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Factors
affecting viscosity
From everyday experience, it should be common knowledge that viscosity
varies with temperature. Honey and syrups can be made to flow more
readily when heated. Engine oil and hydraulic fluids thicken
appreciably on cold days and significantly affect the performance of
cars and other machinery during the winter months. In general, the
viscosity of a simple liquid decreases with increasing temperature (and
vice versa). As temperature increases, the average speed of the
molecules in a liquid increases and the amount of time they spend "in
contact" with their nearest neighbors decreases. Thus, as temperature
increases, the average intermolecular forces decrease. The exact
manner in which the two quantities vary is nonlinear and changes
abruptly when the liquid changes phase.
Viscosity is normally independent of pressure, but liquids under
extreme pressure often experience an increase in viscosity. Since
liquids are normally incompressible, an increase in pressure doesn't
really bring the molecules significantly closer together. Simple
models of molecular interactions won't work to explain this behavior
and, to my knowledge, there is no generally accepted more complex
model that does. The liquid phase is probably the least well
understood of all the phases of matter. |
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