Note on thixotropy

Goodeve and Whitfield (1837-39) studied the equilibrium between the spontaneous building of an internal structure and its breakdown. The apparent viscosity of a thixotropic system was measured at different rates of shear. If r)r represent the apparent viscosity and extrapolated residual viscosity, respectively, then we have, 

If Tl and 1/5 are plotted as ordinate and abscissa, we get curves of different types. The intercept on the K-axis will give the value of T)rt, i.e., resistance of flow at extremely high shear ratio. It has been observed that thixotropy of carbon black dispersion is reduced on adding 3% linoleic acid. These curves also show the effect of thixotropy in the absence and presence of linoleic acid, respectively. 

Several theories have been proposed to account for thixotropy which are given below  

Thixotropic gelation may be considered as a form of flocculation and is sensitive to all kinds of additions. The time of solidification (T) is strongly dependent on the concentration (c) of the electrolyte, as seen in the empirical equation given by Schalek and Szegvari (1923), 

It has been shown that thixotropic gelation is a slow coagulation. Time of solidification is also changed with a change in the pH of the solution as mentioned by Freundlich. It is seen that alcohol promotes the thixotropic gelation and after evaporating the alcohol, the sol returns to the gel state. 

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