The Viscosity of Particulate Systems

Abstract

Our emphasis thus far has been primarily directed toward uniform, that is, translational, fluid-particle motions. In this chapter we shall examine phenomena arising from shearing motion of the fluid relative to the suspended solids. We adopt the point of view that, in a sense, a fluid-particle suspension may be regarded as a continuum. This attitude seems reasonable provided that the particle dimensions are very small compared with the dimensions of the apparatus containing the suspension. Thus, among other things, we shall seek to determine the apparent viscosity of such a suspension. Problems of suspension viscosity are important not only for the macroscopic particles involved in many industrial separation and reaction processes, but also in connection with the very small particles commonly described as colloidal, whose size approaches the molecular dimensions of the suspending fluid medium. The same basic variables characterize suspension viscosity as characterize sedimentation rates, namely: (a) the nature of the fluid; (b) the nature of the suspended particles; (c) the concentration of suspended particles; (d) the motion of particles and fluid-the shearing field of the latter being the prime distinguishing characteristic. Because of the small size of particles involved in viscosity problems, other properties, such as internal flexibility and ease of deformation, may also be important.