Rheological behavior of suspensions flocculated by weak bridging of polymer coils

A polymer coil with weak affinity for the solid surface makes a flexible br idge between particles by reversible adsorption. When the coil size is comp arable to particle diameter in solution, the suspensions flocculated by rev ersible bridging show shear-thickening flow in a narrow range of shear rate s. The force generated by the rapid extension of bridges within the lifetim e is responsible for the shear thickening. Although it is expected that the bridges become more flexible with increasing coil size, the polymers with high molecular weights do not necessarily give rise to shear thickening. Th e striking shear thickening is induced by polymers with molecular weights, M-w, of 2.5 x 10(5) and 4.5 x 10(5). The viscosity enhancement in the shear -thickening region is considerably suppressed for polymers with M-w = 7.5 x 10(5). From the sedimentation experiments, a large portion of the segments of a coil with M-w, = 7.5 x 105 is adsorbed in train on the particle surfa ce. The addition of small amounts of surfactant leads to an increase in the fraction of loops at the expense of trains and the bridges become more fle xible. As a result, the flow of suspensions in the presence of surfactant b ecomes shear thickening. The rheology of suspensions flocculated by reversi ble bridging can be modified by a balance between loop and train fractions in a bridge. (C) 1999 Academic Press.