MadSci Network: Chemistry

Re: Diferenc btwen 'Newtonian fluids'&'highly elastic constant viscosity fluid'

Date: Wed Jun 11 19:30:02 2003
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Chemistry
ID: 1051891497.Ch

Not knowing the specific fluids in use, it is hard to tell the exact difference. However, what I can do is explain the difference between newtonian fluids and other non-newtonian fluids.

Newtonian fluids have a linear relationship between shear stress and rate of shear. They are the most common fluids such as water, and alcohol, air, and other fluids with a simple composition. This relationship is not only linear, but also, the shear stress aproaches zero as the shear rate aproaches zero.

Other types of fluids exhibit other relationships between rate of shear and shear stress than the zero intercept linear of newtonian fluids. Such fluids include:
*Bingham plastic: Resists a small shear stress but flows easily under a large shear stress, eg. such as toothpaste
*Pseudo-plastic: as the shear stress is increased, the rate of shear progresses linearly initially, but then gradually increases much more quickly for each unit of increase in shear force. Most non-newtonian fluids fall in this category. The viscocity decreases with increased rate of shear.
*Dilatant fluids: Viscosity increases with increasing rate of shear. Examples of dilatant fluids are thick suspensions of solids, such as sand and starch. These are not so common.

In addition to this range of relationships between rate of shear and viscosity, another parameter of a fluid to consider is how its elasticity affects flow.

For example: a pseudo plastic fluid (where an increase in shear force decreases viscosity) may also be highly elastic. In this case, the fluid's apparent viscosity will seem to lag behind a change in shear rate. This fluid would then be classified as thixotropic.

If instead a dilatant fluid is also elastic, the opposite happens: upon a suden change in shear rate, the apparent viscosity will not increase immediately, but only after a momentary lag. This fluid would then be called Rheopectic.

A very good reference for all this can be found at: I/NonNewtonian.htm

Your mad scientist,

-Aurelio R. Ramos

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