Studies in thixotropic gelation - I—The mechanism of thixotropic gelation

Thixotropic gels can be converted into sols by applying a suitable mechanical stress,e. g. , by shaking, and set to gels again at a definite rate when the stress is removed. We define a gel as any system consisting of a disperse phase in a liquid dispersion medium, having a measurable rigidity. The actual transition, in a thixotropic system, from sol to gel or from gel to sol is not in general accompanied by any change in volume, although the gel itself may subsequently contract (syneresis). The average distance between the constituent particles is therefore the same in the sol and the gel. It is necessary to explain how, under the circumstances, rigidity can develop in the previously fluid sol. The most widely accepted theory appears to be that the particles in the gel are surrounded by thick envelopes of orientated water molecules, or “lyospheres”; that these envelopes are destroyed by shaking and re-form on allowing to stand; and that rigidity arises when the lyospheres are large enough to make contact with each other throughout the system.