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Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. [1] For liquids, it corresponds to the informal concept of thickness; for example, syrup has a higher viscosity than water. [2]
Example - Cream – the longer it is whipped, the thicker it gets. Shear Thinning Fluid: Its viscosity decreases with increased stress. Example – Blood, Tomato sauce. Dilatant or shear thickening Fluid: Its viscosity increases with increased stress. Example – Oobleck (a mixture of cornstarch and water), Quicksand.
is the dynamic viscosity, i.e., a measure of the fluids' resistance to shearing flows L {\displaystyle L} is the characteristic length of the system ν = μ ρ {\displaystyle \nu ={\frac {\mu }{\rho }}} is the kinematic viscosity – it measures the ratio of dynamic viscosity to the density of the fluid
A built-in density measurement based on the oscillating U-tube principle allows the determination of kinematic viscosity from the measured dynamic viscosity employing the relation =, where: ν is the kinematic viscosity (mm 2 /s), η is the dynamic viscosity (mPa·s), ρ is the density (g/cm 3).
The viscosity of the sample is then calculated using the following equation: = ˙ where is the sample viscosity, and is the force applied to the sample to pull it apart. Much like the Meissner-type rheometer, the SER rheometer uses a set of two rollers to strain a sample at a given rate. [ 31 ]
The dilute gas viscosity contribution to the total viscosity of a fluid will only be important when predicting the viscosity of vapors at low pressures or the viscosity of dense fluids at high temperatures. The viscosity model for dilute gas, that is shown above, is widely used throughout the industry and applied science communities.
In this equation w s is the sediment settling velocity, g is acceleration due to gravity, and D is mean sediment diameter. is the kinematic viscosity of water, which is approximately 1.0 x 10 −6 m 2 /s for water at 20 °C.
Increasing temperature results in a decrease in viscosity because a larger temperature means particles have greater thermal energy and are more easily able to overcome the attractive forces binding them together. An everyday example of this viscosity decrease is cooking oil moving more fluidly in a hot frying pan than in a cold one.