Page 266 - Introduction to Colloid and Surface Chemistry

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.