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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]
The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature.
Dynamic viscosity is a material property which describes the resistance of a fluid to shearing flows. It corresponds roughly to the intuitive notion of a fluid's 'thickness'. For instance, honey has a much higher viscosity than water. Viscosity is measured using a viscometer. Measured values span several orders of magnitude.
The poise is often used with the metric prefix centi-because the viscosity of water at 20 °C (standard conditions for temperature and pressure) is almost exactly 1 centipoise. [3] A centipoise is one hundredth of a poise, or one millipascal-second (mPa⋅s) in SI units (1 cP = 10 −3 Pa⋅s = 1 mPa⋅s). [4] The CGS symbol for the centipoise ...
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.
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 ]
A viscometer (also called viscosimeter) is an instrument used to measure the viscosity of a fluid.For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used.
Volume viscosity (also called bulk viscosity, or second viscosity or, dilatational viscosity) is a material property relevant for characterizing fluid flow. Common symbols are ζ , μ ′ , μ b , κ {\displaystyle \zeta ,\mu ',\mu _{\mathrm {b} },\kappa } or ξ {\displaystyle \xi } .