Biomedical subjects

G Foffi

Publications and source records attributed to G Foffi.

11 recordsLinked to original sources

PubMed2005

Arrested phase separation in a short-ranged attractive colloidal system: a numerical study.

We numerically investigate the competition between phase separation and dynamical arrest in a colloidal system interacting via a short-ranged attractive potential. Equilibrium fluid configurations are quenched at two different temperatures below the critical temperature and followed during their time evolution. At the lowest studied T, the phase-separation process is interrupted by the formation of an attractive glass in the dense phase. At the higher T, no arrest is observed and the phase-separation process proceeds endlessly in the simulated time window. The final structure of the glass retains memory of the interrupted phase-separation process in the form of a frozen spinodal decomposition peak, whose location and amplitude is controlled by the average packing fraction. We also discuss the time evolution of the nonergodicity parameter, providing evidence of a progressively decreasing localization length on increasing the packing fraction. Finally, we confirm that the reported results are independent of the microscopic dynamics.

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PubMed2005

Scaling of dynamics with the range of interaction in short-range attractive colloids.

We numerically study the dependence of the dynamics on the range of interaction Delta for the short-range square well potential. We find that, for small Delta, dynamics scale exactly in the same way as thermodynamics, both for Newtonian and Brownian microscopic dynamics. For interaction ranges from a few percent down to the Baxter limit, the relative location of the attractive-glass line and the liquid-gas line does not depend on Delta. This proves that, in this class of potentials, disordered arrested states (gels) can be generated only as a result of a kinetically arrested phase separation.

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PubMed2004

Star polymers: a study of the structural arrest in the presence of attractive interactions.

Simulations 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.

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PubMed2004

Aging in short-ranged attractive colloids: a numerical study.

We study the aging dynamics in a model for dense simple liquids, in which particles interact through a hard-core repulsion complemented by a short-ranged attractive potential, of the kind found in colloidal suspensions. In this system, at large packing fractions, kinetically arrested disordered states can be created both on cooling (attractive glass) and on heating (repulsive glass). The possibility of having two distinct glasses, at the same packing fraction, with two different dynamics offers the unique possibility of comparing-within the same model-the differences in aging dynamics. We find that, while the aging dynamics of the repulsive glass is similar to the one observed in atomic and molecular systems, the aging dynamics of the attractive glass shows novel unexpected features.

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PubMed2004

alpha-Relaxation processes in binary hard-sphere mixtures.

Molecular-dynamics simulations are presented for two correlation functions formed with the partial density fluctuations of binary hard-sphere mixtures in order to explore the effects of mixing on the evolution of glassy dynamics upon compressing the liquid into high-density states. Partial-density-fluctuation correlation functions for the two species are reported. Results for the alpha-relaxation process are quantified by parameters for the strength, stretching, and time scale, where the latter varies over almost four orders of magnitude upon compression. The parameters exhibit an appreciable dependence on the wave vector, and this dependence is different for the correlation function referring to the smaller and that for the larger species. These features are shown to be in semiquantitative agreement with those calculated within the mode-coupling theory for ideal liquid-glass transitions.

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PubMed2003

Mixing effects for the structural relaxation in binary hard-sphere liquids.

We report extensive molecular-dynamics-simulation results for binary mixtures of hard spheres for different size disparities and different mixing percentages, for packing fractions up to 0.605, and over a characteristic time interval spanning up to 5 orders in magnitude. We explore the changes in the evolution of glassy dynamics due to mixing and discover two opposite scenarios: For large size disparity, increasing the mixing percentage of small particles leads to a speed up of long-time dynamics, while small disparity leads to a slowing down. These results agree with predictions based on the mode-coupling theory for ideal-glass transitions.

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PubMed2003

Structural arrest in dense star-polymer solutions.

The dynamics of star polymers is investigated via extensive molecular and Brownian dynamics simulations for a large range of functionality f and packing fraction eta. The calculated isodiffusivity curves display both minima and maxima as a function of eta and minima as a function of f. Simulation results are compared with theoretical predictions based on different approximations for the structure factor. In particular, the ideal glass transition line predicted by mode-coupling theory is shown to exactly track the isodiffusivity curves, offering a theoretical understanding for the observation of disordered arrested states in star-polymer solutions.

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PubMed2002

Confirmation of anomalous dynamical arrest in attractive colloids: a molecular dynamics study.

Previous theoretical simulation and experimental studies have indicated that particles with a short-ranged attraction exhibit a range of dynamical arrest phenomena. These include very pronounced reentrance in the dynamical arrest curve, a logarithmic singularity in the density correlation functions, and the existence of "attractive" and "repulsive" glasses. Here we carry out extensive molecular dynamics calculations on dense systems interacting via a square-well potential. This is one of the simplest systems with the required properties, and may be regarded as canonical for interpreting the phase diagram, and now also the dynamical arrest. We confirm the theoretical predictions for reentrance, logarithmic singularity, and give a direct evidence of the existence, independent of theory, of two distinct glasses. We now regard the previous predictions of these phenomena as having been established.

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PubMed2002

Evidence for an unusual dynamical-arrest scenario in short-ranged colloidal systems.

Extensive molecular dynamics simulation studies of particles interacting via a short-ranged attractive square-well potential are reported. The calculated loci of constant diffusion coefficient D in the temperature-packing fraction plane show a reentrant behavior, i.e., an increase of diffusivity on cooling, confirming an important part of the high volume-fraction dynamical-arrest scenario earlier predicted by theory for particles with short-ranged potentials. The more efficient localization mechanism induced by the short-range bonding provides, on average, additional free volume as compared to the hard-sphere case and results in faster dynamics.

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PubMed2001

Mechanical properties of a model of attractive colloidal solutions.

We review the nature of glass transitions and the glasses arising from a square-well potential with a narrow and deep well. Our discussion is based on the mode coupling theory (MCT), and the predictions of glasses that we make refer to the "ideal" glasses predicted by this theory. We believe that the square-well system well represents colloidal particles with attractive interactions produced by grafted polymers, or depletion interactions. It has been recently shown that two types of glasses, an attractive and a repulsive one, are predicted by MCT for this model. The former can form at quite low densities. Most of our attention is directed at the mechanical properties of the glasses predicted by this theory. In particular we calculate the elastic shear modulus at zero frequency and the longitudinal stress modulus in the long wavelength limit. Results for both are presented along the glass-liquid transition curves and their interesting behavior is explained in terms of the underlying physics of the system.

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PubMed2000

Higher-order glass-transition singularities in colloidal systems with attractive interactions.

The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third-order bifurcation point, i.e., in a A3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth-order A4 (swallow-tail) singularity at a critical well width. Close to the A3 and A4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.

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