DISPERSION MECHANISM OF COAGULATED PARTIC LES (DOUBLETS) IN SHEAR FIELD IN WATER

The mechanism of dispersion of coagulated particles consisting of two equal-sized spheres (doublet) in a shear field in water is discussed. 1) A definition of dispersibility is proposed as DM=M(b)/M(t). Here, M (b) is the bending moment induced by the drag force on the doublet in a simple shear field, and M(t), is the moment required to separate the two adhering spheres. DM is thus a measure of the easiness of particl e dispersion. A theoretical model is presented which relates DM to the primary particle diameter, shear rate and adhesion force between part icles. 2) Coagulated particles consisting of monodisperse standard PSL (polystyrene latex) spheres (1.1, 1.9 and 5.2 mu m in diameter) were dispersed in a concentric rotating cylinder. During the process, there are two phenomena competing against each other: dispersion and shear coagulation. The relative contribution of these mechanisms is controll ed by the value of DM: Thus, for DM>1, it is found that doublets becom e dispersed; in contrast, when DM<1, shear coagulation overcomes dispe rsion and, as a result, new doublets are formed in addition to those o riginally existing.