Temperature dependence of liquid viscosity

The temperature dependence of liquid viscosity is the phenomenon by which liquid viscosity tends to decrease (or, alternatively, its fluidity tends to increase) as its temperature increases. This can be observed, for example, by watching how cooking oil appears to move more fluidly upon a frying pan after being heated by a stove.

Kinematic viscosity can be estimated as a typical (thermal) velocity times the mean free path. A molecular view of liquids can be used for a qualitative picture of decrease in the shear (or bulk) viscosity of a simple fluid with temperature. As the temperature increases, the thermal velocity increases. However, much more important is the rapid decrease of the mean free path with temperature. The reason for this is that temperature increase releases more and more molecules to move around and interact with any given molecule. The actual process can be quite complex and is typically represented by simplified mathematical or empirical models, some of which are discussed below. The models are valid over limited temperature ranges and for selected materials.

An exponential model for the temperature-dependence of shear viscosity (μ) was first proposed by Reynolds in 1886.

where T is temperature and ${\displaystyle \mu _{0}}$ and ${\displaystyle b}$ are coefficients. See first-order fluid and second-order fluid. This is an empirical model that usually works for a limited range of temperatures.

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