Experimental evidence of mosaic structure in strongly supercooled molecular liquids

Unifying the temperature dependent dynamics of glass formers

Strong changes in bulk properties, such as modulus and viscosity, are observed near the glass transition temperature, Tg, of amorphous materials. For more than a century, intense efforts have been made to define a microscopic origin for these macroscopic changes in properties. Using transition state theory (TST), we delve into the atomic/molecular level picture of how microscopic localized unit relaxations, or "cage rattles," evolve to macroscopic structural relaxations above Tg. Unit motion is broken down into two populations: (1) simultaneous rearrangement occurs among a critical number of units, nα, which ranges from 1 to 4, allowing a systematic classification of glass formers, GFs, that is compared to fragility; and (2) near Tg, adjacent units provide additional free volume for rearrangement, not simultaneously, but within the "primitive" lifetime, τ1, of one unit rattling in its cage. Relaxation maps illustrate how Johari-Goldstein β-relaxations stem from the rattle of nα units. We analyzed a wide variety of glassy materials and materials with a glassy response using literature data. Our four-parameter equation fits "strong" and "weak" GFs over the entire range of temperatures and also extends to other glassy systems, such as ion-transporting polymers and ferroelectric relaxors. The role of activation entropy in boosting preexponential factors to high "unphysical" apparent frequencies is discussed. Enthalpy-entropy compensation is clearly illustrated using the TST approach.

Microscopic Observation of the Anisotropy of the Johari-Goldstein-β Process in Cross-Linked Polybutadiene on Stretching by Time-Domain Interferometry

The strain dependence of the Johari-Goldstein (JG)-β relaxation time, as well as the directional dependence, was systematically investigated for stretched cross-linked polybutadiene using time-domain interferometry. We found that the strain dependence of the JG-β relaxation time is directionally dependent, contrary to expectation: the relaxation time of the JG-β motion, whose displacement is perpendicular to the stretching direction, decreases with stretching, whereas the relaxation time of the parallel JG-β motion changes little. This result is distinct from the previously reported strain dependence of the α relaxation time, where the relaxation time increases isotropically with stretching. Thus, the difference in the strain dependence of the relaxation time between the α and JG-β processes suggests a microscopic origin and requires the modification of the conventional dynamic picture for stretched polymers.

Internal Secondary Relaxation as a Dielectric Probe of Molecular Surroundings

We investigated the secondary relaxation behavior in rotor molecules in a glassy and crystalline state by using the dielectric method. Without changing the molecular source of secondary relaxation, only by modifying the environment around the rotating unit we observed notable variations in spectral parameters. Our results show that internal rotation, like a probe, can sample the immediate surroundings with high sensitivity to molecular-level changes that impact the rotation parameters. Our research offers a new perspective on the dielectric behavior of internal secondary relaxations and challenges the paradigm of their irrelevant nature.

Solid-that-Flows Picture of Glass-Forming Liquids

This perspective article reviews arguments that glass-forming liquids are different from those of standard liquid-state theory, which typically have a viscosity in the mPa·s range and relaxation times on the order of picoseconds. These numbers grow dramatically and become 1012 - 1015 times larger for liquids cooled toward the glass transition. This translates into a qualitative difference, and below the "solidity length" which is roughly one micron at the glass transition, a glass-forming liquid behaves much like a solid. Recent numerical evidence for the solidity of ultraviscous liquids is reviewed, and experimental consequences are discussed in relation to dynamic heterogeneity, frequency-dependent linear-response functions, and the temperature dependence of the average relaxation time.

Tracing the slow Arrhenius process deep in the glassy state-quantitative evaluation of the dielectric relaxation of bulk samples and thin polymer films in the temperature domain

The slow Arrhenius process (SAP) is a dielectric mode connected to thermally activated equilibration mechanisms, allowing for a fast reduction in free energy in liquids and glasses. The SAP, however, is still poorly understood, and so far, this process has mainly been investigated at temperatures above the glass transition. By employing a combination of methods to analyze dielectric measurements under both isochronal and isothermal conditions, we were able to quantitatively reproduce the dielectric response of the SAP of different polymers and to expand the experimental regime over which this process can be observed down to lower temperatures, up to 70 K below the glass transition. Employing thin films of thicknesses varying between 10 and 800 nm, we further verified that the peak shape and activation energy of the SAP of poly(4-bromostyrene) are not sensitive to temperature, nor do they vary upon confinement at the nanoscale level. These observations confirm the preliminary trends reported for other polymers. We find that one single set of parameters-meaning the activation barrier and the pre-exponential factor, respectively, linked to the enthalpic and entropic components of the process-can describe the dynamics of the SAP in both the supercooled liquid and glassy states, in bulk and thin films. These results are discussed in terms of possible molecular origins of the slow Arrhenius process in polymers.

The slow Arrhenius process in small organic molecules

Equilibration, the complex set of molecular rearrangements leading to more stable states, is usually dominated by density fluctuations, occurring through the structural (α-)relaxation, whose timescale quickly increases upon cooling. Growing evidence shows, however, that equilibration can be reached also through an alternative pathway provided by the Slow Arrhenius process (SAP), a molecular mode slower than the structural processes in the liquid state and faster in glass. The SAP, widely observed in polymers, has not yet been reported in small molecules, probably because of the larger experimental difficulties in handling these systems. Here, we report the presence of the SAP in three different molecular glassformers, by investigating these systems in the thin film geometry via dielectric spectroscopy. These results reinforce the idea that the SAP is a universal feature of liquid and glassy dynamics.

Size-dependent vitrification in metallic glasses

Reducing the sample size can profoundly impact properties of bulk metallic glasses. Here, we systematically reduce the length scale of Au and Pt-based metallic glasses and study their vitrification behavior and atomic mobility. For this purpose, we exploit fast scanning calorimetry (FSC) allowing to study glassy dynamics in an exceptionally wide range of cooling rates and frequencies. We show that the main α relaxation process remains size independent and bulk-like. In contrast, we observe pronounced size dependent vitrification kinetics in micrometer-sized glasses, which is more evident for the smallest samples and at low cooling rates, resulting in more than 40 K decrease in fictive temperature, Tf, with respect to the bulk. We discuss the deep implications on how this outcome can be used to convey glasses to low energy states.

Johari-Goldstein β relaxation in glassy dynamics originates from two-scale energy landscape

Supercooled liquids undergo complicated structural relaxation processes, which have been a long-standing problem in both experimental and theoretical aspects of condensed matter physics. In particular, past experiments widely observed for many types of molecular liquids that relaxation dynamics separated into two distinct processes at low temperatures. One of the possible interpretations is that this separation originates from the two-scale hierarchical topography of the potential energy landscape; however, it has never been verified. Molecular dynamics simulations are a promising approach to tackle this issue, but we must overcome laborious difficulties. First, we must handle a model of molecular liquids that is computationally demanding compared to simple spherical models, which have been intensively studied but show only a slower process: α relaxation. Second, we must reach a sufficiently low-temperature regime where the two processes become well-separated. Here, we handle an asymmetric dimer system that exhibits a faster process: Johari-Goldstein β relaxation. Then, we employ the parallel tempering method to access the low-temperature regime. These laborious efforts enable us to investigate the potential energy landscape in detail and unveil the first direct evidence of the topographic hierarchy that induces the β relaxation. We also successfully characterize the microscopic motions of particles during each relaxation process. Finally, we study the correlation between low-frequency modes and two relaxation processes. Our results establish a fundamental and comprehensive understanding of experimentally observed relaxation dynamics in supercooled liquids.

Chloroplasts in plant cells show active glassy behavior under low-light conditions

Plants have developed intricate mechanisms to adapt to changing light conditions. Besides phototropism and heliotropism (differential growth toward light and diurnal motion with respect to sunlight, respectively), chloroplast motion acts as a fast mechanism to change the intracellular structure of leaf cells. While chloroplasts move toward the sides of the plant cell to avoid strong light, they accumulate and spread out into a layer on the bottom of the cell at low light to increase the light absorption efficiency. Although the motion of chloroplasts has been studied for over a century, the collective organelle motion leading to light-adapting self-organized structures remains elusive. Here, we study the active motion of chloroplasts under dim-light conditions, leading to an accumulation in a densely packed quasi-2D layer. We observe burst-like rearrangements and show that these dynamics resemble systems close to the glass transition by tracking individual chloroplasts. Furthermore, we provide a minimal mathematical model to uncover relevant system parameters controlling the stability of the dense configuration of chloroplasts. Our study suggests that the meta-stable caging close to the glass transition in the chloroplast monolayer serves a physiological relevance: Chloroplasts remain in a spread-out configuration to increase the light uptake but can easily fluidize when the activity is increased to efficiently rearrange the structure toward an avoidance state. Our research opens questions about the role that dynamical phase transitions could play in self-organized intracellular responses of plant cells toward environmental cues.

Arriving at the most plausible interpretation of the dielectric spectra of glycerol with help from quasielastic γ-ray scattering time-domain interferometry

Glycerol is one of the glass-forming liquids selected by Robert H. Cole in 1950 to start his study of molecular dynamics by dielectric spectroscopy. Seventy-one years have gone by and remarkably no consensus has been reached on the nature and identity of the relaxation processes observed in the dielectric spectra. The macroscopic dielectric relaxation data allow different interpretations to yield contrasting results, and it is not possible to determine which one is most plausible. Coming to the rescue is the application of the nuclear γ-resonance time-domain interferometry (TDI) to glycerol by Saito et al. [Phys. Rev. E 105, L012605 (2022)10.1103/PhysRevE.105.L012605]. Their microscopic TDI data potentially can decide which interpretation of the dielectric spectra of glycerol is most plausible. The attempt was made by Saito et al., but there is a problem in their analysis of the dielectric data of glycerol and hence their conclusion is untenable. In this paper, we critically compare four major interpretations with the TDI data in an effort to identify the most plausible interpretation of the relaxation processes constituting the dielectric spectra of glycerol.

Q-dependent collective relaxation dynamics of glass-forming liquid Ca0.4K0.6(NO3)1.4 investigated by wide-angle neutron spin-echo

The relaxation behavior of glass formers exhibits spatial heterogeneity and dramatically changes upon cooling towards the glass transition. However, the underlying mechanisms of the dynamics at different microscopic length scales are not fully understood. Employing the recently developed wide-angle neutron spin-echo spectroscopy technique, we measured the Q-dependent coherent intermediate scattering function of a prototypical ionic glass former Ca_0.4K_0.6(NO₃)_1.4, in the highly viscous liquid state. In contrast to the structure modulated dynamics for Q < 2.4 Å⁻¹, i.e., at and below the structure factor main peak, for Q > 2.4 Å⁻¹, beyond the first minimum above the structure factor main peak, the stretching exponent exhibits no temperature dependence and concomitantly the relaxation time shows smaller deviations from Arrhenius behavior. This finding indicates a change in the dominant relaxation mechanisms around a characteristic length of 2π/(2.4 Å⁻¹) ≈ 2.6 Å, below which the relaxation process exhibits a temperature independent distribution and more Arrhenius-like behavior.

Length scale dependence is important for understanding the collective relaxation dynamics in glass-forming liquids. Here, the authors find in liquid Ca_0.4K_0.6(NO₃)_1.4 a change in the dominant relaxation mechanisms around 2.6 Å, below which the relaxation process exhibits a temperature independent distribution and more Arrhenius-like behavior.

Microscopic observation of hidden Johari-Goldstein-β process in glycerol

The Johari-Goldstein-β (JG-β) process is widely observed in a variety of glass-forming systems and recognized as an intrinsic process in deeply supercooled and glassy states. However, in some systems, e.g., glycerol, a clear sign of the JG-β process is often not apparent; for example, an isolated JG-β peak may not be observed in the dielectric relaxation spectrum. In this study, we directly investigated the angstrom-scale dynamics of glycerol through quasielastic scattering experiments using time-domain interferometry. The relaxation times of the local motions start to decouple from the timescale of the diffusion process and follow the established behavior of the JG-β process. This finding microscopically indicates the existence of the hidden JG-β process in glycerol. In addition, we succeeded in determining the decoupling temperature of the JG-β process by using the spatial-scale selectivity of the quasielastic scattering technique.

Microscopic understanding of the Johari-Goldstein β relaxation gained from nuclear γ-resonance time-domain-interferometry experiments

Traditionally the study of dynamics of glass-forming materials has been focused on the structural α relaxation. However, in recent years experimental evidence has revealed that a secondary β relaxation belonging to a special class, called the Johari-Goldstein (JG) β relaxation, has properties strongly linked to the primary α relaxation. By invoking the principle of causality, the relation implies the JG β relaxation is fundamental and indispensable for generating the α relaxation, and the properties of the latter are inherited from the former. The JG β relaxation is observed together with the α relaxation mostly by dielectric spectroscopy. The macroscopic nature of the data allows the use of arbitrary or unproven procedures to analyze the data. Thus the results characterizing the JG β relaxation and the relation of its relaxation time τ_{β} to the α-relaxation time τ_{α} obtained can be equivocal and controversial. Coming to the rescue is the nuclear resonance time-domain-interferometry (TDI) technique covering a wide time range (10^{-9}-10^{-5}s) and a scattering vector q range (9.6-40nm^{-1}). TDI experiments have been carried out on four glass formers, ortho-terphenyl [M. Saito et al., Phys. Rev. Lett. 109, 115705 (2012)10.1103/PhysRevLett.109.115705], polybutadiene [T. Kanaya et al., J. Chem. Phys. 140, 144906 (2014)10.1063/1.4869541], 5-methyl-2-hexanol [F. Caporaletti et al., Sci. Rep. 9, 14319 (2019)10.1038/s41598-019-50824-7], and 1-propanol [F. Caporaletti et al., Nat. Commun. 12, 1867 (2021)10.1038/s41467-021-22154-8]. In this paper the TDI data are reexamined in conjunction with dielectric and neutron scattering data. The results show the JG β relaxation observed by dielectric spectroscopy is heterogeneous and comprises processes with different length scales. A process with a longer length scale has a longer relaxation time. TDI data also prove the primitive relaxation time τ_{0} of the coupling model falls within the distribution of the TDI q-dependent JG β-relaxation times. This important finding explains why the experimental dielectric JG β-relaxation times τ_{β}(T,P) is approximately equal to τ_{0}(T,P) as found in many glass formers at various temperature T and pressure P. The result, τ_{β}(T,P)≈τ_{0}(T,P), in turn explains why the ratio τ_{α}(T,P)/τ_{β}(T,P) is invariant to changes of T and pressure P at constant τ_{α}(T,P), the α-relaxation time.

Excess wings and asymmetric relaxation spectra in a facilitated trap model

In a recent computer study, we have shown that the combination of spatially heterogeneous dynamics and kinetic facilitation provides a microscopic explanation for the emergence of excess wings in deeply supercooled liquids. Motivated by these findings, we construct a minimal empirical model to describe this physics and introduce dynamic facilitation in the trap model, which was initially developed to capture the thermally activated dynamics of glassy systems. We fully characterize the relaxation dynamics of this facilitated trap model varying the functional form of energy distributions and the strength of dynamic facilitation, combining numerical results and analytic arguments. Dynamic facilitation generically accelerates the relaxation of the deepest traps, thus making relaxation spectra strongly asymmetric, with an apparent "excess" signal at high frequencies. For well-chosen values of the parameters, the obtained spectra mimic experimental results for organic liquids displaying an excess wing. Overall, our results identify the minimal physical ingredients needed to describe excess processes in the relaxation spectra of supercooled liquids.

The origin of the faster mechanism of partial enthalpy recovery deep in the glassy state of polymers

A novel finding made by Cangialosi and coworkers in the physical aging of several polymers way below the glass transition temperature Tg is that equilibrium recovery occurs by reaching a plateau in the enthalpy with partial enthalpy recovery. This observation points to the existence of a much faster mechanism capable of partial equilibrium recovery deep in the glassy state. A similar phenomenon was found in different glassy materials. The generality of the phenomenon indicates that the faster mechanism of equilibrium recovery is universal and fundamental. In this paper the faster mechanism is identified to be the universal JG β-relaxation having dynamic and thermodynamic properties analogous to the α-relaxation, and thus capable of effecting enthalpy and volume recovery far below Tg in several high-Tg polymers. The JG β-relaxation is also the mechanism responsible for the first step of two steps in the approach to equilibrium found in another polymer with much lower Tg. The Coupling Model is used to explain why the first step transpires far below Tg in some polymers but much closer to Tg in another polymer.