We report measurements of the shear viscosity η in water up to
150MPa and down to 229.5K. This corresponds to more
than 30K supercooling below the melting line. The temperature
dependence is non-Arrhenius at all pressures, but its functional form at
0.1MPa is qualitatively different from that at all pressures above
20MPa. The pressure dependence is non-monotonic, with a
pressure-induced decrease of viscosity by more than 50 % at low temperature.
Combining our data with literature data on the self-diffusion coefficient
Ds of water, we check the Stokes-Einstein relation which, based on
hydrodynamics, predicts constancy of Dsη/T, where T is the
temperature. The observed temperature and pressure dependence of Dsη/T is analogous to that obtained in simulations of a realistic water
model. This analogy suggests that our data are compatible with the existence of
a liquid-liquid critical point at positive pressure in water.Comment: 11 pages, 8 figures, 7 tables, 1 supplementary figure. Summary of
main changes: the abstract and conclusion were modified, minor edits were
made to all figures for clarity, one table and the supplementary figure were
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