ABSTRACT
D. Thixotropy The structure formation that endows a colloidal suspension with a yield stress also leads to the phenomenon of thixotropy. or reversible rheological changes. if the structure can be broken down by A ow but structural recovery is not instantaneous: very rapid recovery reveals itself through shear thinning alone. lf a suspension is only weakly flocculated, the bonds holding the floes together are weak. and these can be disrupted. in a progressive manner. by flow. On cessation of
flow, Brownian motion helps to reestablish the flocculated rest state. although this process may take anywhere from a few minutes to a few hours to he complete. These structural rearrangements manifest themselves in a variety of ways during rheological testing. During shearing at a constant shear rate, for example, the shear stress continues to decrease with time and reaches a constant value only after an extended period of time. Lf shearing is stopped upon reaching equilibrium but then resumed, the curve of shear stress versus time lies below the earlier curve, unless a sufficient rest period is given hetwcen the two different runs. For the same reason, if the shear rate is ramped up and down, a thixotropic loop is obtained. This is shown in Fig. I 0.21 for the shearing of a latex wall paint 1671; the area within the loop is taken to be a measure of the thixotropy of the sample.
