SIZE EFFECTS ON THE SHEAR-THICKENING BEHAVIOR OF SUSPENSIONS FLOCCULATED BY POLYMER BRIDGING

The polymer coil whose size is comparable in solution to particle diameter makes a flexible bridge between two particles. The suspensions flocculated by the bridging mechanism show shear thickening at high shear rates when both the particle and polymer concentrations are increased above some critical levels. The boundary for appearance of shear thickening is analyzed in terms of a percolation process. The shear thickening results from the three-dimensional network structure consisting of particles bridged by flexible polymer coils. Since the intrinsic mechanism of shear thickening is the restoring forces of extended bridges in shear fields, the most important variable affecting the flow behavior is bridging conformation. The effects of particle and coil sizes on the shear-thickening behavior are examined in relation to bridging conformation. The shear thickening takes place for suspensions prepared using particles with diameters of 80-125 nm and polymers with molecular weights of 2.5 x 10(5)-4.5 x 10(5). The appearance of shear thickening is determined by a delicate balance between flexibility of bridge and adsorption affinity.