UNUSUAL THIXOTROPIC PROPERTIES OF AQUEOUS DISPERSIONS OF LAPONITE RD

The rheological properties of aqueous dispersions of the synthetic hec torite clay Laponite RD that form gel-like structures have been invest igated. Special emphasis has been laid on the phenomenon of thixotropy . Structural recovery at rest after steady shear is considered a funda mental thixotropic process and has been characterized by small amplitu de oscillatory shear measurements. A special sealing technique was use d 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 singl e 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 concent ration and mechanical pre-treatment of the material. This type of kine tics has not been reported so far for the thixotropic recovery of any clay dispersion and it does not fit the common definition of thixotrop y. The reorganization of the gel structure determining this thixotropi c phenomenon is interpreted as a cooperative self delaying process sim ilar to the aging of glassy polymers or precipitation from supersatura ted solid solutions. (C) 1996 Academic Press, Inc.