arXiv subjects

F. Sciortino

Publications and source records attributed to F. Sciortino.

72 records · Page 4Linked to original sources

arXiv2002

Evidence for Unusual Dynamical Arrest Scenario in Short Ranged Colloidal Systems

Extensive molecular dynamics simulation studies of particles interacting via a short ranged attractive square-well (SW) potential are reported. The calculated loci of constant diffusion coefficient $D$ in the temperature-packing fraction plane show a re-entrant 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.

cond-mat.soft↗

arXiv2001

Dynamics and Configurational Entropy in the LW Model for Supercooled Orthoterphenyl

We study thermodynamic and dynamic properties of a rigid model of the fragile glass forming liquid orthoterphenyl. A system of N=343 molecules is considered in a wide range of densities and temperatures, reaching simulation times up to 1 microsecond. Working within the inherent structures thermodynamic formalism, we present results for the temperature and density dependence of the number, depth and shape of the basins of the potential energy surface. We evaluate the configurational entropy of the system and we study the connection between thermodynamical and dynamical properties. We confirm that the temperature dependence of the configurational entropy and of the diffusion constant, as well as the inverse of the characteristic structural relaxation time, are strongly connected in supercooled states.

cond-mat.soft↗

arXiv2001

Mode-Coupling Theory of Colloids with Short-range Attractions

Within the framework of the mode-coupling theory of super-cooled liquids, we investigate new phenomena in colloidal systems on approach to their glass transitions. When the inter-particle potential contains an attractive part, besides the usual repulsive hard core, two intersecting liquid-glass transition lines appear, one of which extends to low densities, while the other one, at high densities, shows a re-entrant behaviour. In the glassy region a new type of transition appears between two different types of glasses. The complex phenomenology can be described in terms of higher order glass transition singularities. The various glass phases are characterised by means of their viscoelastic properties. The glass driven by attractions has been associated to particle gels, and the other glass is the well known repulsive colloidal glass. These correspondences, in associations with the new predictions of glassy behaviour mean that such phenomena may be expected in colloidal systems with, for example, strong depletion or other short-ranged attractive potentials.

cond-mat.soft↗

arXiv2001

Kinetic Arrest Originating in Competition between\linebreak Attractive Interaction and Packing Force

We discuss the situation where attractive and repulsive portions of the inter-particle potential both contribute significantly to glass formation. We introduce the square-well potential as prototypical model for this situation, and {\it reject} the Baxter as a useful model for comparison to experiment on glasses, based on our treatment within mode coupling theory. We present explicit results for various well-widths, and show that, for narrow wells, there is a useful analytical formula that would be suitable for experimentalist working in the field of colloidal science. We raise the question as to whether, in a more exact treatment, the sticky sphere limit might have an infinite glass transition temperature, or a high but finite one.

cond-mat.soft↗

arXiv2001

Slowed Relaxational Dynamics Beyond the Fluctuation-Dissipation Theorem

To describe the slow dynamics of a system out of equilibrium, but close to a dynamical arrest, we generalize the ideas of previous work to the case where time-translational invariance is broken. We introduce a model of the dynamics that is reasonably general, and show how all of the unknown parameters of this model may be related to the observables or to averages of the noise. One result is a generalisation of the Fluctuation Dissipation Theorem of type two (FDT2), and the method is thereby freed from this constraint. Significantly, a systematic means of implementing the theory to higher order is outlined. We propose the simplest possible closure of these generalized equations, following the same type of approximations that have been long known for the equilibrium case of Mode Coupling Theory (MCT). Naturally, equilibrium MCT equations are found as a limit of this generalized formalism. %We indicate that, within the same general %framework, it should be possible to make higher level approximations, %leading to more general applicability.

cond-mat.dis-nn↗

arXiv2001

Off-equilibrium dynamics in the energy landscape of a simple model glass

The aging dynamics of a simple model glass is numerically investigated observing how it takes place in the potential energy landscape $V$. Partitioning the landscape in basins of minima of $|\nabla V|^2$, we are able to elucidate some interesting topological properties of the aging process. The main result is the characterization of the long time behavior as a jump dynamics between basins of attraction of minima. Moreover we extract some information about the landscape itself, determining quantitatively few parameters describing it, such as the mean energy barrier value and the mean square distance between adjacent minima.

cond-mat.soft↗

arXiv2001

Gaussian density fluctuations and Mode Coupling Theory for supercooled liquids

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 the canonical mean-field theory of the fluid dynamics.

cond-mat.soft↗

arXiv2000

Dynamics in a supercooled molecular liquid: Theory and Simulations

We report extensive simulations of liquid supercooled states for a simple three-sites molecular model, introduced by Lewis and Wahnstr"om [L. J. Lewis and G. Wahnstr"om, Phys. Rev. E 50, 3865 (1994)] to mimic the behavior of ortho-terphenyl. The large system size and the long simulation length allow to calculate very precisely --- in a large q-vector range --- self and collective correlation functions, providing a clean and simple reference model for theoretical descriptions of molecular liquids in supercooled states. The time and wavevector dependence of the site-site correlation functions are compared with detailed predictions based on ideal mode-coupling theory, neglecting the molecular constraints. Except for the wavevector region where the dynamics is controlled by the center of mass (around 9 nm-1), the theoretical predictions compare very well with the simulation data. ~

cond-mat.soft↗

arXiv2000

Saddles in the energy landscape probed by supercooled liquids

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multi-dimension configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddles order and potential energy are well defined functions of the temperature T. Moreover, decreasing T, the saddle order vanishes at the same temperature (T_MCT) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T_MCT: it marks the transition from a dynamics between basins of saddles (T>T_MCT) to a dynamics between basins of minima (T<T_MCT).

cond-mat.dis-nn↗

arXiv2000

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, and the predictions of glasses that we make refer to the `ideal' glasses predicted by that 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 glass, 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 behaviour is explained in terms of the underlying physics of the system.

cond-mat.soft↗

arXiv2000

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 A_3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail) singularity at a critical well width. Close to the A_3 and A_4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.

cond-mat.soft↗

arXiv2000

The Free Energy Surface of Supercooled Water

We present a detailed analysis of the free energy surface of a well characterized rigid model for water in supercooled states. We propose a functional form for the liquid free energy, supported by recent theoretical predictions [Y. Rosenfeld and P. Tarazona, Mol. Phys. {\bf 95}, 141 (1998)], and use it to locate the position of a liquid-liquid critical point at $T_{C'} = 130 \pm 5$~K, $P_{C'}=290\pm 30$MPa, and $ρ_{C'} = 1.10 \pm 0.03$~g/cm$^3$. The observation of the critical point strengthens the possibility that SPC/E water may undergo a liquid-liquid phase transition. Finally, we discuss the possibility that the approach to the liquid-liquid critical point could be pre-empted by the glass transition.

cond-mat.stat-mech↗

arXiv2000

Test of molecular mode coupling theory: A first resume

We report recent progress on the test of mode coupling theory for molecular liquids (MMCT) for molecules of arbitrary shape. The MMCT equations in the long time limit are solved for supercooled water including all molecular degrees of freedom. In contrast to our earlier treatment of water as a linear molecule, we find that the glass transition temperature $T_c$ is overestimated by the theory as was found in the case of simple liquids. The nonergodicity parameters are calculated from the "full" set of MMCT-equations truncated at $l_{co}=2$. These results are compared $(i)$ with the nonergodicity parameters from MMCT with $l_{co}=2$ in the "dipole" approximation $n=n'=0$ and the diagonalization approximation $n=n'=0$,$l=l'$ and $(ii)$ with the corresponding results from a MD-simulation. This work supports the possibility that a reduction to the most prominent correlators may constitute a valid approximation for solving the MMCT equations for rigid molecules.

cond-mat.soft↗

arXiv2000

Molecular Correlations in a Supercooled Liquid

We present static and dynamic properties of molecular correlation functions S_{lmn,l'm'n'}(q,t) in a simulated supercooled liquid of water molecules, as a preliminary effort in the direction of solving the molecular mode coupling theory (MMCT) equations for supercooled molecular liquids. The temperature and time dependence of various molecular correlation functions, calculated from 250 ns long molecular dynamics simulations, show the characteristic patterns predicted by MMCT and shed light on the driving mechanism responsible for the slowing down of the molecular dynamics. We also discuss the symmetry properties of the molecular correlation functions which can be predicted on the basis of the C_{2v}-symmetry of the molecule. The analysis of the MD--results for the static correlators S_{lmn,l'm'n'}(q) reveals that additional relationships between correlators with different signs of n and n' exist. We prove that for molecules with C_{rv}-symmetry this unexpected result becomes exact at least for high temperatures.

cond-mat.stat-mech↗

arXiv1999

Thermodynamics of supercooled liquids in the inherent structure formalism: a case study

In this article we review the thermodynamics of liquids in the framework of the inherent structure formalism. We then present calculations of the distribution of the basins in the potential energy of a binary Lennard-Jones mixture as a function of temperature. The comparison between the numerical data and the theoretical formalism allows us to evaluate the degeneracy of the inherent structures in a bulk system and to estimate the energy of the lowest energy disordered state (the Kauzmann energy). We find that, around the mode-coupling temperature, the partition function of the liquid is approximated well by the product of two loosely coupled partition functions, one depending on the inherent structures quantities (depth of the basins and their degeneracy) and one describing the free energy of the liquid constrained in one typical basin.

cond-mat.soft↗

arXiv1999

Inherent Structure Entropy of Supercooled Liquids

We present a quantitative description of the thermodynamics in a supercooled binary Lennard Jones liquid via the evaluation of the degeneracy of the inherent structures, i.e. of the number of potential energy basins in configuration space. We find that for supercooled states, the contribution of the inherent structures to the free energy of the liquid almost completely decouples from the vibrational contribution. An important byproduct of the presented analysis is the determination of the Kauzmann temperature for the studied system. The resulting quantitative picture of the thermodynamics of the inherent structures offers new suggestions for the description of equilibrium and out-of-equilibrium slow-dynamics in liquids below the Mode-Coupling temperature.

cond-mat.stat-mech↗

arXiv1999

Molecular mode-coupling theory for supercooled liquids: Application to water

We present mode-coupling equations for the description of the slow dynamics observed in supercooled molecular liquids close to the glass transition. The mode-coupling theory (MCT) originally formulated to study the slow relaxation in simple atomic liquids, and then extended to the analysis of liquids composed by linear molecules, is here generalized to systems of arbitrarily shaped, rigid molecules. We compare the predictions of the theory for the $q$-vector dependence of the molecular nonergodicity parameters, calculated by solving numerically the molecular MCT equations in two different approximation schemes, with ``exact'' results calculated from a molecular dynamics simulation of supercooled water. The agreement between theory and simulation data supports the view that MCT succeeds in describing the dynamics of supercooled molecular liquids, even for network forming ones.

cond-mat.soft↗

arXiv1998

Ideal glass-glass transitions and logarithmic decay of correlations in a simple system

We calculate the ideal-glass-transition line for adhesive hard spheres in the temperature-volume-fraction plane within the framework of the mode-coupling theory. We find two intersecting lines, controlled by the hard-core and the adhesive part of the potential respectively, giving rise to two different mechanisms for structural arrest. In the glass region we identify the presence of a glass-glass-transition line ending in a cusp bifurcation which causes, even in the close by liquid region, a logarithmic decay of correlations.

cond-mat.soft↗