92168.pdf

Rheology 255

Shear stress, a

Figure 9,5 A thixotropic loop

9.5) can be obtained by measuring the non-equilibrium shear stress as

the shear rate is first increased and then decreased in a standard way.

Solutions of high polymers are, in general, thixotropic to a certain

extent; intermolecular attractions and entanglements are overcome

and the extent of solvent immobilisation is reduced on shearing,

while Brownian motion restores the system to its original condition

when left to stand. The classical examples of thixotropic behaviour

are given by the weak gel systems, such as flocculated sols of iron(III)

oxide, alumina and many clays (particularly bentonite clays), which

can be liquefied' on shaking and 'solidify' on standing. Thixotropy is

particularly important in the paint industry, as it is desirable that the

paint should flow only when being brushed on to the appropriate

surface (high rate of shear) and immediately after brushing.

Rheopexy

Rheopexy is time-dependent shear-thickening, and is sometimes
observed as an acceleration of thixotropic recovery - for example,
bentonite clay suspensions often set only slowly on standing but quite
rapidly when gently disturbed,

irreversible phenomena

Shearing sometimes leads to an irreversible breaking (rheodestruction)
of linkages between the structural elements of a material - e.g. with
dehydrated silica gel networks.