Relation between bridging conformation and rheology of suspensions

Abstract

The rheological properties of suspensions flocculated by polymer bridging are studied as a function of adsorption affinity of polymer for the particle surfaces. Polymer adsorption is irreversible under ordinary conditions, because the polymer chain may attach to the surface at many points and not be able to desorb simultaneously from all sites. When both the particle and polymer concentrations exceed some critical levels, the suspensions elastically respond to small deformation at very low frequencies. Since the flocs are considered to consist of sites (particles) connected by bonds (bridges), the elastic responses due to the three-dimensional network of unbounded flocs are analyzed in terms of site—bond percolation. With decreasing adsorption affinity of polymer chain, the fraction of loops increases at the expense of trains which are in direct contact with surface. The decrease in the fraction of segments in trains causes the adsorption—desorption process to reversibly occur by Brownian motion. As a result, the polymer bridges are constantly forming, breaking and reforming in a quiescent state. When the adsorption affinity of polymer is decreased by the addition of surfactant, the viscosity decreases at low shear rates and increases at high shear rates. Therefore, the viscosity behavior changes from shear-thinning to Newtonian flow. The suspensions in which the particles are bridged by a flexible coil show viscosity profiles consisting of low-shear-rate Newtonian flow, shear-thickening flow at moderate shear rates, and shear-thinning flow at high shear rates. Although the extension of polymer coil connecting particles in shear fields is an intrinsic mechanism, the network structure is essential for shear-thickening flow of suspensions. The elastic properties of highly deformable network are also analyzed by percolation theory. The scaling analysis shows that the critical exponent is 4.0 for network constructed by irreversible bridges and 1.8 for highly elastic network by reversible bridges. The difference in critical behavior for elastic percolation may be attributed to the difference in local elastic constants.