Glass Transition In Supercooled Liquids Research Articles

Molecular heterogeneities in the thermal expansivity of polyalcohols.

Density is the key quantity for nearly all the numerous theories of the (dynamic) glass transition of supercooled liquids and melts. As mean field quantity, it is used to describe correlations and heterogeneities between regions consisting of several molecules. In contrast, the question how density is created by the interactions (i.e., bonds) within a molecule and to its nearest neighbors is almost unexplored. To investigate this for the example of a homologous series of polyalcohols (glycerol, threitol, xylitol, and sorbitol), Fourier-Transform InfraRed (FTIR) spectroscopy is carried out in a wide range of temperatures from far above to far below the calorimetric glass transition Tg. This enables us to determine the potentials and hence the bond lengths of specific intramolecular and intermolecular interactions. While the former has an expansion coefficient of (∼0.1 pm/100 K) with only smooth changes, the latter shows a 30-40 times stronger response with pronounced kinks at Tg. A comparison with the overall expansion based on mass density reveals that one has to separate between strong (OH⋅⋅⋅O) and weak (CH⋅⋅⋅O) intermolecular hydrogen (H)-bridges. Despite the fact that the latter dominates glassy dynamics, their expansivity is 5 times smaller than that of the weak H-bridges. It is to be expected that such heterogeneities on intramolecular and intermolecular scales are a general phenomenon in liquids and glassy systems demonstrating especially the necessity of atomistic simulations.

Thermodynamic and Kinetic Transitions of Liquids in Nanoconfinement.

Core principles of chemistry are ubiquitously invoked to shed light on the nature of molecular level interactions in nanoconfined fluids, which play a pivotal role in a wide range of processes in geochemistry, biology, and engineering. A detailed understanding of the physicochemical processes involved in the flow, structural transitions, and freezing or melting behavior of fluids confined within nanometer-sized pores of solid materials is thus of enormous importance for both basic research and technological applications.This Account provides a perspective on new insights into the thermodynamic and kinetic transitions of nanoconfined fluids in their stable and metastable forms. After briefly introducing the unique properties of mesoporous silicas from the SBA, MCM, and FDU families that serve as the confinement matrices, combining highly ordered single and bimodal mesopore architectures with tunable pore sizes in the ∼2-15 nm range and narrow size distributions, recent studies on melting/freezing behavior of water confined in these host matrices are reviewed. While differential scanning calorimetry (DSC) reveals a linear relationship between melting point depression and pore size (independent of the pore shape), as predicted by the Gibbs-Thomson relation, variable temperature 2H wide-line nuclear magnetic resonance (NMR) spectroscopy studies confirm the core-shell model of water and give evidence for a layer-by-layer freezing mechanism, which gives rise to an apparent fragile-to-strong transition in the solidification dynamics.In contrast to the freezing/melting behavior of water, the effect of nanoconfinement on the glass transition of supercooled liquids is nonuniversal and the glass transition temperature Tg can either increase or decrease with the dimensionality and extent of confinement. This nonuniversal behavior is exemplified by the two glass-forming molecular liquids, glycerol and ortho-terphenyl (OTP). While glycerol shows an increase in Tg and a pronounced slowdown of the rotational dynamics of the constituent molecules due to a change in the molecular packing between the bulk and the confined liquid, OTP displays a linear and confining-media-dependent depression of Tg with increased confinement that is strongly influenced by the pore-liquid interface characteristics.This Account concludes with a focus on recent experimental evidence of extreme spatial and dynamical heterogeneity in both freezing and glass transition processes. This discovery was enabled by the unique mesoporous structures of SBA-16 and FDU-5, possessing bimodal architectures with two interconnected pore types of different size and shape (spherical and cylindrical). For the very first time, two melting points for water and two glass transitions for supercooled OTP, corresponding to a specific pore type, were observed. Collectively, these observations strongly suggest a close mechanistic connection between the local fluctuations in the structure and dynamics of nanoconfined liquids. While the findings reviewed in this Account provide new insights into thermodynamic and kinetic transitions of fluids, there remain many unanswered questions regarding the effects of nanoconfinement on the fundamental properties of fluids, which offer exciting future opportunities in chemical research.

Glass Transition in Supercooled Liquids with Medium-Range Crystalline Order.

The origin of the rapid dynamical slowdown in glass forming liquids in the growth of static length scales, possibly associated with identifiable structural ordering, is a much debated issue. Growth of medium range crystalline order (MRCO) has been observed in various model systems to be associated with glassy behavior. Such observations raise the question of whether molecular mechanisms for the glass transition in liquids with and without MRCO are the same. In this study we perform extensive molecular dynamics simulations of a number of glass forming liquids and show that the static and dynamic properties of glasses with MRCO are different from those of other glass forming liquids with no predominant local order. We also resolve an important issue regarding the so-called point-to-set method for determining static length scales, and demonstrate it to be a robust method for determining static correlation lengths in glass formers.

Thermodynamic Comparison and the Ideal Glass Transition of Antiferromagnetic Ising Model on Multi-branched Husimi and Cubic Recursive Lattice with the Identical Coordination Number**Supported by National Natural Science Foundation of China under Grant No. 11505110

Two types of recursive lattices with the identical coordination number but different unit cells (2-D square and 3-D cube) are constructed and the antiferromagnetic Ising model is solved exactly on them to study the stable and metastable states. A multi-branched structure of the 2-D plaquette model, which we introduced in this work, makes it possible to be an analog to the cubic lattice. Two solutions of each model can be found to exhibit the crystallization of liquid, and the ideal glass transition of supercooled liquid respectively. Based on the solutions, the thermodynamics on both lattices, e.g. the free energy, energy density, and entropy of the supercooled liquid, crystal, and liquid state of the model are calculated and compared with each other. Interactions between particles farther away than the nearest neighbor distance and multi-spins interactions are taken into consideration, and their effects on the thermal behavior are examined. The two lattices show comparable properties on the thermodynamics, which proves that both of them are practical to describe the regular 3-D case, especially to locate the ideal glass transition, while the 2-D multi-branched plaquette model is less accurate with the advantage of simpler formulation and less computation time consumption.

Subcritical transition to turbulence: What we can learn from the physics of glasses

In this note, we discuss possible analogies between the subcritical transition to turbulence in shear flows and the glass transition in supercooled liquids. We briefly review recent experimental and numerical results, as well as theoretical proposals, and compare the difficulties arising in assessing the divergence of the turbulence lifetime in subcritical shear flow with that encountered for the relaxation time in the study of the glass transition. In order to go beyond the purely methodological similarities, we further elaborate on this analogy and propose a simple model for the transition to turbulence, inspired by the random energy model (a standard model for the glass transition), with the aim to possibly foster yet-unexplored directions of research in subcritical shear flows.

Theoretical perspective on the glass transition and amorphous materials

We provide a theoretical perspective on the glass transition in molecular liquids at thermal equilibrium, on the spatially heterogeneous and aging dynamics of disordered materials, and on the rheology of soft glassy materials. We start with a broad introduction to the field and emphasize its connections with other subjects and its relevance. The important role played by computer simulations to study and understand the dynamics of systems close to the glass transition at the molecular level is spelled out. We review the recent progress on the subject of the spatially heterogeneous dynamics that characterizes structural relaxation in materials with slow dynamics. We then present the main theoretical approaches describing the glass transition in supercooled liquids, focusing on theories that have a microscopic, statistical mechanics basis. We describe both successes and failures, and critically assess the current status of each of these approaches. The physics of aging dynamics in disordered materials and the rheology of soft glassy materials are then discussed, and recent theoretical progress is described. For each section, we give an extensive overview of the most recent advances, but we also describe in some detail the important open problems that, we believe, will occupy a central place in this field in the coming years.

Evidence of α fluctuations in myoglobin's denaturation in the high temperature region: Average relaxation time from an Adam–Gibbs perspective

In this work, we derive an analytical expression for the relaxation time τ as a function of temperature T for myoglobin protein (Mb, PDB:1MBN) in the high temperature limit ( T > T g = 200 K). The method is based on a modified version of the Adam–Gibbs theory (AG theory) for the glass transition in supercooled liquids and an implementation of differential geometry techniques. This modified version of the AG theory takes into account that the entropic component in protein's denaturation has two major sources: a configurational contribution Δ S c due to the unfolding of the highly ordered native state N and a hydration contribution Δ S hyd arising from the exposure of non-polar residues to direct contact with solvent polar molecules. Our results show that the configurational contribution Δ S c is temperature-independent and one order of magnitude smaller than its hydration counterpart Δ S hyd in the temperature range considered. The profile obtained for log τ( T) from T = 200 K to T = 300 K exhibits a non-Arrhenius behavior characteristic of α relaxation mechanisms in hydrated proteins and glassy systems. This result is in agreement with recent dielectric spectroscopy data obtained for hydrated myoglobin, where at least two fast relaxation processes in the high temperature limit have been observed. The connection between the relaxation process calculated here and the experimental results is outlined.

Glass formation and glass transition in supercooled liquids, with insights from study of related phenomena in crystals

We divide glass and viscous liquid sciences into two major research areas, the first dealing with how to avoid crystals and so access the viscous liquid state, and the second dealing with how liquids behave when no crystals form. We review some current efforts to elucidate each area, looking at strategies for vitrification of monatomic metals in the first, and the origin of the property ‘fragility’ in the second. Essential in the first is the ‘ideal glassformer’ concept. Essential in the second is the non-trivial behavior of the glassformer thermodynamics. We explore the findings on non-exponential relaxation and dynamic heterogeneities in viscous liquids, emphasizing the way in which direct excitation of the configurational modes has helped differentiate configurational from non-configurational contributions to the excess heat capacity. We then propose a scheme for understanding the relation between inorganic network and non-network glassformers which includes the anomalous case of water as an intermediate. In a final section we examine the additional insights to be gained by study of the glass-like, ergodicity-breaking transitions that occur in disordering crystals. Here we highlight systems in which the background thermodynamics is understood because the ergodic behavior is a lambda transition. Water, and the classical network glassformers, appear to be attenuated versions of the latter type of cooperative transition.

Effects of disorder in mullite: Molecular dynamics simulation and energy landscape analysis

Molecular dynamics are carried out to elucidate the consequences of the distribution of oxygen vacancies and tetrahedally bound cations in mullite. The results are compared with those of sillimanite, an alumino silicate with a mullite-type structure that differs from mullite in that it contains no oxygen vacancies and has a fully ordered structure. The results show that disorder in mullite leads to differences in the behaviors of mullite and sillimanite at high temperature. An energy landscape analysis is carried out, which shows that a high-temperature transition occurs in mullite that is similar to the glass transition in supercooled liquids, whereas no transition is observed in sillimanite.

Critical Dynamics of Dimers: Implications for the Glass Transition

The Adam-Gibbs view of the glass transition relates the relaxation time to the configurational entropy, which goes continuously to zero at the so-called Kauzmann temperature. We examine this scenario in the context of a dimer model with an entropy-vanishing phase transition and stochastic loop dynamics. We propose a coarse-grained master equation for the order parameter dynamics which is used to compute the time-dependent autocorrelation function and the associated relaxation time. Using a combination of exact results, scaling arguments, and numerical diagonalizations of the master equation, we find nonexponential relaxation and a Vogel-Fulcher divergence of the relaxation time in the vicinity of the phase transition. Since in the dimer model the entropy stays finite all the way to the phase transition point and then jumps discontinuously to zero, we demonstrate a clear departure from the Adam-Gibbs scenario. Dimer coverings are the "inherent structures" of the canonical frustrated system, the triangular Ising antiferromagnet. Therefore, our results provide a new scenario for the glass transition in supercooled liquids in terms of inherent structure dynamics.

Mode-coupling theory and the glass transition in supercooled liquids

Mode-coupling theory is an approach to the study of complex behavior in the supercooled liquids which developed from the idea of a nonlinear feedback mechanism. From the coupling of slowly decaying correlation functions the theory predicts the existence of a characteristic temperature ${T}_{c}$ above the experimental glass transition temperature ${T}_{g}$ for the liquid. This article discusses the various methods used to obtain the model equations and illustrates the effects of structure on dynamics and scaling behavior over different time scales using a wave-vector-dependent model. It compares the theoretical predictions, experimental observations, and computer simulation results, and also considers phenomenological extensions of mode-coupling theory. Numerical solutions of the model equations to study the dynamics from a nonperturbative approach are also reviewed. The review looks briefly at recent observations from landscape studies of model systems of structural glasses and their relation to the mode-coupling temperature ${T}_{c}$. The equations for the mean-field dynamics driven by the $p$-spin interaction Hamiltonian are similar to those of mode-coupling theory for structural glasses. Related developments in the nonequilibrium dynamics and generalization of the fluctuation-dissipation relation for the structural glasses are briefly touched upon. The review ends with a summary of the open questions and possible future direction of the field.

Evidence for "fragile" glass-forming behavior in the relaxation of Coulomb frustrated three-dimensional systems.

We show by means of a Monte Carlo simulation study that three-dimensional models with long-range frustration display the generic phenomena seen in fragile glass-forming liquids. Due to their properties (absence of quenched disorder, physical motivation in terms of structural frustration, and tunable fragility), these systems appear as promising minimal theoretical models for describing the glass transition of supercooled liquids.

The viscousslowing down of supercooled liquids as a temperature-controlledsuper-Arrhenius activated process: a description in terms offrustration-limited domains

We propose that the salient feature to be explained about the glasstransition of supercooled liquids is the temperature-controlledsuper-Arrhenius activated nature of the viscous slowing down, morestrikingly seen in weakly bonded, fragile systems. In the light ofthis observation, the relevance of simple models of sphericallyinteracting particles and that of models based on free-volumecongested dynamics are questioned. Finally, we discuss how the mainaspects of the phenomenology of supercooled liquids, including thecrossover from Arrhenius to super-Arrhenius activated behaviour and theheterogeneous character of the α-relaxation, can be described by anapproach based on frustration-limited domains.

The jammingtransition of granular media

A statistical mechanical approach to granular material is proposed.Using lattice models from standard statistical mechanics and resultsfrom a mean-field replica approach we find a jamming transition ingranular media closely related to the glass transition insupercooled liquids. These models reproduce the logarithmicrelaxation in granular compaction and reversible-irreversiblelines, in agreement with experimental data. The models also exhibitaging effects and breakdown of the usual fluctuation-dissipationrelation. It is shown that the glass transition may be responsiblefor the logarithmic relaxation and may be related to the cooperativeeffects underlying many phenomena exhibited by granular materialssuch as the Reynolds transition.

Equilibrium andageing dynamics of simple models for glasses

We analyse the properties of the energy landscape of finite-size fully connected p-spin-like models. In thethermodynamic limit the high-temperature phase is described by theschematic mode-coupling theory of supercooled liquids. In thislimit, the barriers between different basins are infinite below thecritical dynamical temperature at which the ergodicity is broken ininfinite time. We show that finite-size fully connectedp-spin-like models, where activated processes are possible, exhibit properties typical of real supercooled liquid when both arenear the critical glass transition. Our results support theconclusion that fully connected p-spin-like models are the naturalstatistical mechanical models for studying the glass transition insupercooled liquids.

A modifiedfacilitated kinetic Ising model for supercooled liquids

A modification is considered of the well-known Fredrickson-Andersenfacilitated kinetic Ising model for studying the glasstransition in supercooled liquids. In this modification, which wecall the cluster facilitated kinetic Ising model (CluFKI) model, theflip of a spin between the liquid-like regions represented by A orup states and the solid-like regions represented by B or down statescan occur (i.e. is facilitated) only if in the cluster containingthe spin and those adjacent to it there are at least n spins inthe A state. As a result, the transition from a solid-like to aliquid-like region requires more free volume than the reversetransition, which is physically very reasonable. Computercalculations for the n = 1 CluFKI model, as well as a qualitativeanalysis, show that as the temperature T of the system decreases,and so the parameter x = E/T increases, the fraction fA ofspins in state A when the system is in a metastable steady statedecreases until it reaches zero for a critical value xc.Correspondingly, the mean relaxation time ⟨τ⟩ ofthe system's memory function increases rapidly, and ⟨τ⟩→∞ as x→xc. Thisbehaviour is similar to that for the α-relaxation insupercooled liquids as the glass transition temperature isapproached.

Are mean-field spin-glass models relevant for the structural glass transition?

We analyze the properties of the energy landscape of finite-size fully connected p-spin-like models whose high-temperature phase is described, in the thermodynamic limit, by the schematic Mode Coupling Theory of super-cooled liquids. We show that finite-size fully connected p-spin-like models, where activated processes are possible, do exhibit properties typical of real super-cooled liquid when both are near the critical glass transition. Our results support the conclusion that fully connected p-spin-like models are the natural statistical mechanical models for studying the glass transition in super-cooled liquids.

Glass transition from a slush to a real glass in charge-density-wave compound TaS 3

We have found by broad-band dielectric spectroscopy of charge density wave (CDW) compound TaS 3 that the main relaxation time representing CDW viscosity shows a critical slowing down in the range 55-50 K. At lower temperatures two additional processes appear that we assign to the defects in CDW superlattice. Based on very strong similarity with the glass transition in supercooled liquids, we propose a consistent scenario of the glass transition from a CDW slush to a real CDW glass. The length/time scales involved imply the existence of a new family of glasses in systems with the long-range order such as CDW.

Observation of relaxation phenomena in thermophysical properties of metals

Studies of relaxation phenomena related to thermophysical properties of metals are reviewed. They provide important information not available from other techniques. Observation of relaxation phenomena was used in studying ordering in alloys, glass transition in Supercooled liquids, and equilibration of point defects in a crystal lattice. Opportunities to observe the equilibration phenomena in materials are provided by subsecond thermophysical measurements. A simple method is proposed for a straightforward determination of the defect contribution to the enthalpy of metals.

Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals

Neutron scattering studies in (KBr)1−x(KCN)x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)1−x(KCN)x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)0.43(KCN)0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids.