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Viscosité, auto-diffusion et leur découplage dans des solutions aqueuses de glycérol ainsi que dans l’eau pure surfondue sous pression

Abstract : Water is the most common liquid for us, but a great deal of its properties are still not well understood. Water exhibits numerous anomalies compared to other liquids and they are enhanced in the supercooled state, that when water is still liquid below the meltin point.During this word, glycerol is added to water (between 10% to 50% in mass) to lower the melting point and the homogeneous nucleation point. Two properties are measured : viscosity and self-diffusion. The first is a macroscopic property related to flow resistance. The second is linked to relative motion of molecules in the liquid. Viscosity and self-diffusion are linked by the Stokes-Einstein equation (SER) at usuals temperature and pressure in liquid. The viscosity of water-glycerol solution is measured by Differentiel Dynamic Microscopy. Diffusion coefficient of colloids in the solution is measured and allows us to deduce viscosity of the solution. Viscosity raises as temperature raises. At given temperature, viscosity raises in respect with glycerol’s concentration. Viscosity’s behavior according to temperature depends on the concentration. At low concentration (< 20%) it behaves the same way as pure water. For higher concentrations, other laws are more suitable to describe viscosity. Henceforth, adding glycerol modifies supercooled solutions’ dynamic. Mutual diffusion of molecules is related to their mobility. It is characterized by self-diffusion in a pure liquid and intra-diffusion in mixtures. Intra-diffusion in glycerol solutions are measured with concentration ranging from 1% to 50% in mass thanks to NMR-PGSE. Water and glycerol diffusions raise with temperature, whatever the concentration. At high temperature, diffusion decreases as concentration raises. At low temperature and low concentration (1% in glycerol), glycerol’s diffusion decreases more rapidly than for other concentration as temperature is lowered. Water seems to have the same tendancy in similar conditions. Knowing viscosity and intra-diffusion, it is possible to get SER’s evolution according to temperature. The SER is always violated for water molecules as soon as temperature is lower than 273 K. Glycerol molecules behave the same way when their concentration is 30% and 50%. For 10%, the SER is unexpectedly observed. At 1% the SER is violated again, but in opposite way compared to what is usually observed. Microscopic interpretation of these observations remains elusive. We also performed viscosity and self-diffusion measurements on pure water under pressure up to 150 MPa. SER is also tested for temperatures down to 228 K, allowing comparison with recent numerical simulations
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Romain Berthelard. Viscosité, auto-diffusion et leur découplage dans des solutions aqueuses de glycérol ainsi que dans l’eau pure surfondue sous pression. Mécanique des fluides [physics.class-ph]. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSE1182⟩. ⟨tel-02485182⟩



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