And yet if flows!

Amine Dehaoui, Bruno Issenmann et Frédéric Caupin (équipe Liquides et Interfaces), have published an article "Viscosity of deeply supercooled water and its coupling to microscopic diffusion" in the journal Proceedings of the National Academy of Sciences USA.

>Liquid water can exist below 0°C in a supercooled state, but the slightest disturbance triggers the nucleation and growth of a crystal. This is what happens to the drops of freezing rain when they reach the ground. For this reason, properties of supercooled water are extremely difficult to measure. Yet they exhibit many anomalies which are keys to our understanding of water and its complex network of hydrogen bonds. Measuring the flow rate of water in a small tube, the viscosity of water has been obtained down to -24°C by Hallett in 1963, and down to -35°C by Osipov et al. in 1977. However, doubts have been raised about the latter data. Researchers at ILM used another method: they followed spontaneous movements of polystyrene beads of 400 nm diameter suspended in water and subjected to the random impacts of water molecules. The amplitude of this Brownian motion is related to the bead size, to temperature, and to the liquid viscosity, as shown by Albert Einstein in 1905. The new data confirm the results of Hallett but reveal a bias in those of Osipov et al. Water at -34°C is 14 times more viscous than at room temperature. The comparison with diffusion properties of water molecules reveals that viscosity decouples from translational diffusion, while it remain coupled to rotational diffusion. According to computer simulations, the decoupling between viscosity and translation might be due to the existence in supercooled water of the same dynamical heterogeneities as in a liquid near its glass transition: the fastest molecules are not uniformly distributed, but clustered in regions of space fluctuating in time.


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