Flow curves which I have obtained for 1% (w/v) suspensions of kaolinite clay, montmorillonite clay and silica microspheres all reveal shear thickening behaviour (an increase in viscosity) at higher shear rates. As I understand it, this is to be expected but I would like to know why it happens.
Under shear certain suspensions like montmorillonite clay periodic colloid structures also called tactoids are formed. Such structures are responsible for shear thickning. For paints and certain other applications such thickening is useful.
Shear thickening can be associated with jamming of the colloids, and shear can also cause concentration variations across the sample which can lead to shear banding.
You are essentially dealing with a two-phase system in which there is a dispersed phase that lives inside a suspending medium which acts as a continuous phase. Spatial arrangement and distribution, agglomeration and de-agglomeration during shear are responsible for shear-thickening and shear-thinning behavior. The basic knowledge on such type of systems can be got from the book
Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999).
There is a chapter on constitutive theories and equations of suspensions where the effects of shape, concentration, dimensions, and size distributions of the particles have been discussed.
Thanks Aroon, you're book was very helpful. There is a reference in particular from you're book that I would very much like to see but I can't seem to find it myself. It is reference number 141 from chapter 4 - 141. Govier, G.W. and Winning, M.D. (1948) AIChE Meeting, Montreal, Quebec.
Is there any chance you could forward me a copy if you have it?
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