It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity η on temperature T . In an Arrhenius plot, ln η vs 1∕T , a strong liquid shows linear behavior whereas a fragile liquid exhibits an upward curvature [super-Arrhenius (SA) behavior], a situation customarily described by using the Vogel-Fulcher-Tammann law. Here we analyze existing data of the transport coefficients of 84 glassforming liquids. We show the data are consistent, on decreasing temperature, with the onset of a well-defined dynamical crossover η × , where η × has the same value, η × ≈ 10 3 Poise, for all 84 liquids. The crossover temperature, T × , located well above the calorimetric glass transition temperature T g , marks significant variations in the system thermodynamics, evidenced by the change of the SA-like T dependence above T × to Arrhenius behavior below T × . We also show that below T × the familiar Stokes-Einstein relation D∕T ∼ η −1 breaks down and is replaced by a fractional form D∕T ∼ η −ζ , with ζ ≈ 0.85. dynamical arrest | dynamic transition | supercooled liquids

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Using NMR, we measure the proton chemical shift ␦, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because ␦ is directly connected to the magnetic shielding tensor, we discuss the data in terms of the local hydrogen bond geometry and order. We argue that the derivative ؊(٢ ln ␦/٢T)P should behave roughly as the constant pressure specific heat CP(T), and we confirm this argument by detailed comparisons with literature values of CP(T) in the range 290 -370 K. We find that ؊(٢ ln ␦/٢T)P displays a pronounced maximum upon crossing the locus of maximum correlation length at Ϸ240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that CP(T) displays a maximum on crossing the Widom line.configurational specific heat ͉ nuclear magnetic resonance ͉ proteins ͉ proton chemical shift U nlike most fluids, water displays anomalies in thermodynamical properties such as compressibility, isobaric heat capacity, and thermal expansion coefficient, and their explanation on molecular basis remains a challenge (1-3). One hypothesis that has received support from various theoretical studies (4-7) is the liquid-liquid (LL) critical point hypothesis, but a proper test can be obtained only by studying the properties of liquid water well below its homogeneous nucleation temperature, T H ϭ 231 K. This is made possible by confining water inside nanoporous structures so small that the liquid cannot freeze.Among recent findings concerning water's dynamical properties at these low temperatures are (8-13): a fragile-to-strong crossover and the violation of the Stokes-Einstein relation, related to the crossing of the Widom line and to the existence of a low-density-liquid-like (LDL-like) local structure. The Widom line is the locus of maximum correlation length in the one-phase region beyond the liquid-liquid critical point, where thermodynamic response functions take their maximum values (12, 13). Scattering experiments (using neutrons and x-rays) have given precise values of the pair correlation function (PCF), providing important benchmarks for testing models of its structure. The PCF represents only an isotropically averaged measure of structure. Thus, in many cases, PCFs may not faithfully reproduce the subtle hydrogen bond geometry responsible for water's thermal anomalies. Our goal in this study is to provide additional information on the local hydrogen bond geometry and, in particular, the average number of the possible configurations of the local molecular hydrogen bonding geometry, by measuring the NMR proton chemical shift ␦. If a water molecule in a dilute gas is taken to be an isolated-state reference, the chemical shift ␦ accounts for the change of the value of the magnetic shielding with respect to that of such a reference. Hence the chemical shift is related to the ''non-dilute'' or ''virial'' interaction of a water molecule with its surroundings, providing a picture of the intermolecular geometry (14-19). Originally, it has been proposed, especially in the high ...

Using nuclear magnetic resonance and quasi-elastic neutron scattering spectroscopic techniques, we obtain experimental evidence of a well-defined dynamic crossover temperature T(L) in supercooled water. We consider three different geometrical environments: (i) water confined in a nanotube (quasi-one-dimensional water), (ii) water in the first hydration layer of the lysozyme protein (quasi-two-dimensional water), and (iii) water in a mixture with methanol at a methanol molar fraction of x = 0.22 (quasi-three-dimensional water). The temperature predicted using a power law approach to analyze the bulk water viscosity in the super-Arrhenius regime defines the fragile-to-strong transition and the Stokes-Einstein relation breakdown recently observed in confined water. Our experiments show that these observed processes are independent of the system dimension d and are instead caused by the onset of an extended hydrogen-bond network that governs the dynamical properties of water as it approaches dynamic arrest.