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L Berthier

Publications and source records attributed to L Berthier.

13 recordsLinked to original sources
PubMed2005

Direct experimental evidence of a growing length scale accompanying the glass transition.

Understanding glass formation is a challenge, because the existence of a true glass state, distinct from liquid and solid, remains elusive: Glasses are liquids that have become too viscous to flow. An old idea, as yet unproven experimentally, is that the dynamics becomes sluggish as the glass transition approaches, because increasingly larger regions of the material have to move simultaneously to allow flow. We introduce new multipoint dynamical susceptibilities to estimate quantitatively the size of these regions and provide direct experimental evidence that the glass formation of molecular liquids and colloidal suspensions is accompanied by growing dynamic correlation length scales.

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PubMed2004

Heterogeneous dynamics of coarsening systems.

We show by means of experiments, theory, and simulations that the slow dynamics of coarsening systems displays dynamic heterogeneity similar to that observed in glass-forming systems. We measure dynamic heterogeneity via novel multipoint functions which quantify the emergence of dynamic, as opposed to static, correlations of fluctuations. Experiments are performed on a coarsening foam using time-resolved correlation, a recently introduced light scattering method. Theoretically we study the Ising model, and present exact results in one dimension, and numerical results in two dimensions. For all systems the same dynamic scaling of fluctuations with domain size is observed.

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PubMed2003

Shear localization in a model glass.

Using molecular dynamics simulations, we show that a simple model of a glassy material exhibits the shear localization phenomenon observed in many complex fluids. At low shear rates, the system separates into a fluidized shear band and an unsheared part. The two bands are characterized by a very different dynamics probed by a local intermediate scattering function. Furthermore, a stick-slip motion is observed at very small shear rates. Our results, which open the possibility of exploring complex rheological behavior using simulations, are compared to recent experiments on various soft glasses.

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PubMed2001

Real-space application of the mean-field description of spin-glass dynamics.

The out of equilibrium dynamics of finite dimensional spin glasses is considered from a point of view going beyond the standard "mean-field theory" versus "droplet picture" debate of the past decades. The main predictions of both theories concerning the spin-glass dynamics are discussed. It is shown, in particular, that predictions originating from mean-field ideas concerning the violations of the fluctuation-dissipation theorem apply quantitatively, provided one properly takes into account the role of a spin-glass coherence length, which plays a central role in the droplet picture. Dynamics in a uniform magnetic field is also briefly discussed.

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PubMed2001

Glassy systems under time-dependent driving forces: application to slow granular rheology.

We study the dynamics of a glassy model with infinite range interactions externally driven by an oscillatory force. We find a well-defined transition in the (temperature-amplitude-frequency) phase diagram between (i) a "glassy" state characterized by the slow relaxation of one-time quantities, aging in two-time quantities and a modification of the equilibrium fluctuation-dissipation relation; and (ii) a "liquid" state with a finite relaxation time. In the glassy phase, the degrees of freedom governing the slow relaxation are thermalized to an effective temperature. Using Monte Carlo simulations, we investigate the effect of trapping regions in phase space on the driven dynamics. We find that it alternates between periods of rapid motion and periods of trapping. These results confirm the strong analogies between the slow granular rheology and the dynamics of glasses. They also provide a theoretical underpinning to earlier attempts to present a thermodynamic description of moderately driven granular materials.

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PubMed2001

Phase separation in a homogeneous shear flow: morphology, growth laws, and dynamic scaling.

We numerically investigate the influence of a homogeneous shear flow on the spinodal decomposition of a binary mixture by solving the Cahn-Hilliard equation in a two-dimensional geometry. Several aspects of this much studied problem are clarified. Our numerical data show unambiguously that, in the shear flow, the domains have on average an elliptic shape. The time evolution of the three parameters describing this ellipse is obtained for a wide range of shear rates. For the lowest shear rates investigated, we find the growth laws for the two principal axis R perpendicular (t) approximately const, R parallel (t) approximately t, while the mean orientation of the domains with respect to the flow is inversely proportional to the strain. This implies that when hydrodynamics is neglected, a shear flow does not stop the domain growth process. We also investigate the possibility of dynamic scaling, and show that only a nontrivial form of scaling holds, as predicted by a recent analytical approach to the case of a nonconserved order parameter. We show that a simple physical argument may account for these results.

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PubMed2001

Phase separation in a chaotic flow.

The phase separation between two immiscible liquids advected by a bidimensional velocity field is investigated numerically by solving the corresponding Cahn-Hilliard equation. We study how the spinodal decomposition process depends on the presence-or absence-of Lagrangian chaos. A fully chaotic flow, in particular, limits the growth of domains, and for unequal volume fractions of the liquids, a characteristic exponential distribution of droplet sizes is obtained. The limiting domain size results from a balance between chaotic mixing and spinodal decomposition, measured in terms of Lyapunov exponent and diffusivity constant, respectively.

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PubMed2000

Fluctuation-dissipation relation in a sheared fluid.

In a fluid out of equilibrium, the fluctuation-dissipation theorem (FDT) is usually violated. Using molecular dynamics simulations, we study in detail the relationship between correlation and response functions in a fluid driven into a stationary nonequilibrium state. Both the high temperature fluid state and the low temperature glassy state are investigated. In the glassy state, the violation of the FDT is similar to the one observed previously in an aging system in the absence of external drive. In the fluid state, violations of the FDT appear only when the fluid is driven beyond the linear response regime, and are then similar to those observed in the glassy state. These results are consistent with the picture obtained earlier from theoretical studies of driven mean-field disordered models, confirming the similarity between these models and simple glassy systems.

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PubMed2000

Dynamic ultrametricity in spin glasses.

We investigate the dynamics of spin glasses from the "rheological" point of view, in which aging is suppressed by the action of small, nonconservative forces. The different features can be expressed in terms of the scaling of relaxation times with the magnitude of the driving force, which plays the role of the critical parameter. Stated in these terms, ultrametricity loses much of its mystery and can be checked rather easily. This approach also seems a natural starting point to investigate what would be the real-space structures underlying the hierarchy of time scales. We study in detail the appearance of this many-scale behavior in a mean-field model, in which dynamic ultrametricity is clearly present. A similar analysis is performed on numerical results obtained for a three-dimensional spin glass: In that case, our results are compatible with either that dynamic ultrametricity is absent or that it develops so slowly that even in experimental time-windows it is still hardly observable.

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PubMed2000

A two-time-scale, two-temperature scenario for nonlinear rheology

We investigate a general scenario for "glassy" or "jammed" systems driven by an external, nonconservative force, analogous to a shear force in a fluid. In this scenario, the drive results in the suppression of the usual aging process, and the correlation and response functions become time translation invariant. The relaxation time and the response functions are then dependent on the intensity of the drive and on temperature. We investigate this dependence within the framework of a dynamical closure approximation that becomes exact for disordered, fully connected models. The relaxation time is shown to be a decreasing function of the drive ("shear thinning" effect). The correlation functions below the glass transition temperature (Tc) display a two-time-scale relaxation pattern, similar to that observed at equilibrium slightly above Tc. We also study the violation of the fluctuation-dissipation relationship in the driven system. This violation is very reminiscent of the one that takes place in a system aging below Tc at zero drive. It involves, in particular the appearance of a two-temperature regime, in the sense of an effective fluctuation-dissipation temperature [L. F. Cugliandolo, J. Kurchan, and L. Peliti, Phys. Rev. E 55, 3898 (1997)]. Although our results are, in principle, limited to the closure relations that hold for mean-field models, we argue that a number of the salient features are not inherent to the approximation scheme, and may be tested in experiments and simulations.

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PubMed1999

Isolation of isolectins from Vitis vinifera L. Cv. Chardonnay grape berries.

A lectin fraction from Chardonnay grape juice has been isolated by affinity chromatography on a column of p-aminophenyl beta-D-glucoside-derivatized agarose. The lectin fractions agglutinate rabbit and human erythrocytes without serological specificity. None of the usual monosaccharides, glycosides, or glycoproteins inhibit the hemagglutinating activity. Erythroagglutination is only inhibited by nitrophenyl glycosides, p-nitrophenyl beta-D-glucoside being the strongest inhibitor. In SDS-PAGE in the presence of 2-mercaptoethanol and gel filtration HPLC, the lectin fraction gave a single band or peak corresponding to M(r) 13.2-11.9 kDa, thus indicating it to be a monomer. Three bands were observed by isoelectric focusing with pI values of 4.1, 4. 4, and 4.9. The isolectins seem to be glycoproteins since they are bound on a concanavalin A-Sepharose column.

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