Dynamic Crossover Phenomenon Research Articles

Mesoscopic two-point collective dynamics of glass-forming liquids.

The collective density-density and hydrostatic pressure-pressure correlations of glass-forming liquids are spatiotemporally mapped out using molecular dynamics simulations. It is shown that the sharp rise of structural relaxation time below the Arrhenius temperature coincides with the emergence of slow, nonhydrodynamic collective dynamics on mesoscopic scales. The observed long-range, nonhydrodynamic mode is independent of wave numbers and closely coupled to the local structural dynamics. Below the Arrhenius temperature, it dominates the slow collective dynamics on length scales immediately beyond the first structural peak in contrast to the well-known behavior at high temperatures. These results highlight a key connection between the qualitative change in mesoscopic two-point collective dynamics and the dynamic crossover phenomenon.

New paradigm for configurational entropy in glass-forming systems

We show that on cooling towards glass transition configurational entropy exhibits more significant changes than predicted by classic relation. A universal formula according to Kauzmann temperature {T}_{K} is given: S={S}_{0}{t}^{n}, where t=left(T-{T}_{K}right)/T. The exponent n is hypothetically linked to dominated local symmetry. Such a behaviour is coupled to previtreous evolution of heat capacity Delta {C}_{P}^{config.}left(Tright)=left(nC/Tright){left(1-{T}_{K}/Tright)}^{n-1} associated with finite temperature singularity. These lead to generalised VFT relation, for which the basic equation is retrieved. For many glass-formers, basic VFT equation may have only an effective meaning. A universal-like reliability of the Stickel operator analysis for detecting dynamic crossover phenomenon is also questioned. Notably, distortions-sensitive and derivative-based analysis focused on previtreous changes of configurational entropy and heat capacity for glycerol, ethanol and liquid crystal is applied.

Pressure-Related Universal Previtreous Behavior of the Structural Relaxation Time and Apparent Fragility

The report presents the evidence for the ‘universal’ previtreous behavior of the reciprocal of the pressure related apparent fragility. This finding was the base for deriving the ‘model-free’ dependence for the pressure evolution of the structural relaxation time. All these led to new conclusions for the dynamic crossover phenomenon. Finally, the behavior of electric conductivity in the previtreous domain is discussed. The experimental evidence covers 8*OCB (liquid crystal), EPON 828 (resin) and diisobutyl phthalate, propylene carbonate (low molecular weight liquids). Experiments were carried up to extreme P~ 2.2 GPa, in the ‘bulk’ measurement capacitor. The report contains a summary of applied so far descriptions for the pressure evolution of dynamic properties on approaching the glass transition.

Five-fold local symmetry in metallic liquids and glasses**Project supported by the National Natural Science Foundation of China (Grant Nos. 51271195 and 51271197), the National Basic Research Program of China (Grant No. 2015CB856800), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China (Grnat No. 16XNLQ01).

The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as periodicity for crystals, for establishing the structure–property relationship in amorphous materials. In this paper, we briefly review the recent developments of the five-fold local symmetry in metallic liquids and glasses and the understanding of the structure–property relationship based on this parameter. Experimental evidence demonstrates that five-fold local symmetry is found to be general in metallic liquids and glasses. Comprehensive molecular dynamics simulations show that the temperature evolution of five-fold local symmetry reflects the structural evolution in glass transition in cooling process, and the structure–property relationship such as relaxation dynamics, dynamic crossover phenomena, glass transition, and mechanical deformation in metallic liquids and glasses can be well understood base on the simple and general structure parameter of five-fold local symmetry.

Structural signatures evidenced in dynamic crossover phenomena in metallic glass-forming liquids

Molecular dynamics simulations were performed to investigate dynamic evolution in metallic glass-forming liquids during quenching from high temperature above melting point down to supercooled region. Two crossover temperatures TA and TS (TA > TS) are identified, and their physical meanings are clarified. TA and TS are found to be not only the sign of dynamic crossover phenomena but also the manifestation of two key structure correlation lengths ξs. As temperature decreases below TA, ξs goes beyond the nearest-neighbor distance, resulting in the Arrhenius-to-non-Arrhenius transition of structural relaxation time and the failure of Stokes-Einstein (SE) relation. As TS is traversed, the increase rate of ξs reaches the maximum, leading to the simultaneous appearance of dynamical heterogeneity and fractional SE relation. It is further found that structure correlation increases much faster than dynamic correlation, playing a role of structural precursor for dynamic evolution in liquids. Thus, a structural link is established for deeper understanding dynamic crossover phenomena.

Dynamic crossover in deeply cooled water confined in MCM-41 at 4 kbar and its relation to the liquid-liquid transition hypothesis.

With quasi-elastic neutron scattering, we study the single-particle dynamics of the water confined in a hydrophilic silica material, MCM-41, at 4 kbar. A dynamic crossover phenomenon is observed at 219 K. We compare this dynamic crossover with the one observed at ambient pressure and find that (a) above the crossover temperature, the temperature dependence of the characteristic relaxation time at ambient pressure exhibits a more evident super-Arrhenius behavior than that at 4 kbar. Especially, at temperatures below about 230 K, the relaxation time at 4 kbar is even smaller than that at ambient pressure. This feature is different from many other liquids. (b) Below the crossover temperature, the Arrhenius behavior found at ambient pressure has a larger activation energy compared to the one found at 4 kbar. We ascribe the former to the difference between the local structure of the low-density liquid (LDL) phase and that of the high-density liquid (HDL) phase, and the latter to the difference between the strength of the hydrogen bond of the LDL and that of the HDL. Therefore, we conclude that the phenomena observed in this paper are consistent with the LDL-to-HDL liquid-liquid transition hypothesis.

Dynamics of a globular protein and its hydration water studied by neutron scattering and MD simulations

This review article describes our neutron scattering experiments made in the past four years for the understanding of the single-particle (hydrogen atom) dynamics of a protein and its hydration water and the strong coupling between them. We found that the key to this strong coupling is the existence of a fragile-to-strong dynamic crossover (FSC) phenomenon occurring at around TL = 225±5 K in the hydration water. On lowering of the temperature toward FSC, the structure of hydration water makes a transition from predominantly the high density form (HDL), a more fluid state, to predominantly the low density form (LDL), a less fluid state, derived from the existence of a liquid–liquid critical point at an elevated pressure. We show experimentally that this sudden switch in the mobility of hydration water on Lysozyme, B-DNA and RNA triggers the dynamic transition, at a temperature TD = 220 K, for these biopolymers. In the glassy state, below TD, the biopolymers lose their vital conformational flexibility resulting in a substantial diminishing of their biological functions. We also performed molecular dynamics (MD) simulations on a realistic model of hydrated lysozyme powder, which confirms the existence of the FSC and the hydration level dependence of the FSC temperature. Furthermore, we show a striking feature in the short time relaxation (β-relaxation) of protein dynamics, which is the logarithmic decay spanning 3 decades (from ps to ns). The long time α-relaxation shows instead a diffusive behavior, which supports the liquid-like motions of protein constituents. We then discuss our recent high-resolution X-ray inelastic scattering studies of globular proteins, Lysozyme and Bovine Serum Albumin. We were able to measure the dispersion relations of collective, intra-protein phonon-like excitations in these proteins for the first time. We found that the phonon energies show a marked softening and at the same time their population increases substantially in a certain wave vector range when temperature crosses over the TD. Thus the increase of biological activities above TD has positive correlation with activation of slower and large amplitude collective motions of a protein.

Hydration-dependent dynamic crossover phenomenon in protein hydration water.

The characteristic relaxation time τ of protein hydration water exhibits a strong hydration level h dependence. The dynamic crossover is observed when h is higher than the monolayer hydration level hc=0.2-0.25 and becomes more visible as h increases. When h is lower than hc, τ only exhibits Arrhenius behavior in the measured temperature range. The activation energy of the Arrhenius behavior is insensitive to h, indicating a local-like motion. Moreover, the h dependence of the crossover temperature shows that the protein dynamic transition is not directly or solely induced by the dynamic crossover in the hydration water.

An insight into the dynamic crossover phenomenon in alcohols from the fluidity equation

Recently, we have introduced an equation offering a good description of the viscosity–temperature relationship for ionic liquids. The equation makes used of fluidity that varies smoothly with temperature rather than sharply varying viscosity. This three-parameter fluidity has a clear physical foundation based on the dynamic crossover in the viscosity induced by temperature. It is applied to a series of normal aliphatic alcohols, methanol through n-dodecanol, proposing a good correlation for temperature dependence of the viscosity for the alcohols. Using the parameters of the equation, the dynamic crossover temperature (Tx) characteristic of glass forming liquid is calculated, which is found to be close to the values obtained by power law equation based on the mode coupling theory and the values reported from derivative analysis of the dielectric spectroscopy. The exponent of the fluidity equation is found to be universal for all studied alcohols (ϕ=0.312), and then it is applied as a two-parameter equation. In addition, a universal behavior is found for the viscosity when the temperature is reduced by the crossover temperature for each alcohol. Finally, another recent proposed equation which correlates the surface tension and viscosity of a liquid is accurately applied for the alcohols.

Computer simulations of dynamic crossover phenomena in nanoconfined water **Dedicated to Professor Sow-Hsin Chen on the occasion of his 75th birthday.

In order to study dynamic crossover phenomena in nanoconfined water we performed a series of molecular dynamics (MD) computer simulations of water clusters adsorbed in zeolites, which are microporous crystalline aluminosilicates containing channels and cavities of nanometric dimensions. We used a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. In addition, the full flexibility of the aluminosilicate framework was included in the calculations. Previously reported and new simulations of water confined in a number of different types of zeolites in the temperature range 100–300 K and at various coverage are discussed in connection with the experimental data. Dynamic crossover phenomena are found in all the considered cases, in spite of the different shape and size of the clusters, even when the confinement hinders the formation of tetrahedral hydrogen bonds for water molecules. Hypotheses about the possible dynamic crossover mechanisms are proposed.

The Dynamic Response Function χ T(Q,t) of Confined Supercooled Water and its Relation to the Dynamic Crossover Phenomenon

Abstract We have made a series of Quasi-Elastic Neutron Scattering (QENS) studies of supercooled water confined in 3-D and 1-D geometries, specifically, interstitial water in aged cement paste (3-D) and water confined in MCM-41-S and Double Wall Nano Tube DWNT (1-D). In addition, we also include the cases of hydration water on protein surface and other biopolymer surfaces (pseudo 2-D). By analyzing the QENS spectra using Relaxing Cage Model (RCM), we are able to extract accurately the self-intermediate scattering function of hydrogen atoms FH(Q,t), at low-Q as a function of temperature T, showing an α-relaxation process at long time. We can then construct the Dynamic Response Function χT(Q,t) = -dFH(Q,t)/dT. χT(Q,t) as a function of t at constant Q shows a single peak at the characteristic α-relaxation time 〈τ〉, the amplitude of which grows as we approach the dynamic crossover temperature TL observed before in each of these geometries. However, the peak height of χT(Q,t) decreases after passing the crossover temperature TL. We make an argument to relate the occurrence of the extremum of the peak height in χT to the existence of the dynamic crossover temperature in each of these cases.

Dynamics of a globular protein and its hydration water studied by neutron scattering and MD simulations

This review article describes our neutron scattering experiments made in the past four years for the understanding of the single-particle (hydrogen atom) dynamics of a protein and its hydration water and the strong coupling between them. We found that the key to this strong coupling is the existence of a fragile-to-strong dynamic crossover (FSC) phenomenon occurring at aroundTL= 225±5 K in the hydration water. On lowering of the temperature toward FSC, the structure of hydration water makes a transition from predominantly the high density form (HDL), a more fluid state, to predominantly the low density form (LDL), a less fluid state, derived from the existence of a liquid–liquid critical point at an elevated pressure. We show experimentally that this sudden switch in the mobility of hydration water on Lysozyme, B-DNA and RNA triggers the dynamic transition, at a temperatureTD= 220 K, for these biopolymers. In the glassy state, belowTD, the biopolymers lose their vital conformational flexibility resulting in a substantial diminishing of their biological functions. We also performed molecular dynamics (MD) simulations on a realistic model of hydrated lysozyme powder, which confirms the existence of the FSC and the hydration level dependence of the FSC temperature. Furthermore, we show a striking feature in the short time relaxation (β-relaxation) of protein dynamics, which is the logarithmic decay spanning 3 decades (from ps to ns). The long timeα-relaxation shows instead a diffusive behavior, which supports the liquid-like motions of protein constituents. We then discuss our recent high-resolution X-ray inelastic scattering studies of globular proteins, Lysozyme and Bovine Serum Albumin. We were able to measure the dispersion relations of collective, intra-protein phonon-like excitations in these proteins for the first time. We found that the phonon energies show a marked softening and at the same time their population increases substantially in a certain wave vector range when temperature crosses over theTD. Thus the increase of biological activities aboveTDhas positive correlation with activation of slower and large amplitude collective motions of a protein.

Evidence of dynamic crossover phenomena in water and other glass-forming liquids:experiments, MD simulations and theory

In a recent quasi-elastic neutron scattering experiment on water confined in a Portland cementpaste, we find that this 3D confined water shows a dynamic crossover phenomenon atTL = 227 ± 5 K. The DSC heat-flow scan upon cooling and an independent measurement of specific heatat constant pressure of confined water in silica gel show a prominent peak at the sametemperature. We show in this paper that this type of behavior is common to many otherglassy liquids, which also show the crossover temperature in coincidence with thetemperature of a small specific heat peak. We also demonstrate with MD simulations thatthe dynamic crossover phenomenon in confined water is an intrinsic propertyof bulk water, and is not due to the confinement effect. Recently, an extendedversion of the mode coupling theory (MCT) including the hopping effect wasdeveloped. This theory shows that, instead of a structural arrest transition atTC predicted by the idealized MCT, a fragile-to-strong dynamic crossover phenomenon takes place insteadat TC, confirming both the experimental and the numerical results. The coherent and incoherentα relaxation times can be scaled with the calculated viscosity, showing thesame crossover phenomenon. We thus demonstrated with experiments,simulations and theory that a genuine change of dynamical behavior ofboth water and many glassy liquids happens at the crossover temperatureTL, which is 10–30% higher than the calorimetric glass transition temperatureTg.

Neutron scattering studies of dynamic crossover phenomena in a coupled system of biopolymer and its hydration water

We have observed a Fragile-to-Strong Dynamic Crossover (FSC) phenomenon of the α-relaxation time and self-diffusion constant in hydration water of three biopolymers: lysozyme, B-DNA and RNA. The mean squared displacement (MSD) of hydrogen atoms is measured by Elastic Neutron Scattering (ENS) experiments. The α-relaxation time is measured by Quasi-Elastic Neutron Scattering (QENS) experiments and the self-diffusion constant by Nuclear Magnetic Resonance (NMR) experiments. We discuss the active role of the FSC of the hydration water in initiating the dynamic crossover phenomenon (so-called glass transition) in the biopolymer. The latter transition controls the flexibility of the biopolymer and sets the low temperature limit of its biofunctionality. Finally, we show an MD simulation of a realistic hydrated powder model of lysozyme and demonstrate the agreement of the MD simulation with the experimental data on the FSC phenomenon in the plot of logarithm of the α-relaxation time vs. 1/T.

Dynamic susceptibility of supercooled water and its relation to the dynamic crossover phenomenon

We study the dynamic susceptibility chi_{T}(Q,t) of deeply supercooled water by means of quasielastic neutron scattering and molecular dynamics simulations. Both techniques show an increase in the peak height of chi_{T}(Q,t) as the temperature is lowered toward the dynamic crossover temperature T_{L} . Below T_{L} , the peak height decreases steadily. We attribute this phenomenon to the change in slope of the Arrhenius plot of the translational relaxation time at T_{L} . In contrast, the peak height of the calculated four-point correlation function chi_{4}(Q,t) , directly related to the size of dynamic heterogeneity, increases toward and below T_{L} .

The Low-Temperature Dynamic Crossover Phenomenon in Protein Hydration Water: Simulations vs Experiments

A super-Arrhenius-to-Arrhenius dynamic crossover phenomenon has been observed in the translational alpha-relaxation time and in the inverse of the self-diffusion constant both experimentally and by simulations for lysozyme hydration water in the temperature range of TL = 223 +/- 2 K. MD simulations are based on a realistic hydrated powder model, which uses the TIP4P-Ew rigid molecular model for the hydration water. The convergence of neutron scattering, nuclear magnetic resonance and molecular dynamics simulations supports the interpretation that this crossover is a result of the gradual evolution of the structure of hydration water from a high-density liquid to a low-density liquid form upon crossing of the Widom line above the possible liquid-liquid critical point of water.

Evidence of the existence of the low-density liquid phase in supercooled, confined water

By confining water in a nanoporous structure so narrow that the liquid could not freeze, it is possible to study properties of this previously undescribed system well below its homogeneous nucleation temperature TH = 231 K. Using this trick, we were able to study, by means of a Fourier transform infrared spectroscopy, vibrational spectra (HOH bending and OH-stretching modes) of deeply supercooled water in the temperature range 183 < T < 273 K. We observed, upon decreasing temperature, the building up of a new population of hydrogen-bonded oscillators centered around 3,120 cm(-1), the contribution of which progressively dominates the spectra as one enters into the deeply supercooled regime. We determined that the fractional weight of this spectral component reaches 50% just at the temperature, TL approximately 225 K, where the confined water shows a fragile-to-strong dynamic cross-over phenomenon [Ito, K., Moynihan, C. T., Angell, C. A. (1999) Nature 398:492-494]. Furthermore, the fact that the corresponding OH stretching spectral peak position of the low-density-amorphous solid water occurs exactly at 3,120 cm(-1) [Sivakumar, T. C., Rice, S. A., Sceats, M. G. (1978) J. Chem. Phys. 69:3468-3476.] strongly suggests that these oscillators originate from existence of the low-density-liquid phase derived from the occurrence of the first-order liquid-liquid (LL) phase transition and the associated LL critical point in supercooled water proposed earlier by a computer molecular dynamics simulation [Poole, P. H., Sciortino, F., Essmann, U., Stanley, H. E. (1992) Nature 360:324-328].