The Present Status and Prospect of Suspension Rheology.

Particles dispersed in a liquid tend to form flocs due to attractive forces between particles. The rheological properties of suspensions depend on the dynamic structures of flocs in shear fields. Main task of suspension rheology is to establish the quantitative relation among the mechanical properties of particle bond, geometrical structures of flocs, and macroscopic rheology. For understanding of current status of suspension rheology, the fundamental properties of ordinary flocculated suspensions are summarized. The most important aspect is that the particle bonds are not broken down in a quiescent state. Hence, the gross structure of flocs may be statistically invariant. The relation between rheology and floc structure is discussed on the basis of fractal and percolation concepts. The suspensions flocculated by polymers show interesting rheological behavior. For example, the suspensions flocculated by reversible bridging are Newtonian at low shear rates and shear-thickening at high shear rates, whereas the ordinary flocculated suspensions are shear-thinning in a wide range of shear rates due to the progressive breakdown of flocs. Since the particle-particle interactions are strongly influenced by adsorption affinity for surfaces, the flocculation induced by polymer has great potential as a new technique in rheology control of suspensions. The unique rheological behavior can be directly connected with the mechanical properties of the bonds between two primary particles. The physical quantities which determine the floc structures and rheological properties are the transverse and central components of attractive force between particles. The importance of vector nature of particle bonds is demonstrated, with a perspective of quantitative understanding of suspension rheology.