Glass transition and reversible gelation in asymmetric binary mixtures: A study by mode coupling theory and molecular dynamics.

The glass transition and the binodals of asymmetric binary mixtures are investigated from the effective fluid approach in the mode coupling theory and by molecular dynamics. Motivated by previous theoretical predictions, the hard-sphere mixture and the Asakura-Oosawa models are used to analyze experimental results from the literature, relative to polystyrene spheres mixed either with linear polymers or with dense microgel particles. In agreement with the experimental observations, the specificity of the depletant particles is shown to favor lower density gels. It further favors equilibrium gelation by reducing also the tendency of the system to phase separate. These results are confirmed by a phenomenological modification of the mode coupling theory in which the vertex functions are computed at an effective density lower than the actual one. A model effective potential in asymmetric mixtures of hard particles is used to further check this phenomenological modification against molecular dynamics simulation.

α-relaxation, shear viscosity, and elastic moduli of hard-particle fluids from a mode-coupling theory with a retarded vertex.

The recently proposed modification of the mode-coupling theory (MCT) in which the static structure used in the vertex is computed at a lower density than the actual one is tested on several dynamics-related properties. The predictions from this modified version of MCT calibrated on the one-component hard-sphere fluid are found in very good agreement with simulation data for one-component and binary hard-sphere fluids. They are also relevant for the stress moduli for models with attractive tails beyond the hard core. The clear improvement observed on several properties should give a new impetus to the use of MCT as a quantitative tool.

Bond lifetime and diffusion coefficient in colloids with short-range interactions.

We use molecular dynamics simulations to study the influence of short-range structures in the interaction potential between hard-sphere-like colloidal particles. Starting from model potentials and effective potentials in binary mixtures computed from the Ornstein-Zernike equations, we investigate the influence of the range and strength of a possible tail beyond the usual core repulsion or the presence of repulsive barriers. The diffusion coefficient and mean "bond" lifetimes are used as indicators of the effect of this structure on the dynamics. The existence of correlations between the variations of these quantities with the physical parameters is discussed to assess the interpretation of dynamics slowing down in terms of long-lived bonds. We also discuss the question of a universal behaviour determined by the second virial coefficient B ((2)) and the interplay of attraction and repulsion. While the diffusion coefficient follows the B ((2)) law for purely attractive tails, this is no longer true in the presence of repulsive barriers. Furthermore, the bond lifetime shows a dependence on the physical parameters that differs from that of the diffusion coefficient. This raises the question of the precise role of bonds on the dynamics slowing down in colloidal gels.

Non-ergodicity transition and multiple glasses in binary mixtures: on the accuracy of the input static structure in the mode coupling theory.

We examine the question of the accuracy of the static correlation functions used as input in the mode coupling theory (MCT) of non-ergodic states in binary mixtures. We first consider hard-sphere mixtures and compute the static pair structure from the Ornstein-Zernike equations with the Percus-Yevick closure and more accurate ones that use bridge functions deduced from Rosenfeld's fundamental measures functional. The corresponding MCT predictions for the non-ergodicity lines and the transitions between multiple glassy states are determined from the long-time limit of the density autocorrelation functions. We find that while the non-ergodicity transition line is not very sensitive to the input static structure, up to diameter ratios D(2)/D(1) = 10, quantitative differences exist for the transitions between different glasses. The discrepancies with the more accurate closures become even qualitative for sufficiently asymmetric mixtures. They are correlated with the incorrect behavior of the PY structure at high size asymmetry. From the example of ultra-soft potential it is argued that this issue is of general relevance beyond the hard-sphere model.

Mode-coupling theory for the glass transition: test of the convolution approximation for short-range interactions.

We reexamine the convolution approximation commonly used in the mode-coupling theory (MCT) of nonergodic states of classical fluids. This approximation concerns the static correlation functions used as input in the MCT treatment of the dynamics. Besides the hard-sphere model, we consider interaction potentials that present a short-range tail, either attractive or repulsive, beyond the hard core. By using accurate static correlation functions obtained from the fundamental measures functional for hard spheres, we show that the role of three-body direct correlations can be more significant than what is inferred from previous simple ansatzs for pure hard spheres. This may in particular impact the location of the glass transition line and the nonergodicity parameter.

Effect of residual attractive interactions in size asymmetric colloidal mixtures: Theoretical analysis and predictions.

We analyze the influence of residual attractions on the static and some dynamic properties of size asymmetric mixtures of "hard-sphere-like" colloids. These attractions, usually neglected in the theoretical analysis, are characterized by a very short range and a moderate strength reflecting the underlying microscopic structure of the colloidal particles. Their effect on the potentials of mean force is analyzed from analytical expressions obtained from low density expansions. The effective potential of the big particle fluid is next considered. An analytical expression is proposed for estimating the deviation with respect to the hard sphere depletion potential. This case is compared to that of mixtures with noninteracting depletants. The important consequences on the binodals and the glass transition lines of the effective fluid are discussed in both cases. This study is next extended to other properties-the specific heat and the low shear viscosity-which incorporate contributions from the two components of the binary mixture.

Gelation and phase coexistence in colloidal suspensions with short-range forces: generic behavior versus specificity.

The interplay between physical gelation and equilibrium phase transitions in asymmetric binary mixtures is analyzed from the effective fluid approach, in which the big particles interact via a short-range effective attraction beyond the core due to the depletion mechanism. The question of the universality of the scenario for dynamical arrest is then addressed. The comparison of the phase diagrams of the hard-sphere mixture and the Asakura-Oosawa models at various size ratios shows that strong specificity is observed for nonideal depletants. In particular, equilibrium gelation, without the competition with fluid-fluid transition is possible in mixtures of hard-sphere colloids. This is interpreted from the specificities of the effective potential, such as its oscillatory behavior and its complex variation with the physical parameters. The consequences on the dynamical arrest and the fluid-fluid transition are then investigated by considering in particular the role of the well at contact and the first repulsive barrier. This is done for the actual effective potential in the hard-sphere mixture and for a square well and shoulder model, which allows a separate discussion of the role of the different parameters, in particular on the localization length and the escape time. This study is next extended to mixtures of "hard-sphere-like" colloids with residual interactions. It confirms the trends relative to equilibrium gelation and illustrates a diversity of the phase behavior well beyond the scenarios expected from simple models.

Equilibrium route to colloidal gelation: mixtures of hard-sphere-like colloids.

The binodals and the nonergodicity lines of a binary mixture of hard-sphere-like particles with a large size ratio are computed for studying the interplay between dynamic arrest and phase separation in depletion-driven colloidal mixtures. Contrary to the case of hard core plus short-range effective attraction, physical gelation without competition with the fluid-phase separation can occur in such mixtures. This behavior due to the oscillations in the depletion potential should concern all simple mixtures with a nonideal depletant, justifying further studies of their dynamic properties.

Equilibrium and glassy states of the Asakura-Oosawa and binary hard sphere mixtures: effective fluid approach.

Motivated by recent experimental results on model binary colloidal mixtures, especially for the glass transition, we investigate the phase diagram of two models of asymmetric binary mixtures: the hard sphere and the Asakura-Oosawa mixtures. This includes the binodals and the glass transition line, computed in the effective one-component representation using the corresponding potentials of mean force at infinite dilution. The reference hypernetted chain approximation is used for computing the static properties and the glass transition line is computed in the mode coupling approximation. The similarities and the differences between the two models are discussed for different size ratios. It is shown that while both models follow a universal behavior at large asymmetry, the hard sphere mixture model leads to more original results at moderate size ratio. These results show that a modeling beyond generic effective potentials might be necessary for an appropriate description of the complete phase diagram.

When mixtures of hard-sphere-like colloids do not behave as mixtures of hard spheres.

The validity of the concept of "hard-sphere-like" particles for mixtures of colloids is questioned from a theoretical point of view. This concerns the class of pseudobinary mixtures in which the nonsteric interactions between the colloids are "residual" (with very small range and moderate strength). It is shown that contrary to common expectation, such interactions may have unexpected consequences on the theoretical phase diagram. The distinction between this situation and true solute-solvent mixtures is emphasized.

Is the binary hard-sphere mixture a good reference system for sterically stabilized colloids?

The relevance of the hard-sphere mixture model as a starting point for the study of sterically stabilized colloids is discussed. Two physical situations are distinguished: true molecular solvent-colloid mixtures, and pseudobinary mixtures of two supramolecular objects. For the former, the limitation of the hard-sphere mixture model are recalled. Its potential use as a reference system for perturbation treatments is then analyzed. The accuracy of the latter is tested numerically. This study shows that the hard-sphere mixture is, in general, not a good reference system for sterically stabilized colloids in molecular solvents. For pseudobinary mixtures, the potential of mean force between the bigger solutes induced by the smaller ones is considered. The influence of a very short-range heteroattraction is discussed.

[Value and limitations of coronary artery imaging with the MRI navigator technique. Comparison with coronary angiography results in 37 patients]. / Intérêt et limites actuelles de l'imagerie des artères coronaires avec la technique d'IRM du navigateur. Comparaison avec les données coronaro-angiographiques chez 37 patients.

The introduction of a non-invasive method of imaging the coronary arteries would be a great advance in daily cardiological practice. The authors report their experience of imaging the coronary arteries with 1 Tesla MRI using the "navigator technique". Twenty-five sections 1.2 mm thick, focused on the proximal left coronary artery, were acquired with a 512 matrix, without injecting contrast during normal respiration with a tolerance on the portion of the right diaphragmatic cupola of 5 mm. Analysis of the coronary segments included in the field of view was performed on native sections after curve reconstruction and on targetedMIP series. A comparison of the results with respect to conventional coronary angiography showed a relatively limited visualisation of the proximal coronary segments because, in addition to the impossibility of carrying out the investigation in 24% of cases (faulty cardiac or respiratory synchronisation, poor signal/noise ratio), only 93% of the left main coronary and 75% of the proximal left anterior descending arteries could be visualised. In the analyzable segments, the diagnostic performances were modest with a global sensitivity of 60.8% and specificity of 91%. With the exception of the left main coronary artery, the sensitivities observed did not make MRI of the coronary arteries a rival to conventional coronary angiography. These limited performances may be explained by the lack of rapidity of the sequences of acquisition compared to the rapid motion of the structures under investigation whose dimensions are 5 to 10 times smaller than their amplitude of excursion. Technical developments are regularly accomplished in this domain, especially 3rd generation sequences in apnoea with injection of contrast media. At present, despite some results reported in the literature, angio-MRI of the coronary arteries cannot be used reliably to guide clinical decisions in coronary artery disease with the exception of some situations like congenital abnormalities of the coronary arteries, non-invasive follow-up of coronary aneurysms or analysis of the left main coronary artery.

Validity of the perturbation theory for hard particle systems with very-short-range attraction.

Motivated by recent studies of colloidal systems at the effective one-component level, the validity of the first-order perturbation theory of classical systems of hard particles interacting via short-range potentials is investigated. The influence of the physical parameters on the accuracy of the perturbation theory is examined. It is shown that this simple method is intrinsically appropriate to describe the fluid-solid transition. Concerning the fluid-fluid one, the first-order perturbation theory provides acceptable results when the interaction range is not too small. For very-short-range potentials it systematically leads to an unphysical fluid-fluid transition. In the case of the depletion interaction between hard sphere solutes such a transition is found even at moderate size asymmetry. It is finally shown that the perturbation theory is not more appropriate for an extended solid than for a liquid with the same density, thus making difficult a quantitative description of the isostructural solid-solid transition.