Simple Glass Formers Research Articles

Local Dynamical Heterogeneity in Simple Glass Formers.

We study the local dynamical fluctuations in glass-forming models of particles embedded in d-dimensional space, in the mean-field limit of d→∞. Our analytical calculation reveals that single-particle observables, such as squared particle displacements, display divergent fluctuations around the dynamical (or mode-coupling) transition, due to the emergence of nontrivial correlations between displacements along different directions. This effect notably gives rise to a divergent non-Gaussian parameter, α_{2}. The d→∞ local dynamics therefore becomes quite rich upon approaching the glass transition. The finite-d remnant of this phenomenon further provides a long sought-after, first-principle explanation for the growth of α_{2} around the glass transition that is not based on multiparticle correlations.

Probing Small-Angle Molecular Motions with EPR Spectroscopy: Dynamical Transition and Molecular Packing in Disordered Solids

Disordered molecular solids present a rather broad class of substances of different origin—amorphous polymers, materials for photonics and optoelectronics, amorphous pharmaceutics, simple molecular glass formers, and others. Frozen biological media in many respects also may be referred to this class. Theoretical description of dynamics and structure of disordered solids still does not exist, and only some phenomenological models can be developed to explain results of particular experiments. Among different experimental approaches, electron paramagnetic resonance (EPR) applied to spin probes and labels also can deliver useful information. EPR allows probing small-angle orientational molecular motions (molecular librations), which intrinsically are inherent to all molecular solids. EPR is employed in its conventional continuous wave (CW) and pulsed—electron spin echo (ESE)—versions. CW EPR spectra are sensitive to dynamical librations of molecules while ESE probes stochastic molecular librations. In this review, different manifestations of small-angle motions in EPR of spin probes and labels are discussed. It is shown that CW-EPR-detected dynamical librations provide information on dynamical transition in these media, similar to that explored with neutron scattering, and ESE-detected stochastic librations allow elucidating some features of nanoscale molecular packing. The possible EPR applications are analyzed for gel-phase lipid bilayers, for biological membranes interacting with proteins, peptides and cryoprotectants, for supercooled ionic liquids (ILs) and supercooled deep eutectic solvents (DESs), for globular proteins and intrinsically disordered proteins (IDPs), and for some other molecular solids.

Structural order as a genuine control parameter of dynamics in simple glass formers

Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity. Its rationalisation by subtle changes in the liquid structure has been long debated but remains elusive, due to intrinsic difficulty in detecting the underlying complex structural ordering. Here we report that structural order parameter characterising local packing capability can well describe the glassy dynamics not only macroscopically but also microscopically, no matter whether it is driven by temperature or density. A Vogel-Fulcher-Tammann (VFT)-like relation is universally identified between the structural relaxation time and the order parameter for supercooled liquids with isotropic interactions. More importantly, we find such an intriguing VFT-like relation to be statistically valid even at a particle level, between spatially coarse-grained structural order and microscopic particle-level dynamics. Such a unified description of glassy dynamics based solely on structural order is expected to contribute to the ultimate understanding of the long-standing glass-transition problem.

Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers

Glass formers show motional processes over an extremely broad range of timescales, covering more than ten orders of magnitude, meaning that a full understanding of the glass transition needs to comprise this tremendous range in timescales. Here we report simultaneous dielectric and neutron spectroscopy investigations of three glass-forming liquids, probing in a single experiment the full range of dynamics. For two van der Waals liquids, we locate in the pressure–temperature phase diagram lines of identical dynamics of the molecules on both second and picosecond timescales. This confirms predictions of the isomorph theory and effectively reduces the phase diagram from two to one dimension. The implication is that dynamics on widely different timescales are governed by the same underlying mechanisms.

Microscopic origin of the fragile to strong crossover in supercooled water: The role of activated processes.

We perform an accurate analysis of the density self-correlation functions of TIP4P/2005 supercooled water on approaching the region of the liquid-liquid critical point. In a previous work on this model, we provided evidence of a fragile to strong crossover of the dynamical behavior in the deep supercooled region. The structural relaxation follows the Mode Coupling theory in the fragile region and then deviates from Mode Coupling regime to a strong Arrhenius behavior. This crossover is particularly important in water because it is connected to the thermodynamics of the supercooled region. To better understand the origin of this crossover, we compute now the Van Hove self-correlation functions. In particular we aim at investigating the presence and the role of the hopping phenomena that are the cause of the fragile to strong crossover in simple liquids. In TIP4P/2005 water, we find hopping processes too and we analyze how they depend on temperature and density upon approaching the fragile to strong crossover and the Mode Coupling ideal crossover temperature. Our results show that water behaves like a simple glass former. After an initial ballistic regime, the cage effect dominates the mild supercooled region, with diffusion taking place at long time. At the fragile to strong crossover, we find that hopping (activated) processes start to play a role. This is evidenced by the appearance of peaks in the Van Hove correlation functions. In the deep supercooled regime, our analysis clearly indicates that activated processes dominate the dynamics. The comparison between the Van Hove functions and the radial distribution functions allows to better understand the mechanism of hopping phenomena in supercooled water and to connect their onset directly with the crossing of the Widom Line.

Locally favoured structures and dynamic length scales in a simple glass-former

We investigate the static and dynamic properties of a weakly polydisperse hard sphere system in the deeply supercooled state, i.e. at densities higher than that corresponding to the mode-coupling transition. The structural analysis reveals the emergence of icosahedral locally favoured structures, previously only found in trace quantities. We present a new approach to probe the shape of dynamically heterogeneous regions, which is insensitive to particle packing effects that can hamper such analysis. Our results indicate that the shape of the dynamically heterogeneous regions changes only weakly and moreover hint that the often-used four-point correlation length may exhibit a growth in excess of that which our method identifies. The growth of the size of the dynamically heterogeneous regions appears instead to be in line with the one of structural and dynamic propensity correlations.

Growing timescales and lengthscales characterizing vibrations of amorphous solids

Low-temperature properties of crystalline solids can be understood using harmonic perturbations around a perfect lattice, as in Debye's theory. Low-temperature properties of amorphous solids, however, strongly depart from such descriptions, displaying enhanced transport, activated slow dynamics across energy barriers, excess vibrational modes with respect to Debye's theory (i.e., a boson peak), and complex irreversible responses to small mechanical deformations. These experimental observations indirectly suggest that the dynamics of amorphous solids becomes anomalous at low temperatures. Here, we present direct numerical evidence that vibrations change nature at a well-defined location deep inside the glass phase of a simple glass former. We provide a real-space description of this transition and of the rapidly growing time- and lengthscales that accompany it. Our results provide the seed for a universal understanding of low-temperature glass anomalies within the theoretical framework of the recently discovered Gardner phase transition.

Hopping and the Stokes-Einstein relation breakdown in simple glass formers.

One of the most actively debated issues in the study of the glass transition is whether a mean-field description is a reasonable starting point for understanding experimental glass formers. Although the mean-field theory of the glass transition--like that of other statistical systems--is exact when the spatial dimension d → ∞, the evolution of systems properties with d may not be smooth. Finite-dimensional effects could dramatically change what happens in physical dimensions,d = 2, 3. For standard phase transitions finite-dimensional effects are typically captured by renormalization group methods, but for glasses the corrections are much more subtle and only partially understood. Here, we investigate hopping between localized cages formed by neighboring particles in a model that allows to cleanly isolate that effect. By bringing together results from replica theory, cavity reconstruction, void percolation, and molecular dynamics, we obtain insights into how hopping induces a breakdown of the Stokes-Einstein relation and modifies the mean-field scenario in experimental systems. Although hopping is found to supersede the dynamical glass transition, it nonetheless leaves a sizable part of the critical regime untouched. By providing a constructive framework for identifying and quantifying the role of hopping, we thus take an important step toward describing dynamic facilitation in the framework of the mean-field theory of glasses.

Nanoconfinement of Ionic Liquid and Polymers in Supported Thin Films: A Time-of-Flight Secondary Ion Mass Spectrometry Study of Surface Diffusion

To gain insights into properties of a room-temperature ionic liquid in confined geometry, 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide ([emim][Tf2N]) adspecies interacting with various substrates was investigated as a function of temperature by using time-of-flight secondary ion mass spectrometry. The monolayer of adspecies is incorporated into substrates of porous Si, self-assembled monolayers, and Li[Tf2N] at 180–190 K that is slightly greater than the bulk glass transition temperature, Tg, of [emim][Tf2N]. In contrast to simple glass formers, no surface diffusivity is observed for [emim][Tf2N] in the sub-Tg region. The uptake of [emim][Tf2N] into polymer films is associated with the surface mobility of polymer segments rather than adspecies. The mobility of poly(methyl methacrylate) is not quenched by hydrogen bonds with the substrate, as evidenced by that the uptake onset of [emim][Tf2N] is almost independent of the polymer film thickness.

"Pre-Peak" in the Structure Factor of Simple Molecular Glass Formers

X-ray investigations of three fragile simple molecular glass forming liquids (m-toluidine, m-cresol, methylhydrazine) are presented, in the stable, undercooled and glassy state. Upon decreasing temperature, a ''pre-peak'' appears in the structure factor of all the investigated compounds in very similar conditions. It reveals a reversible intermediate range order of the liquid. A standard interpretation in terms of microcrystalline organization would lead to spatial repeat distances typically of 10 Angstrom and correlation length at most similar to 50 Angstrom at T-g. The features of this organization may be discussed in crystallographic terms from a comparison with the spectra of the crystalline varieties.

Geometrical Frustration and Static Correlations in a Simple Glass Former

We study the geometrical frustration scenario of glass formation for simple hard-sphere models. We find that the dual picture in terms of defects brings little insight and no theoretical simplification for the understanding of the slowing down of relaxation, because of the strong frustration characterizing these systems. The possibility of a growing static length is furthermore found to be physically irrelevant in the regime that is accessible to computer simulations.

Rayleigh-Brillouin light-scattering study of a simple glass former: Evidence of locally favored structures

The Rayleigh-Brillouin light scattering is studied in the glass-forming picoline with emphasis to the temperature dependence of the Landau-Placzek ratio. It was found that the Landau-Placzek ratio is in good agreement with the theoretical prediction at T>T(A), where TA is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature. Below T(A) the Landau-Placzek ratio exceeds significantly the theoretical prediction. The finding is interpreted as the evidence of the locally favored structures, which presence in an equilibrium glass-forming liquid below T(A).

Evolution of excess wing and β-process in simple glass formers

Dielectric loss spectra of glass forming liquids are analyzed, with emphasis on systems for which a peak due to a secondary relaxation is not immediately obvious. Thus, glass formers are considered for which the high-frequency flank of the alpha-relaxation peak appears to be dominated by a so-called wing contribution. It is shown that even for such supercooled liquids the shape of the alpha-peak has to be characterized by two parameters. By performing a series of aging experiments it is demonstrated that the high-frequency flank of the alpha-relaxation, assumed to follow a power-law behavior, is superimposed by contributions from an excess wing and from a beta-relaxation peak. In particular, the excess wing, previously associated with either the alpha- or the beta-relaxation, is identified as a feature that evolves in its own right. It is argued that excess wing and beta-relaxation are always present albeit with relative strengths that may vastly differ from glass former to glass former.

Dynamic Arrest in Polymer Melts: Competition between Packing and Intramolecular Barriers

We present molecular dynamics simulations of a simple model for polymer melts with intramolecular barriers. We investigate structural relaxation as a function of the barrier strength. Dynamic correlators can be consistently analyzed within the framework of the mode coupling theory of the glass transition. Control parameters are tuned in order to induce a competition between general packing effects and polymer-specific intramolecular barriers as mechanisms for dynamic arrest. This competition yields unusually large values of the so-called mode coupling theory exponent parameter and rationalizes qualitatively different observations for simple bead-spring and realistic polymers. The systematic study of the effect of intramolecular barriers presented here also establishes a fundamental difference between the nature of the glass transition in polymers and in simple glass formers.

The measurement of mechanical properties of glycerol, m-toluidine, and sucrose benzoate under consideration of corrected rheometer compliance: An in-depth study and review

Determination of the mechanical response of materials can be fraught with error if rheometer compliance is not properly taken into account. The resulting inaccuracies in the determined mechanical properties of the materials of interest can result in mistakes in material modeling, design, and theory. In the present work, we build on our previous report [K. Schroter, S. A. Hutcheson, X. Shi, A. Mandanici, and G. B. McKenna, J. Chem. Phys. 125, 214507 (2006)] and investigate the effects of instrument compliance that result from use of a commercial rotary rheometer and its fixtures on the determination of the dynamic shear and stress relaxation responses of glycerol, m-toluidine, and sucrose benzoate near to the glass transition regime. We revisit the procedure for compliance corrections presented in earlier work and correct dynamic shear data for these materials. We also present a new correction procedure to obtain shear stress relaxation curves from data that was obtained using this instrument. In addition, we broaden our consideration of compliance effects to materials, such as polymer melts, that have lower moduli than the simple glass formers previously considered. We discuss the possible errors for the viscoelastic response of glass-forming liquids and polymer melts or rubber networks that have been reported in the literature. A major purpose of the present work is to alert the community to possible problems in modulus values and relaxation functions obtained for a large number of materials for which rotary rheometers were used in a range where material stiffness (modulus and geometry effects) was comparable to the rheometer stiffness. Finally, we include recommendations for both experimental protocol and instrument design to avoid, minimize, or correct for compliance effects.

Transition from single-molecule to cooperative dynamics in a simple glass former: Raman line-shape analysis

Parameters for orientational and inhomogeneous broadening are found from the line shape analysis of the Raman spectrum of the glass former alpha-picoline during cooling from low-viscous to glassy state. The orientational phase loss time tau(OPL), extracted from the analysis, coincides with the alpha relaxation time at T > T(A), where T(A) is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the alpha-relaxation time dependence on temperature. At lower temperatures tau(OPL) (T) continues the Arrhenius behavior, in contrast to the alpha-relaxation time. The width of inhomogeneous broadening of the Raman line decreases noticeably as temperature increases in the temperature range T(g) < T < T(A), approaching to zero at T approximately T(A). The findings evidence the transition of molecular dynamics from individual to cooperative at T = T(A).

Dynamic shear modulus of glycerol: Corrections due to instrument compliance

A recent article by Shi et al. [J. Chem. Phys.123, 174507 (2005)] reports results from mechanical measurements on three simple inorganic glass formers: glycerol, m-toluidine, and sucrose benzoate. The experiments carried out were stress relaxation, aging, and dynamic (all in shear) using a torsional rheometer, an advanced rheometric expansion system (TA Instruments). The original force rebalance transducer (2KFRT) supplied with the system was replaced with a custom-made load cell (Sensotec) that had a capacity of 20 000 g cm in torque and 5000 g in normal force. The replacement of the load cell was done due to the belief that the main source of compliance in this instrument was from the 2KFRT. With this assumption, the authors published their results for the three materials of interest and compared their results with the techniques of Schroter and Donth [J. Chem. Phys.113, 9101 (2000)] for the measurements on glycerol and reported important differences. These differences were disputed by one of the present authors (Schroter), and the present report shows that the results from Schroter and Donth are correct. We show that the reasons have to do with the instrument compliance being greater than originally thought by Shi et al. Here we examine the effects of platen diameter/geometry on the glycerol dynamic moduli, describe a means to correct dynamic data, present a revised comparison of the corrected data with that of Schroter and Donth, and provide a discussion of future work and conclusions.

Correlation of fragility and Poisson’s ratio: Difference between metallic and nonmetallic glass formers

It is shown that fragility of supercooled metallic liquids correlates with the Poisson's ratio of the respective metallic glasses. However, the correlation differs from that found previously for simple nonmetallic glass formers [V. N. Novikov and A. P. Sokolov, Nature 431, 961 (2004)]. The observed difference is assigned to the contribution of the free electron gas to the bulk modulus in metallic glasses.

Particle mobility and metabasin exploration in model glass-formers

In this work, we assess the relevance of different range particle displacements and string motions to the metabasin (MB) dynamics of a simple glass former. We find that MB transitions are triggered by the occurrence of massive rearrangements termed as “democratic clusters” or d-clusters which last on average less than 1% of the α-relaxation time. Thus, at variance from previous investigations that focused on large displacements and strings, our work points to the essential role of medium-range particle displacements: we find that d-clusters are compact objects since the local neighborhoods of the most mobile particles exhibit significant rearrangements with a clear prevalence of medium-range displacements (of around 20% of the interparticle distance). This is in contrast to the string-like movements for which we demonstrate that many of them fully develop in isolation (their local environment being mostly insensitive to them) and at times different from that of MB transitions, being thus irrelevant to the α-relaxation. Additionally, from the analysis of the MB dynamics, preliminary support to the dynamics facilitation scenario is provided.

Democratic Particle Motion for Metabasin Transitions in Simple Glass Formers

We use molecular dynamics computer simulations to investigate the local motion of the particles in a supercooled binary liquid. Using the concept of the distance matrix, we find that the alpha relaxation corresponds to a small number of crossings from one metabasin to a neighboring one. Each crossing is very rapid and involves the collective motion of O(40) particles that form a relatively compact cluster, whereas stringlike motions seem not to be relevant for these transitions. These compact clusters are thus potential candidates for the cooperatively rearranging regions proposed a long time ago by Adam and Gibbs.