Shear thickening and thinning effects

It is not uncommon to observe a situation where a slowly stirred polymer solution will have an apparent low viscosity. However, if the liquid is now subjected to a sustained shear field then the liquid will exhibit an increased viscosity. This phenomenon, called shear thickening, is caused by elongational viscosity creating interactions which do not exist in the equilibrium polymer solution. 

A closely related phenomenon is thixotropy. This is the property of some polymer solutions to show a time dependent reduction in viscosity when sheared. The longer the fluid is subjected to the shear stress, the lower its viscosity becomes. So a thixotropic fluid is one which takes a finite time to attain its equilibrium viscosity in shear after being subjected to a step change in shear rate. In 

These two effects are particularly important in what are called thixotropic paints or coatings. In the can the paint is a semi-soUd gel, but when stirred for a time and then applied by a brush or roller it spreads like a liquid, reverting to the semi-soHd consistency when the appHcator is removed. 

So aU these technological effects are controlled by the rate at which the polymer chains are able to execute movement and rearrangement in response to external stresses. 

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