Ask A Scientist Materials Science Archive

name         Jay
status       other
grade        other
location     CA

Question -   How does the temperature relate to viscosity?
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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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