Configurational entropy and collective modes in normal and supercooled liquids (original) (raw)
Related papers
Irreversible reorganization in a supercooled liquid originates from localized soft modes
Nature Physics, 2008
The transition of a fluid to a rigid glass upon cooling is a common route of transformation from liquid to solid that embodies the most poorly understood features of both phases 1,2,3 . From the liquid perspective, the puzzle is to understand stress relaxation in the disordered state. From the perspective of solids, the challenge is to extend our description of structure and its mechanical consequences to materials without long range order. Using computer simulations, we show that the localized low frequency normal modes of a configuration in a supercooled liquid are causally correlated to the irreversible structural reorganization of the particles within that configuration. We also demonstrate that the spatial distribution of these soft local modes can persist in spite of significant particle reorganization. The consequence of these two results is that it is now feasible to construct a theory of relaxation length scales in glass-forming liquids without recourse to dynamics and to explicitly relate molecular properties to their collective relaxation.
Relaxation dynamics of glasses along a wide stability and temperature range
Scientific Reports, 2016
While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states.
Soft Modes and Nonaffine Rearrangements in the Inherent Structures of Supercooled Liquids
Physical Review Letters, 2014
We find that the hierarchical organization of the potential energy landscape in a model supercooled liquid can be related to a change in the spatial distribution of soft normal modes. For groups of nearby minima, between which fast relaxation processes typically occur, the localization of the soft modes is very similar. The spatial distribution of soft regions changes, instead, for minima between which transitions relevant to structural relaxation occur. This may be the reason why the soft modes are able to predict spatial heterogeneities in the dynamics. Nevertheless, the very softest modes are only weakly correlated with dynamical heterogeneities, and instead show higher statistical overlap with regions in the local minima that would undergo non-affine rearrangements if subjected to a shear deformation. This feature of the supercooled liquid is reminiscent of the behavior of nonaffine deformations in amorphous solids, where the very softest modes identify the loci of plastic instabilities. PACS numbers: 61.43.Fs,64.70.Q-,05.20.Jj
Dynamics and thermodynamics of supercooled liquids and glasses from a model energy landscape
Physical Review B, 2001
The dynamics and thermodynamics of a model potential-energy surface are analyzed with regard to supercooling and glass formation. Relaxation is assumed to be mediated by pathways that connect groups of local minima. The dynamics between these groups is treated via transition state theory using appropriate densities of states consistent with the thermodynamics of the model, with a general expression for the free energy barrier. Nonergodicity is admitted by successive disconnection of regions that no longer contribute to the partition function as a function of the observation time scale. The model exhibits properties typical of supercooled liquids and glasses spanning the whole range of ''fragile'' and ''strong'' behavior. Non-Arrhenius dynamics, characteristic of ''fragile'' glass formers, are observed when the barriers to relaxation increase as the potential energy decreases, but only if the observation time scale is long enough. For a fixed observation time, fragility generally increases as the free energy barriers decrease and vibrational frequencies increase. We associate higher vibrational frequencies with systems that have more local minima, and hence when the model exhibits dynamic fragility we usually see a large change in the heat capacity at the glass transition. However, in some regions of parameter space the expected correlations between dynamics and thermodynamics are not present.
A single relaxation time description for glasses and their liquid state
arXiv: Soft Condensed Matter, 2016
We use relaxation data taken out of equilibrium on glasses of different stability and equilibrium relaxation times from the supercooled liquid to propose a common description for both liquid and glass states. Using ultrafast scanning calorimetry, the accessible timescales of the glass are expanded to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-temperature regime above the conventional devitrification temperature heating at 10 K/min. Surprisingly, both the liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws derived for the liquid. This work provides a strong connection between the glass and liq...
Entropy Scaling of Molecular Dynamics in a Prototypical Anisotropic Model near the Glass Transition
Mysteries of dynamics and thermodynamics of molecular systems in the vicinity of the boundary between thermodynamically nonequilibrium glassy and metastable supercooled liquid states are still incompletely discovered and their theoretical and simulation models are imperfect despite many previous efforts. Among them, the role of total system entropy, configurational entropy, and excess entropy in the temperature-pressure or temperature-density dependence of global molecular dynamics timescale relevant to the glass transition is an essential topic intensively studied for over half of a century. By exploiting a well-known simple ellipsoidal model recently successfully applied to simulate the supercooled liquid state and the glass transition, we gain a new insight into the different views on the relationship between entropy and relaxation dynamics of glass formers, showing the molecular grounds for the entropy scaling of global molecular dynamics timescale.
Excitations Are Localized and Relaxation Is Hierarchical in Glass-Forming Liquids
2011
For several atomistic models of glass formers, at conditions below their glassy dynamics onset temperatures, Tmathrmo{T_\mathrm{o}}Tmathrmo, we use importance sampling of trajectory space to study the structure, statistics and dynamics of excitations responsible for structural relaxation. Excitations are detected in terms of persistent particle displacements of length aaa. At supercooled conditions, for aaa of the order of or smaller than a particle diameter, we find that excitations are associated with correlated particle motions that are sparse and localized, occupying a volume with an average radius that is temperature independent and no larger than a few particle diameters. We show that the statistics and dynamics of these excitations are facilitated and hierarchical. Excitation energy scales grow logarithmically with aaa. Excitations at one point in space facilitate the birth and death of excitations at neighboring locations, and space-time excitation structures are microcosms of heterogeneous dynamics at larger scales. This nature of dynamics becomes increasingly dominant as temperature TTT is lowered. We show that slowing of dynamics upon decreasing temperature below TmathrmoT_\mathrm{o}Tmathrmo is the result of a decreasing concentration of excitations and concomitant growing hierarchical length scales, and further that the structural relaxation time tau\tautau follows the parabolic law, log(tau/taumathrmo)=J2(1/Tâ1/Tmathrmo)2\log(\tau / \tau_\mathrm{o}) = J^2(1/T - 1/T_\mathrm{o})^2log(tau/taumathrmo)=J2(1/Tâ1/Tmathrmo)2, for T<TmathrmoT<T_\mathrm{o}T<Tmathrmo, where JJJ, taumathrmo\tau_\mathrm{o}taumathrmo and TmathrmoT_\mathrm{o}Tmathrmo can be predicted quantitatively from dynamics at short time scales. Particle motion is facilitated and directional, and we show this becomes more apparent with decreasing TTT. We show that stringlike motion is a natural consequence of facilitated, hierarchical dynamics.
Physical Review Letters, 2010
We identify the pattern of microscopic dynamical relaxation for a two dimensional glass forming liquid. On short timescales, bursts of irreversible particle motion, called cage jumps, aggregate into clusters. On larger time scales, clusters aggregate both spatially and temporally into avalanches. This propagation of mobility, or dynamic facilitation, takes place along the soft regions of the systems, which have been identified by computing isoconfigurational Debye-Waller maps. Our results characterize the way in which dynamical heterogeneity evolves in moderately supercooled liquids and reveal that it is astonishingly similar to the one found for dense glassy granular media.
Journal of Non-Crystalline Solids, 2009
The structural relaxation dynamics of two molecular glass forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S C , as predicted by Adam and Gibbs (AG), was also investigated. As S C is not directly accessible by experiments, it was estimated, following a common procedure in literature, from the excess entropy S exc of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T-P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S exc and S C was found, with different proportionality factors in isothermal and isobaric conditions. Using such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature.