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Temperature dependence of liquid viscosity
The temperature dependence of liquid viscosity is the phenomenon by which liquid viscosity tends to fall (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. It is usually expressed by one of the following models:
Additional recommended knowledge
The model is based on the assumption that the fluid flow obeys the Arrhenius equation for molecular kinetics:
where T is temperature, μ0 is a coefficient, E is the activation energy and R is the universal gas constant. A first-order fluid is another name for a power-law fluid with exponential dependence of viscosity on temperature.
The Williams-Landel-Ferry model or WLF for short is usually used for polymer melt's or other fluids that have a glass transition temperature.
The model is:
where T-temperature, C1, C2, Tr and μ0 are empiric parameters (only three of them are independent from each other).
If one selects the parameter Tr based on the glass transition temperature, then the parameters C1, C2 become very similar for the wide class of polymers. Typically, if Tr is set to match the glass transition temperature Tg, we get
Van Krevelen recommends to choose
Using such universal parameters allows one to guess the temperature dependence of a polymer by knowing the viscosity at a single temperature.
In reality the universal parameters are not that universal, and it is much better to fit the WLF parameters from the experimental data.
The Seeton Fit is based on curve fitting the viscosity dependence of many liquids (refrigerants, hydrocarbons and lubricants) versus temperature and applies over a large temperature and viscosity range:
Viscosity of water equation accurate to within 2.5% from 0 °C to 370 °C:
μ (Temp)= 2.414*10^-5 (N·s/m²) * 10^(247.8 K/(Temp - 140 K))
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Temperature_dependence_of_liquid_viscosity". A list of authors is available in Wikipedia.|