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Name: Jay
Status: Other
Grade: Other
Location: CA
Country: N/A
Date: August 2005


Question:
How does the temperature relate to viscosity?



Replies:
The temperature dependence of viscosity sometimes can be complicated because many factors enter into determining the viscosity of a material or a solution. However, over small temperature intervals, say from about 0 C. to 60 C. plots of: log(viscosity) vs. t C. will be nearly linear (for aqueous polymer solutions), where viscosity is expressed in centipoise. For other types of viscosity measuring instruments where the flow through an orifice under the force of gravity is measured vs. time the temperature dependence can be more complex because the density of the solution is also changing. Simple models for the viscosity suggest that a plot of: log(viscosity) vs. 1/T where T is in kelvins should be linear. The slope of such a plot gives a sort of "activation energy" for viscous flow although that is rather empirical, rather than theoretically justified. Keep in mind that viscosity is a complicated phenomenon depending upon a number of variables in addition to temperature, not all of which are easily controlled. Some of these include: concentration, shear rate, type of solution (polymer vs. low molecular weight solute), solvent type, ionic strength (if that is applicable), type of instrument, instrument settings---that is the short list. Like other "dynamic" properties of liquids and solutions, viscosity is a tricky property to measure.

Vince Calder



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