The rheological properties of suspensions of rigid particles

Abstract

Experimental and theoretical work on the rheological properties of suspensions are reviewed. Attention is focused on systems consisting of rigid, neutrally buoyant particles suspended in Newtonian fluids; no restrictions, however, are placed on the concentration of the particles or on the forces acting in the suspension. The assumption that an effective viscosity depending solely on the volume fraction of the particles suffices to describe the rheology of suspensions is examined and shown to be inadequate. Indeed, the experimental evidence strongly supports the view that suspensions behave macroscopically as non-Newtonian fluids whose rheological properties are influenced by a large number of factors; these factors are listed. The various theories that have been put forward to explain the flow of suspensions are discussed, with particular emphasis being placed on the nature of the approximations made, so that purely empirical formulas can be clearly separated from those having a theoretical basis. Suggestions for future work, both theoretical and experimental, are provided. D. J. JEFFREY and ANDREAS ACRIVOS Department of Chemical EngineeringStanford University Stanford, California 94305 SCOPEA well-known and long-standing problem in fluid mechanics has been the calculation of the effective viscosity of a suspension. In recent years it has become clear that many of the complex phenomena associated with a flowing suspension cannot be explained by using a classical Newtonian description of a fluid with an effective viscosity. Thus, suspensions have to be treated as nowNewtonian fluids whose rheological (flow) properties are influenced by a large number of variables. This review presents some of the reasons why such non-Newtonian behavior occurs and describes the variables that must be included in any proposed theory for such behavior. The discussion is restricted to suspensions of rigid, neutrally buoyant particles in Newtonian fluids and thereby excludes emulsions, reinforced plastics, etc., but otherwise no restrictions are placed on the scope of the review. CONCLUSIONS AND SIGNIFICANCEThe volume fraction of the particles in a suspension (volume occupied by particles per unit volume of suspension) has often been assumed to be the only variable that influences the observed rheological properties of the suspension. Experimental evidence is presented to show that this is incorrect and that other factors, such as the shape and size distribution of the particles, the presence of electrical charges, and the type of flow being experienced must be considered. It readily follows then that no formula can exist that will give the effective viscosity or other flow properties solely as a function of the volume fraction of the particles. Of the many expressions that have been advanced for predicting the dependence of the flow properties on the factors referred to above, some are empirical while others rest on sound theoretical foundations. Successful empirical formulas generally contain one or more adjustable parameters which m...

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