FLOCCULATION OF COLLOIDS BY SOLUBLE POLYMERS AND ITS EFFECT ON RHEOLOGY

Flocculation of colloidal suspensions is often induced by the addition of small amounts of polymers. For irreversible bridging, reversible b ridging, and depletion flocculation, the particle interactions are ana lyzed as a function of the adsorption affinity of the polymer chain fo r the particle surfaces. Although the forces between primary particles are attractive in three mechanisms, the lifetime and vector nature of the particle bond is quite different. In bridging flocculation induce d by irreversible adsorption, the polymer bridges are not broken down by thermal energy. The irreversible growth can generate fractal flocs. When the polymer chains do not have strong affinity for the surface, the suspensions are flocculated by reversible bridging. Since the part icle bonds are constantly forming, breaking, and reforming in a quiesc ent state, the flocculation by reversible bridging may be an equilibri um phenomenon. In depletion flocculation, the attractive forces are pr oduced by osmotic pressure of nonadsorbing polymer coils. The process is essentially phase separation, resulting in particle-rich solid and particle-poor fluid phases. The particles in the dense solid phase are arranged in an ordered lattice which shows opal-like iridescence. The differences in flocculation process lead to significant differences i n the rheological behavior of suspensions. In suspensions flocculated by irreversible bridging, the extension and bending of bonds can accum ulate the elastic energy and the relaxation hardly takes place by ther mal energy. When the particle concentration is increased beyond some c ritical value, a three-dimensional network is formed over the system. Therefore, the concentrated suspensions behave as solids at low strain s. The network formation process can be described as a sol-gel transit ion through percolation theory. In reversible bridging, the polymer co ils have an equilibrium conformation. The flow is Newtonian at shear r ates where the time-scale of coil extension in shear fields is much lo nger than that of desorption. The bonds in suspensions flocculated by depletion show little resistance with respect to transverse bending fo rces. Since the strain energy is rapidly dissipated by shear motion of particles, the suspensions can be characterized as fluids. The macros copic rheological responses of heterogeneous suspension systems are di scussed in relation to the mechanical properties of primary bonds and gross floc structures.