Indications for a change of diffusion mechanism in supercooled liquids

Understanding dynamics in coarse-grained models. IV. Connection of fine-grained and coarse-grained dynamics with the Stokes-Einstein and Stokes-Einstein-Debye relations

Applying an excess entropy scaling formalism to the coarse-grained (CG) dynamics of liquids, we discovered that missing rotational motions during the CG process are responsible for artificially accelerated CG dynamics. In the context of the dynamic representability between the fine-grained (FG) and CG dynamics, this work introduces the well-known Stokes-Einstein and Stokes-Einstein-Debye relations to unravel the rotational dynamics underlying FG trajectories, thereby allowing for an indirect evaluation of the effective rotations based only on the translational information at the reduced CG resolution. Since the representability issue in CG modeling limits a direct evaluation of the shear stress appearing in the Stokes-Einstein and Stokes-Einstein-Debye relations, we introduce a translational relaxation time as a proxy to employ these relations, and we demonstrate that these relations hold for the ambient conditions studied in our series of work. Additional theoretical links to our previous work are also established. First, we demonstrate that the effective hard sphere radius determined by the classical perturbation theory can approximate the complex hydrodynamic radius value reasonably well. Furthermore, we present a simple derivation of an excess entropy scaling relationship for viscosity by estimating the elliptical integral of molecules. In turn, since the translational and rotational motions at the FG level are correlated to each other, we conclude that the "entropy-free" CG diffusion only depends on the shape of the reference molecule. Our results and analyses impart an alternative way of recovering the FG diffusion from the CG description by coupling the translational and rotational motions at the hydrodynamic level.

Relaxation and diffusion of an ionic plasticizer in amorphous poly(vinylpyrrolidone)

The present work focuses on the dynamics of the ionic constituents of 1-propyl-3-methyl-imidazolium-bis-(trifluormethylsulfonyl)-imide (PT), a paradigmatic ionic liquid, as an additive in poly(vinylpyrrolidone) (PVP). Hence, the resulting product can be regarded as a polymer electrolyte as well as an amorphous dispersion. Leveraging dielectric spectroscopy and oscillatory shear rheology, complemented by differential scanning calorimetry, the spectral shapes and the relaxation maps of the supercooled PVP-PT mixtures are accessed in their full compositional range. The study also presents dielectric and shear responses of neat PVP with a molecular weight of 2500 g mol-1. We discuss the plasticizing role of the PT additive and the decoupling between ionic dynamics and segmental relaxation in these mixtures. The extracted relaxation times, steady-state viscosities, and conductivities are employed to estimate the translational diffusivities of the ionic penetrants by means of the Stokes-Einstein, Nernst-Einstein, and Almond-West relations. While some of the estimated diffusivities agree with each other, some do not, pointing to the importance of the chosen hydrodynamic approximations and the type of response considered for the analysis. The present extensive dielectric, rheological, and calorimetric study enables a deeper understanding of relaxation and transport of ionic ingredients in polymers, particularly in the slow-dynamics regime which is difficult to access experimentally by direct-diffusivity probes.

Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

The glass formation and the dipolar reorientational motions in deep eutectic solvents (DESs) are frequently overlooked, despite their crucial role in defining the room-temperature physiochemical properties. To understand the effects of these dynamics on the ionic conductivity and their relation to the mechanical properties of the DES, we conducted broadband dielectric and rheological spectroscopy over a wide temperature range on three well-established carboxylic acid-based natural DESs. These are the eutectic mixtures of choline chloride with oxalic acid (oxaline), malonic acid (maline), and phenylacetic acid (phenylaceline). In all three DESs, we observe signs of a glass transition in the temperature dependence of their dipolar reorientational and structural dynamics, as well as varying degrees of motional decoupling between the different observed dynamics. Maline and oxaline display a breaking of the Walden rule near the glass-transition temperature, while the relation between the dc conductivity and dipolar relaxation time in both maline and phenylaceline is best described by a power law. The glass-forming properties of the investigated systems not only govern the orientational dipolar motions and rheological properties, which are of interest from a fundamental point of view, but they also affect the dc conductivity, even at room temperature, which is of high technical relevance.

Understanding the glassy dynamics from melting temperatures in binary glass-forming liquids

It is natural to expect that small particles in binary mixtures move faster than large ones. However, in binary glass-forming liquids with soft-core particle interactions, we observe the counterintuitive dynamic reversal between large and small particles along with the increase of pressure by performing molecular dynamics simulations. The structural relaxation (dynamic heterogeneity) of small particles is faster (weaker) than large ones at low pressures, but becomes slower (stronger) above a crossover pressure. In contrast, this dynamic reversal never happens in glass-forming liquids with hard-core interactions. We find that the difference of the effective melting temperatures felt by large and small particles can be used to understand the dynamic reversal. In binary mixtures, we derive effective melting temperatures of large and small particles simply from the conversion of units and find that particles with a higher effective melting temperature usually undergo a slower and more heterogeneous relaxation. The presence (absence) of the dynamic reversal in soft-core (hard-core) systems is simply due to the non-monotonic (monotonic) behavior of the melting temperature as a function of pressure. Interestingly, by manipulating the relative softness between large and small particles, we obtain a special case of soft-core systems, in which large particles always have higher effective melting temperatures than small ones. As a result, the dynamic reversal is totally eliminated. Our work provides another piece of evidence of the underlying connections between the properties of non-equilibrium glass-formers and equilibrium crystal-formers.

Approach to hyperuniformity in a metallic glass-forming material exhibiting a fragile to strong glass transition

We investigate a metallic glass-forming (GF) material (Al₉₀Sm₁₀) exhibiting a fragile-strong (FS) glass-formation by molecular dynamics simulation to better understand this highly distinctive pattern of glass-formation in which many of the usual phenomenological relations describing relaxation times and diffusion of ordinary GF liquids no longer apply, and where instead genuine thermodynamic features are observed in response functions and little thermodynamic signature is exhibited at the glass transition temperature, T_g. Given the many unexpected similarities between the thermodynamics and dynamics of this metallic GF material with water, we first focus on the anomalous static scattering in this liquid, following recent studies on water, silicon and other FS GF liquids. We quantify the "hyperuniformity index" H of our liquid, which provides a quantitative measure of molecular "jamming". To gain insight into the T-dependence and magnitude of H, we also estimate another more familiar measure of particle localization, the Debye-Waller parameter 〈u²〉 describing the mean-square particle displacement on a timescale on the order of the fast relaxation time, and we also calculate H and 〈u²〉 for heated crystalline Cu. This comparative analysis between H and 〈u²〉 for crystalline and metallic glass materials allows us to understand the critical value of H on the order of 10^-3 as being analogous to the Lindemann criterion for both the melting of crystals and the "softening" of glasses. We further interpret the emergence of FS GF and liquid-liquid phase separation in this class of liquids to arise from a cooperative self-assembly process in the GF liquid.

Relaxation time scales of interfacial water upon fluid to ripple to gel phase transitions of bilayers

The slow relaxation of interface water (IW) across three primary phases of membranes is relevant to understand the influence of IW on membrane functions at supercooled conditions. To this objective, a total of ∼16.26μs all-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine lipid membranes are carried out. A supercooling-driven drastic slow-down in heterogeneity time scales of the IW is found at the fluid to the ripple to the gel phase transitions of the membranes. At both fluid-to-ripple-to-gel phase transitions, the IW undergoes two dynamic crossovers in Arrhenius behavior with the highest activation energy at the gel phase due to the highest number of hydrogen bonds. Interestingly, the Stokes-Einstein (SE) relation is conserved for the IW near all three phases of the membranes for the time scales derived from the diffusion exponents and the non-Gaussian parameters. However, the SE relation breaks for the time scale obtained from the self-intermediate scattering functions. The behavioral difference in different time scales is universal and found to be an intrinsic property of glass. The first dynamical transition in the α relaxation time of the IW is associated with an increase in the Gibbs energy of activation of hydrogen bond breaking with locally distorted tetrahedral structures, unlike the bulk water. Thus, our analyses unveil the nature of the relaxation time scales of the IW across membrane phase transitions in comparison with the bulk water. The results will be useful to understand the activities and survival of complex biomembranes under supercooled conditions in the future.

Efficient d-dimensional molecular dynamics simulations for studies of the glass-jamming transition

We develop an algorithm suitable for parallel molecular dynamics simulations in d spatial dimensions and describe its implementation in C++. All routines work in arbitrary d; the maximum simulated d is limited only by available computing resources. These routines include several that are particularly useful for studies of the glass-jamming transition, such as SWAP Monte Carlo and FIRE energy minimization. The scalings of simulation runtimes with the number of particles N and number of simulation threads n_{threads} are comparable to popular molecular dynamics codes such as LAMMPS. The efficient parallel implementation allows simulation of systems that are much larger than those employed in previous high-dimensional glass-transition studies. As a demonstration of the code's capabilities, we show that supercooled d=6 liquids can possess dynamics that are substantially more heterogeneous and experience a breakdown of the Stokes-Einstein relation that is substantially stronger than previously reported, owing at least in part to the much smaller system sizes employed in earlier simulations.

Advances in the study of supercooled water

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Spatial Dimensionality Dependence of Heterogeneity, Breakdown of the Stokes-Einstein Relation, and Fragility of a Model Glass-Forming Liquid

We investigate the heterogeneity of dynamics, the breakdown of the Stokes-Einstein relation and fragility in a model glass forming liquid, a binary mixture of soft spheres with a harmonic interaction potential for spatial dimensions from 3 to 8. The dynamical heterogeneity is quantified through the dynamical susceptibility χ₄ and the non-Gaussian parameter α₂. We find that the fragility, the degree of breakdown of the Stokes-Einstein relation, and the heterogeneity of the dynamics decrease with increasing spatial dimensionality. We briefly describe the dependence of fragility on the density and use it to resolve an apparent inconsistency with previous results.

Dehydration induced dynamical heterogeneity and ordering mechanism of lipid bilayers

Understanding the influence of dehydration on the membrane structure is crucial to control membrane functionality related to domain formation and cell fusion under anhydrobiosis conditions. To this end, we perform all-atom molecular dynamic simulations of 1,2-dimyristoyl-sn-glycero-3-phosphocholine dimyristoylphosphatidylcholine lipid membranes at different hydration levels at 308 K. As dehydration increases, the lipid area per head group decreases with an increase in bilayer thickness and lipid order parameters indicating bilayer ordering. Concurrently, translational and rotational dynamics of interfacial water (IW) molecules near membranes slow down. On the onset of bilayer ordering, the IW molecules exhibit prominent features of dynamical heterogeneity evident from non-Gaussian parameters and one-dimensional van Hove correlation functions. At a fully hydrated state, diffusion constants (D) of the IW follow a scaling relation, D∼τ_α ^-1, where the α relaxation time (τ_α) is obtained from self-intermediate scattering functions. However, upon dehydration, the relation breaks and the D of the IW follows a power law behavior as D∼τ_α ^-0.57, showing the signature of glass dynamics. τ_α and hydrogen bond lifetime calculated from intermittent hydrogen bond auto-correlation functions undergo a similar crossover in association with bilayer ordering on dehydration. The bilayer ordering is accompanied with an increase in fraction of caged lipids spanned over the bilayer surface and a decrease in fraction of mobile lipids due to the non-diffusive dynamics. Our analyses reveal that the microscopic mechanism of lipid ordering by dehydration is governed by dynamical heterogeneity. The fundamental understanding from this study can be applied to complex bio-membranes to trap functionally relevant gel-like domains at room temperature.

Dynamic heterogeneity, cooperative motion, and Johari-Goldstein [Formula: see text]-relaxation in a metallic glass-forming material exhibiting a fragile-to-strong transition

We investigate the Johari-Goldstein (JG) [Formula: see text]-relaxation process in a model metallic glass-forming (GF) material ([Formula: see text]), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG [Formula: see text]-relaxation time [Formula: see text] and the lifetime of the mobile particle clusters [Formula: see text], defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the [Formula: see text]-relaxation time [Formula: see text] and the lifetime of immobile particle clusters [Formula: see text]. Moreover, we find that the average diffusion coefficient D nearly coincides with [Formula: see text] of the smaller atomic species (Al) and that the 'hopping time' associated with D coincides with [Formula: see text] to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG [Formula: see text]-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG [Formula: see text]-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that [Formula: see text] exhibits fragile-to-strong (FS) glass formation, as found in many other metallic GF liquids, but this fact does not greatly alter the geometrical nature of DH in this material and the relation of DH to dynamical properties. On the other hand, the temperature dependence of the DH and dynamical properties, such as the structural relaxation time, can be significantly altered from 'ordinary' GF liquids.

Testing the Stokes-Einstein relation with the hard-sphere fluid model

The Stokes-Einstein (SE) relation has been widely applied to quantitatively describe the Brownian motion. Notwithstanding, here we show that even for a simple fluid, the SE relation may fail over a wide range of the Brownian particle's size. Namely, although the SE relation could be a good approximation for a large enough Brownian particle, a significant error may appear when decreasing the Brownian particle's size down to several hundred times the size of the fluid molecules, and the error increases with the decrease of the Brownian particle's size. The cause is rooted in the fact that the kinetic contribution to the diffusion coefficient is inversely proportional to the squared radius of the Brownian particle. After excluding the kinetic contribution, we show that the applicable range of the SE relation is expanded significantly.

Effects of Glass Transition and Structural Relaxation on Crystal Nucleation: Theoretical Description and Model Analysis

In the application of classical nucleation theory (CNT) and all other theoretical models of crystallization of liquids and glasses it is always assumed that nucleation proceeds only after the supercooled liquid or the glass have completed structural relaxation processes towards the metastable equilibrium state. Only employing such an assumption, the thermodynamic driving force of crystallization and the surface tension can be determined in the way it is commonly performed. The present paper is devoted to the theoretical treatment of a different situation, when nucleation proceeds concomitantly with structural relaxation. To treat the nucleation kinetics theoretically for such cases, we need adequate expressions for the thermodynamic driving force and the surface tension accounting for the contributions caused by the deviation of the supercooled liquid from metastable equilibrium. In the present paper, such relations are derived. They are expressed via deviations of structural order parameters from their equilibrium values. Relaxation processes result in changes of the structural order parameters with time. As a consequence, the thermodynamic driving force and surface tension, and basic characteristics of crystal nucleation, such as the work of critical cluster formation and the steady-state nucleation rate, also become time-dependent. We show that this scenario may be realized in the vicinity and below the glass transition temperature, and it may occur only if diffusion (controlling nucleation) and viscosity (controlling the alpha-relaxation process) in the liquid decouple. Analytical estimates are illustrated and confirmed by numerical computations for a model system. The theory is successfully applied to the interpretation of experimental data. Several further consequences of this newly developed theoretical treatment are discussed in detail. In line with our previous investigations, we reconfirm that only when the characteristic times of structural relaxation are of similar order of magnitude or longer than the characteristic times of crystal nucleation, elastic stresses evolving in nucleation may significantly affect this process. Advancing the methods of theoretical analysis of elastic stress effects on nucleation, for the first time expressions are derived for the dependence of the surface tension of critical crystallites on elastic stresses. As the result, a comprehensive theoretical description of crystal nucleation accounting appropriately for the effects of deviations of the liquid from the metastable states and of relaxation on crystal nucleation of glass-forming liquids, including the effect of simultaneous stress evolution and stress relaxation on nucleation, is now available. As one of its applications, this theoretical treatment provides a new tool for the explanation of the low-temperature anomaly in nucleation in silicate and polymer glasses (the so-called "breakdown" of CNT at temperatures below the temperature of the maximum steady-state nucleation rate). We show that this anomaly results from much more complex features of crystal nucleation in glasses caused by deviations from metastable equilibrium (resulting in changes of the thermodynamic driving force, the surface tension, and the work of critical cluster formation, in the necessity to account of structural relaxation and stress effects) than assumed so far. If these effects are properly accounted for, then CNT appropriately describes both the initial, the intermediate, and the final states of crystal nucleation.

Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

Influence of wall heterogeneity on nanoscopically confined polymers

We investigate via molecular dynamics simulations the behaviour of a polymer melt confined between surfaces with increasing spatial correlation (patchiness) of weakly and strongly interacting sites. Beyond a critical patchiness, we find a dramatic dynamic decoupling, characterized by a steep growth of the longest relaxation time and a constant diffusion coefficient. This arises from dynamic heterogeneities induced by the walls in the adjacent polymer layers, leading to the coexistence of fast and slow chain populations. Structural variations are also present, but they are not easy to detect. Our work opens the way to a better understanding of adhesion, friction, rubber reinforcement by fillers, and many other open issues involving the dynamics of polymeric materials on rough, chemically heterogeneous and possibly "dirty" surfaces.

Non-Gaussianity of the van Hove function and dynamic-heterogeneity length scale

Non-Gaussian nature of the probability distribution of particles' displacements in the supercooled temperature regime in glass-forming liquids are believed to be one of the major hallmarks of glass transition. It has already been established that this probability distribution, which is also known as the van Hove function, shows universal exponential tail. The origin of such an exponential tail in the distribution function is attributed to the hopping motion of particles observed in the supercooled regime. The non-Gaussian nature can also be explained if one assumes that the system has heterogeneous dynamics in space and time. Thus exponential tail is the manifestation of dynamic heterogeneity. In this work we directly show that non-Gaussianity of the distribution of particles' displacements occur over the dynamic heterogeneity length scale and the dynamical behavior course grained over this length scale becomes homogeneous. We study the non-Gaussianity of the van Hove function by systematically coarse graining at different length scales and extract the length scale of dynamic heterogeneity at which the shape of the van Hove function crosses over from non-Gaussian to Gaussian. The obtained dynamic heterogeneity scale is found to be in very good agreement with the scale obtained from other conventional methods.

Power law relationship between diffusion coefficients in multi-component glass forming liquids

The slow down of dynamics in glass forming liquids as the glass transition is approached has been characterised through the Adam-Gibbs relation, which relates relaxation time scales to the configurational entropy. The Adam-Gibbs relation cannot apply simultaneously to all relaxation times scales unless they are coupled, and exhibit closely related temperature dependences. The breakdown of the Stokes-Einstein relation presents an interesting situation to the contrary, and in analysing it, it has recently been shown that the Adam-Gibbs relation applies to diffusion coefficients rather than to viscosity or structural relaxation times related to the decay of density fluctuations. However, for multi-component liquids --the typical cases considered in computer simulations, metallic glass formers, etc.-- such a statement raises the question of which diffusion coefficient is described by the Adam-Gibbs relation. All diffusion coefficients can be consistently described by the Adam-Gibbs relation if they bear a power law relationship with each other. Remarkably, we find that for a wide range of glass formers, and for a wide range of temperatures spanning the normal and the slow relaxation regimes, such a relationship holds. We briefly discuss possible rationalisations of the observed behaviour.

Facile Method for the Investigation of Temperature-Dependent C60 Diffusion in Conjugated Polymers

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C₆₀ molecules from a C₆₀ layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C₆₀ molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C₆₀ can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C₆₀ diffusion is thermally activated and not driven by WFL-type collective motion above T_g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.

ESR evidence of the dynamic crossover in the supercooled liquid states of a series of solid n-alkanes

A study of the rotation dynamics of the spin probe TEMPO by electron spin resonance (ESR) spectroscopy in a series of six short- and medium-sized n-alkanes is presented. In addition to the usual crossover effects connected with solid-liquid phase transitions at temperature Tm and the solid-solid transition at Tss, we find a strong correlation between the change in reorientation at the characteristic ESR temperature TfastX1 < Tss and/or Tm with the ratio TfastX1/Tm = 0.76 ± 0.03 and the characteristic dynamic temperature, TX, obtained from the viscosity data. This indicates the presence of local disordered regions in the crystalline structure which is indeed sensitive to dynamic change at the crossover temperature TX ≅ TfastX1.

Unexpected Crossover in the kinetics of mutarotation in the supercooled region: the role of H-bonds

Intra- and intermolecular studies on the molten L-sorbose have been carried out at variable temperature conditions to determine the crosover temperature (T_c). In addition, isothermal time-dependent FTIR and Raman measurements were performed to probe the pace of mutarotation and activation energy of this reaction in the studied saccharide, which varied from 53–62 kJ/mol up to 177–192 kJ/mol below and above T_c, respectively. To explain the change in activation barrier for the mutarotation a complementary analysis using difference FTIR spectra collected around T_c = 365 K in the hydroxyl region has been done. It was found that the alteration of kinetic parameters and molecular dynamics around T_c are strictly related to the variation in the strength of H-bonds which above T_c are significantly weaken, increasing the freedom of rotation of functional groups and movement of individual molecules. That phenomenon most likely affects the proton transfer, underlying molecular mechanism of mutarotation, which may lead to the significant increase in activation barrier. The new insight into a molecular aspect of the mutarotation around T_c has created an opportunity to better understanding the relationship between physics of condensed matter and the potential role of H-bonds dynamics on the progress of the chemical reaction in highly viscous systems.

Pressure-Induced Glass Transition Probed via the Mobility of Coumarin 1 Fluorescent Molecule

The route to form a glass is generally achieved upon cooling where the slowing down might be interpreted as the trapping of molecules in potential wells. On the other hand, isothermal compression induces a glassy state by modifying the molecular packing ending in jamming. Here, we focus on how isothermal compression perturbs the mobility of a probe molecule in three different host liquids up to the pressure-induced glass transition. By use of the fluorescence recovery technique, the diffusion of the fluorescent molecule Coumarin 1 (C1) is measured in poly(propylene glycol) (PPG-1000M and -2700M), in the fragile van der Waals propylene carbonate (PC), and in hydrogen-bonded methanol and ethanol. High pressures up to 6 GPa are obtained with a diamond anvil cell. In PC at a pressure ∼1.3 GPa close to the glass-transition pressure, the diffusion coefficient of C1 follows an Arrhenius behavior with an ∼5 orders of magnitude increase of the diffusive time. No decoupling from the Stokes-Einstein equation is noticed. A similar exponential behavior is measured in ethanol and methanol but extended to different pressure ranges up to 2.5 and 6.2 GPa, respectively. In PPG-1000M a decoupling from the Stokes-Einstein relation is observed between 0.3 and 0.8 GPa that could be related to a modification of the interaction between polymer segments and the probe molecule. These results might indicate that interaction between probe and dynamic heterogeneities become less important under applied pressure, unlike in the temperature-induced glass transition.

Heterogeneous diffusion, viscosity, and the Stokes-Einstein relation in binary liquids

We investigate the origin of the breakdown of the Stokes-Einstein relation (SER) between diffusivity and viscosity in undercooled melts. A binary Lennard-Jones system, as a model for a metallic melt, is studied by molecular dynamics. A weak breakdown at high temperatures can be understood from the collectivization of motion, seen in the isotope effect. The strong breakdown at lower temperatures is connected to an increase in dynamic heterogeneity. On relevant time scales some particles diffuse much faster than the average or than predicted by the SER. The van Hove self-correlation function allows one to unambiguously identify slow particles. Their diffusivity is even less than predicted by the SER. The time span of these particles being slow particles, before their first conversion to be a fast one, is larger than the decay time of the stress correlation. The contribution of the slow particles to the viscosity rises rapidly upon cooling. Not only the diffusion but also the viscosity shows a dynamically heterogeneous scenario. We can define a "slow" viscosity. The SER is recovered as the relation between slow diffusivity and slow viscosity.

A molecular dynamics simulations study on the relations between dynamical heterogeneity, structural relaxation, and self-diffusion in viscous liquids

We perform molecular dynamics simulations for viscous liquids to study the relations between dynamical heterogeneity, structural (α) relaxation, and self-diffusion. For atomistic models of supercooled water, polymer melts, and an ionic liquid, we characterize the space-time characteristics of dynamical heterogeneity by the degree of deviations from Gaussian displacement statistics (α2), the size of clusters comprising highly mobile particles (S(w)), and the length of strings consisting of cooperatively moving particles (L(w)). Comparison of our findings with previous simulation results for a large variety of viscous liquids, ranging from monoatomic liquids to silica melt, reveals a nearly universal decoupling between the time scales of maximum non-Gaussian parameter (τ(α2)) and the time constant of the α relaxation (τ(α)) upon cooling, explicitly, τ(α2) ∝τ(α)(3/4). Such uniform relation was not observed between the peak times of S(w) or L(w) and τ(α). On the other hand, the temperature-dependent time scale of maximum string length (τ(L)) follows the inverse of the self-diffusion coefficient (D) for various systems at sufficiently low temperatures, i.e., τ(L) ∝ D(-1). These observations are discussed in view of a breakdown of the Stokes-Einstein relation for the studied systems. It is found that the degree of deviation from this relation is correlated with the stretching of the α relaxation.

Crossover from localized to diffusive water dynamics in carbon nanohorns: A comprehensive quasielastic neutron-scattering analysis

Incoherent neutron scattering by water confined in carbon nanohorns was measured with the backscattering spectrometer SPHERES and analyzed in exemplary breadth and depth. Quasielastic spectra admit δ-plus-Kohlrausch fits over a wide q and T range. From the q and T dependence of fitted amplitudes and relaxation times, however, it becomes clear that the fits do not represent a uniform physical process, but that there is a crossover from localized motion at low T to diffusive α relaxation at high T. The crossover temperature of about 210 to 230 K increases with decreasing wave number, which is incompatible with a thermodynamic strong-fragile transition. Extrapolated diffusion coefficients D(T) indicate that water motion is at room temperature about 2.5 times slower than in the bulk; in the supercooled state this factor becomes smaller. At even higher temperatures, where the α spectrum is essentially flat, a few percentages of the total scattering go into a Lorentzian with a width of about 1.6μeV, probably due to functional groups on the surface of the nanohorns.

Time-resolved X-ray Tracking of Expansion and Compression Dynamics in Supersaturating Ion-Networks

Supersaturation of a solution system is a metastable state containing more solute than can be normally solubilized. Moreover, this condition is thermodynamically important for a system undergoing a phase transition. This state plays critical roles in deposition morphology in inorganic, organic, polymer and protein solution systems. In particular, microscopic solution states under supersaturated conditions have recently received much attention. In this report, we observed the dynamic motion of individual ion-network domains (INDs) in a supersaturated sodium acetate trihydrate solution (6.4 M) by using microsecond time-resolved and high accuracy (picometre scale) X-ray observations (diffracted X-ray tracking; DXT). We found that there are femto-Newton (fN) anisotropic force fields in INDs that correspond to an Angstrom-scale relaxation process (continuous expansion and compression) of the INDs at 25 μs time scale. The observed anisotropic force-field (femto-Newton) from DXT can lead to new explanations of how material crystallization is triggered. This discovery could also influence the interpretation of supercooling, bio-polymer and protein aggregation processes, and supersaturated systems of many other materials.

Slow dynamics of a tagged particle in a supercooled liquid

The ergodicity-nonergodicity (ENE) transition of the self-consistent mode-coupling theory (MCT) is marked by the point at which the time correlation of collective density fluctuations is not zero in the long-time limit. The nonergodic state, reaching beyond the ENE transition of simple MCT, is characterized by a finite shear modulus. The MCT, formulated in the current set of papers, predicts that the single-particle density correlation, unlike the collective density correlation, decays to zero at long times on either side of the ENE transition. The self-diffusion coefficient remains finite. This differs from the existing MCT results in which both collective and single-particle correlations are simultaniously frozen at the ENE transition. We discuss in this paper mechanisms by which a sharp fall in self-diffusion coefficient may occur within the present model. This overdamping or the so-called adiabatic approximation for the supercooled state does not maintain microscopic momentum conservation. Within this approximation, the self-diffusion constant approaches zero at the ENE transition point. This approximate result, which is similar to the prediction of the existing MCT models, further illustrates the process of cage formation with increase of density. At a qualitative level, our analysis shows that the self-diffusion process depends on the structure as well as short-time transport properties of the supercooled liquid. We solve the integral equations for the nonergodicity parameters to analyze the full implications of the adiabatic approximation.

Slow Water Dynamics near a Glass Transition or a Solid Interface: A Common Rationale

Performing molecular dynamics simulations, we investigate the enormous slowdowns of water dynamics when approaching a glass transition or a solid interface. We show that both effects can be described on common grounds within a theoretical framework, which was recently proposed by Schweizer et al. and considers coupled local hopping and elastic distortion. For confined water, we correctly describe the variation of the α-relaxation time, τα, as a function of both temperature and position with respect to the interface. Exploiting our knowledge of a cooperative length scale ξ(T) from the confinement studies, we quantitatively rationalize the glassy slowdown, τα(T), and the Stokes-Einstein breakdown of bulk water. For both confined and bulk liquid, variations of the α-relaxation time are intimately related to changes of the cage-rattling amplitude.

Heterogeneous Viscoelasticity: A Combined Theory of Dynamic and Elastic Heterogeneity

We present a heterogeneous version of Maxwell's theory of viscoelasticity based on the assumption of spatially fluctuating local viscoelastic coefficients. The model is solved in coherent-potential approximation. The theory predicts an Arrhenius-type temperature dependence of the viscosity in the vanishing-frequency limit, independent of the distribution of the activation energies. It is shown that this activation energy is generally different from that of a diffusing particle with the same barrier-height distribution, which explains the violation of the Stokes-Einstein relation observed frequently in glasses. At finite but low frequencies, the theory describes low-temperature asymmetric alpha relaxation. As examples, we report the good agreement obtained for selected inorganic, metallic, and organic glasses. At high frequencies, the theory reduces to heterogeneous elasticity theory, which explains the occurrence of the boson peak and related vibrational anomalies.

Shape of dynamical heterogeneities and fractional Stokes-Einstein and Stokes-Einstein-Debye relations in quasi-two-dimensional suspensions of colloidal ellipsoids

We examine the influence of the shape of dynamical heterogeneities on the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations in quasi-two-dimensional suspensions of colloidal ellipsoids. For ellipsoids with repulsive interactions, both SE and SED relations are violated at all area fractions. On approaching the glass transition, however, the extent to which this violation occurs changes beyond a crossover area fraction. Quite remarkably, we find that it is not just the presence of dynamical heterogeneities but their change in the shape from stringlike to compact that coincides with this crossover. On introducing a suitable short-range depletion attraction between the ellipsoids, associated with the lack of morphological evolution of dynamical heterogeneities, the extent to which the SE and SED relations are violated remains unchanged even for deep supercooling.

Fluctuation-dissipation theorem and the dielectric response in supercooled liquids

We consider the correlation between static conductivity and dynamic dielectric relaxation in a number of polar organic liquids. Experimental evidence suggests that in the simple cases the linear dependence between characteristic frequency of relaxation process and the value of static susceptibility is observed. However, this proportionality can be broken due to the appearance of additional relaxation processes (secondary or high-frequency ones) so it can be confused with the "fractional" variant of Debye-Stokes-Einstein relation.

Coherent neutron scattering and collective dynamics on mesoscale

By combining, and modestly extending, a variety of theoretical concepts for the dynamics of liquids in the supercooled regime, we formulate a simple analytic model for the temperature and wavevector dependent collective density fluctuation relaxation time that is measurable using coherent dynamic neutron scattering. Comparison with experiments on the ionic glass-forming liquid Ca-K-NO3 in the lightly supercooled regime suggests the model captures the key physics in both the local cage and mesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxation time. The model is consistent with the idea that the decoupling between diffusion and viscosity is reflected in a different temperature dependence of the collective relaxation time at intermediate wavevectors and near the main (cage) peak of the static structure factor. More generally, our analysis provides support for the ideas that decoupling information and growing dynamic length scales can be at least qualitatively deduced by analyzing the collective relaxation time as a function of temperature and wavevector, and that there is a strong link between dynamic heterogeneity phenomena at the single and many particle level. Though very simple, the model can be applied to other systems, such as molecular liquids.

Effect of compression on the relationship between viscosity and dielectric relaxation time in hydrogen-bonded primary alcohols

High pressure viscosity and dielectric measurements were carried out on two monohydroxy alcohols, 2-ethyl-1-hexanol and 5-methyl-2-hexanol, at room temperature. Analysis of the dielectric relaxation times versus viscosity revealed the breakdown of the Einstein-Debye relation above some characteristic pressure. The failure of the Einstein-Debye relation is a manifestation of pressure induced changes of supramolecular hydrogen bonded structures which occur in these liquids.

Translational diffusion of probe molecules under high pressure: a study by fluorescence recovery after photobleaching technique

We present fluorescence recovery measurements after photobleaching performed under high pressure in liquids that fill square-section fused silica micro-capillaries. These micro-capillaries withstand pressure up to 2500 bar for a wall thickness of about 140 μm and fit easily on the microscope stage. This technique allows the translational diffusion coefficient of fluorescent molecules in liquids to be measured as a function of pressure. When the liquid sample is far from its glass transition the translational diffusive coefficient is in agreement with the Stokes-Einstein equation. As the glass transition is approached by further increasing the pressure, decoupling of the measured diffusion coefficient from the Stokes-Einstein relation is observed. These are the first measurements that combine the fluorescence recovery technique and high hydrostatic pressures. This experimental setup can also be used either with diamond or sapphire anvil cells in order to span a larger pressure range.

Surface wrinkling induced by photofluidization of low molecular azo glasses

Photoinduced surface wrinkling is demonstrated for a low molecular azo compound confined between a substrate and a thin inorganic elastic layer. The wrinkling process is investigated by time-resolved light scattering as well as by two-dimensional autocorrelation analysis of microscopic images. It is shown that the temporal evolution of the wrinkling pattern is directly controlled by the amount of photons absorbed by the sample and that there is no significant dependence of the equilibrium wavelength on irradiation intensity. Finally, the comparison of thermal and photoinduced wrinkling revealed that photoinduced wrinkles are characterized by a narrower mode distribution and less coarsening.

Slow processes in supercooled o-terphenyl: relaxation and decoupling

We mapped the relaxation times of inter- and intramolecular correlations in o-terphenyl by a quasielastic scattering method using nuclear resonant scattering of synchrotron radiation. From the obtained map, we found that the slow β process is decoupled from the α process at 278 K, and this temperature is clearly below the previous decoupling temperature of 290 K, at which the α-relaxation dynamics changes. Then, it was also concluded that sufficient solidlike condition achieved by further cooling from 290 K is required to decouple the slow β process from the α process and, due to the difference of the length scales between the α and the slow β processes, these two averaged relaxation times <τ> are concluded not to cross as an extrapolation assumed so far. Furthermore, evidence of the restricted dynamics of the slow β process could be obtained as an anomalous momentum transfer (q) dependence of <τ>(<τ> ∝q(-2.9)) at 265 K, observed at q values of 18-48  nm(-1).

Nitrogen nuclear spin flips in nitroxide spin probes of different sizes in glassy o-terphenyl: possible relation with α- and β-relaxations

The pulsed electron-electron double resonance (ELDOR) technique was employed to study nitroxide spin probes of three different sizes dissolved in glassy o-terphenyl. A microwave pulse applied to the central hyperfine structure (hfs) component of the nitroxide electron paramagnetic resonance spectrum was followed by two echo-detecting pulses of different microwave frequency to probe the magnetization transfer (MT) to the low-field hfs component. The MT between hfs components is readily related to flips in the nitrogen nuclear spin, which in turn are induced by molecular motion. The MT on the time scale of tens of microseconds was observed over a wide temperature range, including temperatures near and well below the glass transition. For a bulky nitroxide, it was found that MT rates approach dielectric α (primary) relaxation frequencies reported for o-terphenyl in the literature. For small nitroxides, MT rates were found to match the frequencies of dielectric β (secondary) Johari-Goldstein relaxation. The most probable motional mechanism inducing the nitrogen nuclear spin flips is large-angle angular jumps, between some orientations of unequal occupation probabilities. The pulsed ELDOR of nitroxide spin probes may provide additional insight into the nature of Johari-Goldstein relaxation in glassy media and may serve as a tool for studying this relaxation in substances consisting of non-rigid molecules (such as branched polymers) and in heterogeneous and non-polar systems (such as a core of biological membranes).