ABSTRACT
II. UNSTABLE SUSPENSIONS The flow of particulate suspensions is encountered in a myriad of applications involving materials such as filled polymers. paints. mineral slurries, fuels. and building materials. ln order that rheological measurements made on these materials be reproducible in time and be relatable to the microstructure. it is necessary that particles neither settle nor agglomerate during the process of measurement. For colloidal particles. settling is not a concern unJess flocculation takes place under the action of the London-van der Waals forces of attraction due to the large surface-area-to-volume ratio. Particle-particle collisions resulting from Brownian motion as well as any imposed flow field typically lead to the formation
of floes that can settle as a soft, loosely packed sediment under the influence of gravity. Sucb a suspension can be made stable against flocculation by either electrostatic or steric means ]9]. However. if the densities of the two phases are not the same. the suspension will still be unstable against sedimentation: the deflocculated particles will settle in the form of a close-packed sediment that is difficult to redisperse. For large particles, settling is the major concern. since surface forces can safely be neglected: the rate of settling can be calculated theoretically II 0 ]. and settling can often be retarded by the use of a viscous suspending medium. However. even in the case of large particles. shear-induced collisions can result in agglomeration if binding mechanisms such as liquid bridges or mechanical interlocking are present ]11]. Clearly, the ideal situation is one where the suspension is stable against both flocculation as well as sedimentation. This can be ensured in model studies but is obviously not always practical in the case of industrial materials.
