The resistance of a fluid to shear forces, and hence to flow. In a Newtonian fluid, such shear resistance is proportional to the relative velocity between the two surfaces on either side of a layer of fluid, the area in shear, the viscosity of the fluid and the reciprocal of the thickness of the layer of fluid. The value is given by the coefficient of viscosity ]. For normal ranges of temperature,] for a liquid decreases with increase in temperature and is independent of the pressure. Important non-Newtonian fluids include polymer melts, whose viscosities decrease with increasing shear rate pseudo-plastic fluids and dilatant fluids for which the viscosity increases with shear rate. For comparing the viscosities of liquids, various scales have been devised, eg Redwood No. 1 seconds UK, Saybolt Universal seconds US, and Engler degrees Germany,

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