Viscosity of deeply supercooled water and its coupling to molecular diffusion

The viscosity of a liquid measures its resistance to flow, with consequences for hydraulic machinery, locomotion of microorganisms, and flow of blood in vessels and sap in trees. Viscosity increases dramatically upon cooling, until dynamical arrest when a glassy state is reached. Water is a notoriously poor glassformer, and the supercooled liquid crystallizes easily, making the measurement of its viscosity a challenging task. Here we report viscosity of water supercooled close to the limit of homogeneous crystallization. Our values contradict earlier data. A single power law reproduces the 50-fold variation of viscosity up to the boiling point. Our results allow us to test the Stokes–Einstein and Stokes–Einstein–Debye relations that link viscosity, a macroscopic property, to the molecular translational and rotational diffusion, respectively. In molecular glassformers or liquid metals, the violation of the Stokes–Einstein relation signals the onset of spatially heterogeneous dynamics and collective motions. Although the viscosity of water strongly decouples from translational motion, a scaling with rotational motion remains, similar to canonical glassformers.Water, considered as a potential glassformer, has been a long-lasting topic of intense activity. Its possible liquid–glass transition was reported 50 years ago to be in the vicinity of 140?K (1, 2). However, ice nucleation hinders the access to this transition from the liquid side. Bypassing crystallization requires hyperquenching the liquid at tremendous cooling rates, ca. 10⁷?K ? s^?1 (3). As a consequence, many questions about supercooled and glassy water and its glass–liquid transition remain open (4–7).As an example, crystallization of water is accompanied by one of the largest known relative changes in sound velocity, which has been attributed to the relaxation effects of the hydrogen bond network (8, 9). Indeed, whereas the sound velocity is around 1,400 m⋅s−1 in liquid water at 273?K, it reaches around 3,300 m⋅s−1 in ice at 273?K and a similar value in the known amorphous phases of ice at 80?K (10). Such a large jump is usually the signature of a strong glass, i.e., one in which relaxation times or viscosity follow an Arrhenius law upon cooling. However, pioneering measurements on bulk supercooled water by NMR (11) and quasi-elastic neutron scattering (12), as well as recent ones by optical Kerr effect (8, 9), reveal a large super-Arrhenius behavior between 340 and 240?K, similar to what is observed in fragile glassformers (13, 14). The temperature dependence of the relaxation time is well described by a power law (8, 9), as expected from mode-coupling theory (15, 16), which usually applies well to liquids with a small change of sound velocity upon vitrification. Based on these and other observations, it has been hypothesized that supercooled water experiences a fragile-to-strong transition (17). This idea has motivated experimental efforts to measure dynamic properties of supercooled water and has received some indirect support from experiments on nanoconfined water (18–20) and from simulations (21, 22).In usual glassformers, many studies have focused on the coupling or decoupling between the following dynamic quantities: viscosity (η) and self or tracer diffusion coefficients for translation (D_t) and rotation (D_r). If objects as small as molecules were to follow macroscopic hydrodynamics, one would expect that the preceding quantities would be related through the Stokes–Einstein (SE), D_t ∝ T/η, and Stokes–Einstein–Debye (SED), D_r ∝ T/η, relations, where T is the temperature. These relations are indeed obeyed by many liquids at sufficiently high temperature. However, they might break down at low temperature. Pioneering experiments were performed by the groups of Sillescu (23–25) and Ediger (26–28) where a series of molecular glassformers were investigated. SE relation is obeyed at sufficiently high temperature but violated around 1.3T_g, where T_g is the glass transition temperature, thus indicating decoupling between translational diffusion and viscosity. In contrast, it was observed for ortho-terphenyl (23, 24, 26) that rotational diffusion and viscosity remain strongly coupled (i.e., obey the SED relation) even very close to T_g. A corollary is that translational and rotational diffusion decouple from each other at low temperature. These observations imply that deeply supercooled liquids exhibit spatially heterogeneous dynamics (29–31). Dynamic heterogeneities have been confirmed by direct observations of several single fluorescent molecules immersed in ortho-terphenyl (32) or nanorods immersed in glycerol (33). Physically different systems also show analogous behavior. Colloids near the colloidal glass transition violate SE but obey SED (34). In the metallic alloy Zr₆₄Ni₃₆, SE relation is even violated without supercooling, more than 35% above the liquidus temperature (35). This has also been related to the emergence of dynamic heterogeneities (36).For water, SE already breaks down at ambient temperature, which corresponds to around 2.1?T_g (T_g ? 136?K). Molecular dynamics simulations (37–39) have proposed that this occurs concurrently to dynamic heterogeneities caused by a putative liquid–liquid critical point. However, SE and SED also fail by application of high pressure at 400?K (40) where no liquid–liquid transition is expected. To gain more insight, the test of SE and SED in supercooled water deserves further investigation. Translational self-diffusion coefficient D_t (41) and rotational correlation time τ_r (assumed to scale as 1/D_r) (42) have thus been measured down to the homogeneous crystallization temperature (238?K) at ambient pressure. Their comparison reveals a decoupling between rotation and translation that increases with supercooling (42), similar to glassformers. However, viscosity data are needed for a direct test of SE and SED relations. Quite surprisingly, there are only two sets of data for the viscosity η at significant supercooling. Using Poiseuille flow in capillaries, Hallett (43) reached 249.35?K, and Osipov et al. (44) reached 238.15?K. However, the two sets disagree below 251?K, with an 8% difference at 249?K, beyond the reported uncertainties. The measurements in ref. 44 are suspected of errors (45) because of the small capillary diameter used. Here we report η at ambient pressure down to 239.27?K. Our study completes the knowledge of the main dynamic parameters of water down to the homogeneous crystallization limit and allows us to check the coupling of viscosity to molecular translation or rotation, as has been done for usual glassformers.     

The significance of combining World Health Organization and Center for Disease Control criteria to resolve indeterminate human immunodeficiency virus type-1 Western blot results

A total of 466 repeatedly reactive HIV-1 blood samples evidenced by enzyme immunoassay were analyzed by the Western blot method and interpreted according to WHO and CDC criteria. Discordant and indeterminate samples were further analyzed by PCR. When the Western blot result was classified as indeterminate, according to both WHO and CDC criteria, the PCR test was always negative. These findings suggest that samples with double-indeterminate status should be reported as negative.     

Infective endocarditis complicated by fatal cerebral hemorrhage revealing an unrepaired univentricular heart in a 19-year-old woman

Univentricular heart is a complex and rare cyanotic congenital heart disease. When not operated, affected patients exceptionally reach adulthood. We report the unprecedented case of a 19 year-old young woman, admitted to the hospital for a severe deterioration of general status and ultimately diagnosed to have an infective endocarditis with multiple vegetations in a previously undiagnosed univentricular heart of left ventricular morphology, subsequently rapidly complicated by fatal cerebral hemorrhage.