Viscosity and Stokes-Einstein relation in deeply supercooled water under pressure

We report measurements of the shear viscosity. in water up to 150 MPa and down to 229.5 K. This corresponds to more than 30 K supercooling below the melting line. The temperature dependence is non-Arrhenius at all pressures, but its functional form at 0.1 MPa is qualitatively different from that at all pressures above 20 MPa. 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 D-s of water, we check the Stokes-Einstein relation which, based on hydrodynamics, predicts constancy of D-s eta/T, where T is the temperature. The observed temperature and pressure dependence of D-s eta/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.