SHEAR-INDUCED GELATION OF COLLOIDAL DISPERSIONS

Nanometric silica particles, dispersed in water, have been bridged int o long necklaces by adsorbing macromolecules of poly(ethylene oxide). At high coverage of particles by macromolecules the necklaces repel ea ch other and the dispersions are homogeneous solutions; at low coverag e the necklaces bind to each other and a concentrated phase separates from excess water. It is reported here that shear induced gelation and shear induced flocculation are observed near the boundary of the phas e separation region. The structures of these dispersions under shear h ave been observed through small angle neutron scattering. It has been found that, above a critical shear rate, the necklaces connect to each other to form threadlike objects which align along the velocity. At h igher shear these objects associate sideways to form three-dimensional flocs. (C) 1997 The Society of Rheology.