical point. For a suitable choice of potential parameters, namely, when the attractive well is deep and/or large enough, the static structure factor displays a secondary low-Q peak. In this case HNC predictions closely follow the simulation results, whereas MSA results progressively worsen the more pronounced this low-Q peak is. We discuss the appearance of such a peak, also experimentally observed in colloidal suspensions and protein solutions, in terms of the formation of equilibrium clusters in the homogeneous fluid. [ABSTRACT FROM AUTHOR]

The phase diagrams of two protein solutions, lysozyme and gamma-crystallin in water and added salt, are determined within a short-range, pair-potential representation of macroparticle interactions. The results indicate the minimal assumptions about the nature of the microscopic potential allowing a satisfactory reproduction of the phase-diagram topology of globular protein solutions.