Structural Arrest in Dense Star-Polymer Solutions

Giuseppe Foffi, Francesco Sciortino
2003
Article

Résumé

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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https://infoscience.epfl.ch/record/143974?ln=fr

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Giuseppe Foffi, Francesco Sciortino
2003
Article

Résumé

Official source

https://infoscience.epfl.ch/record/143974?ln=fr

À propos de ce résultat

Concepts associés (14)

Concepts associés

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Publications associées (21)

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Publications associées

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Publications associées (21)

Hard sphere-like glass transition in eye lens alpha-crystallin solutions

Nicolas Dorsaz, Giuseppe Foffi

We study the equilibrium liquid structure and dynamics of dilute and concentrated bovine eye lens alpha-crystallin solutions, using small-angle X-ray scattering, static and dynamic light scattering, viscometry, molecular dynamics simulations, and mode-coupling theory. We find that a polydisperse Percus-Yevick hard-sphere liquid-structure model accurately reproduces both static light scattering data and small-angle X-ray scattering liquid structure data from alpha-crystallin solutions over an extended range of protein concentrations up to 290 mg/mL or 49% vol fraction and up to ca. 330 mg/mL for static light scattering. The measured dynamic light scattering and viscosity properties are also consistent with those of hard-sphere colloids and show power laws characteristic of an approach toward a glass transition at alpha-crystallin volume fractions near 58%. Dynamic light scattering at a volume fraction beyond the glass transition indicates formation of an arrested state. We further perform event-driven molecular dynamics simulations of polydisperse hard-sphere systems and use mode-coupling theory to compare the measured dynamic power laws with those of hard-sphere models. The static and dynamic data, simulations, and analysis show that aqueous eye lens alpha-crystallin solutions exhibit a glass transition at high concentrations that is similar to those found in hard-sphere colloidal systems. The alpha-crystallin glass transition could have implications for the molecular basis of presbyopia and the kinetics of molecular change during cataractogenesis.

Natl Acad Sciences2014

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Dynamics of supercooled liquids: Density fluctuations and mode coupling theory

Giuseppe Foffi, Francesco Sciortino

We write equations of motion for density variables that are equivalent to Newton's equations. We then propose a set of trial equations parametrized by two unknown functions to describe the exact equations. These are chosen to best fit the exact Newtonian equations. Following established ideas, we choose to separate these trial functions into a set representing integrable motions of density waves, and a set containing all effects of non-integrability. The density waves are found to have the dispersion of sound waves, and this ensures that the interactions between the independent waves are minimized. Furthermore, it transpires that the static structure factor is fixed by this minimum condition to be the solution of the Yvon-Born-Green equation. The residual interactions between density waves are explicitly isolated in their Newtonian representation and expanded by choosing the dominant objects in the phase space of the system, that can be represented by a dissipative term with memory and a random noise. This provides a mapping between deterministic and stochastic dynamics. Imposing the fluctuation-dissipation theorem allows us to calculate the memory kernel. We write exactly the expression for it, following two different routes, i.e. using explicitly Newton's equations, or instead, their implicit form, that must be projected onto density pairs, as in the development of the well established mode coupling theory. We compare these two ways of proceeding, showing the necessity to enforce a new equation of constraint for the two schemes to be consistent. Thus, while in the first 'Newtonian' representation a simple Gaussian approximation for the random process leads easily to the mean spherical approximation for the statics and to MCT for the dynamics of the system, in the second case higher levels of approximation are required to have a fully consistent theory.

2002

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Gaussian density fluctuations and mode coupling theory for supercooled liquids

Giuseppe Foffi, Francesco Sciortino

The equations of motion for the density modes of a fluid, derived from Newton's equations, are written as a linear generalized Langevin equation. The constraint imposed by the fluctuation-dissipation theorem is used to derive an exact form for the memory function. The resulting equations, solved under the assumption that the noise, and consequently density fluctuations, of the liquid are Gaussian distributed, are equivalent to the random phase approximation for the static structure factor and to the well-known ideal mode coupling theory (MCT) equations for the dynamics. This finding suggests that MCT is a theory of fluid dynamics that becomes exact in a mean-field limit.

2001

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Dynamics of supercooled liquids: Density fluctuations and mode coupling theory

Giuseppe Foffi, Francesco Sciortino

2002