Unusual thixotropic properties of aqueous dispersions of Laponite RD

The rheological properties of aqueous dispersions of the synthetic hectorite clay Laponite RD that form gel-like structures have been investigated. Special emphasis has been laid on the phenomenon of thixotropy. Structural recovery at rest after steady shear is considered a fundamental thixotropic process and has been characterized by small amplitude oscillatory shear measurements. A special sealing technique was used to prevent the evaporation of water during these experiments. After cessation of steady shear \eta*\ increases monotonically with time and even after 16 days no equilibrium viscosity value is reached. A single power law \eta*\similar to t(n) holds within the time regime from 10 to 10(6) s. The exponent n=0.13+/-0.02 is independent of clay concentration and mechanical pre-treatment of the material. This type of kinetics has not been reported so far for the thixotropic recovery of any clay dispersion and it does not fit the common definition of thixotropy. The reorganization of the gel structure determining this thixotropic phenomenon is interpreted as a cooperative self delaying process similar to the aging of glassy polymers or precipitation from supersaturated solid solutions. (C) 1996 Academic Press, Inc.