Shear induced gelation of colloidal dispersions

Nanometric silica particles, dispersed in water, have been bridged into long necklaces by adsorbing macromolecules of poly(ethylene oxide). At high coverage of particles by macromolecules the necklaces repel each other and the dispersions are homogeneous solutions; at low coverage 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 phase separation region. The structures of these dispersions under shear have 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 higher shear these objects associate sideways to form three-dimensional flocs. (C) 1997 The Society of Rheology.