RHEOLOGY AND CONCENTRATED SUSPENSIONS

Rheology and Concentrated Suspensions volume fraction defined by... 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Tsai, S. C. and Zammouri, K. 1988. Role of interparticular van der Waals force in rheology of concentrated suspensions. J. Rheol. 32 737-750. 

Maschmeyer, R. 0. and Hill, C. T. 1977. Rheology of concentrated suspensions of fibers in tube flow. Trans. Soc. Rheol. 21 183-194. 

Bergstrom, L., Rheology of concentrated suspension, in Surface and Colloidal Chemistry in Advanced Ceramic Processing, R.J. Pugh and L. Berstrom, Eds., Marcel Dekker, New York, 1994. 

Brady, J. R, and Bossis, G., The rheology of concentrated suspensions of spheres in simple shear flow by numerical simulations, J. Fluid Mech., 155,105-129 (1985). 

Marti, L, Hofler, O., Fischer, P. Windhab, E.J. 2005. Rheology of Concentrated Suspensions Containing Mixtures of Spheres and Fibres. Rheologica Acta 44 (5) 502-512. 

Van Den Tempel [40] discussed the rheology of concentrated suspensions (carbon dispersions). He mentioned that the viscoelastic properties of a flocculated dispersion, at small or slow deformations, are explained in terms of a network model. The results show that the network is made from chains of preformed aggregates. The aggregation state is controlled by the volume fraction, the particle size, and the total particle concentration. 

J.T. Jenkins and D.F.. McTigue, Viscous Fluctuations and the Rheology of Concentrated Suspensions. Submitted to Journal of Fluid Mechanics (1995)... 

The Newtonian constitutive equation is the simplest equation we can use for viscous liquids. It (and the inviscid fluid, which has negligible viscosity) is the basis of all of fluid mechanics. When faced with a new liquid flow problem, we should try the Newtonian model first. Any other will be more difficult. In general, the Newtonian constitutive equation accurately describes the rheological behavior of low molecular weight liquids and even high polymers at very slow rates of deformation. However, as we saw in the introduction to this chapter (Figures 2.1.2 and 2.1.3) viscosity can be a strong function of the rate of deformation for polymeric liquids, emulsions, and concentrated suspensions. 

Detailed treatments of the rheology of various dispersed systems are available (71—73), as are reviews of the viscous and elastic behavior of dispersions (74,75), of the flow properties of concentrated suspensions (75—82), and of viscoelastic properties (83—85). References are also available that deal with blood red ceU suspensions (69,70,86). 

The rheological properties of a suspension depend upon factors such as the size, shape and concentration of the particles, the stability of the suspension and the viscosity of the medium. Flow properties can be modified by altering the surface charge... 

It is convenient to distinguish between particle or fluid rotation about axes normal and parallel to the direction of relative motion. These two types of motion may be termed respectively top spin and screw motion (Til). Top spin is of more general importance since this corresponds to particle rotation caused by fluid shear or by collision with rigid surfaces. Workers concerned with suspension rheology and allied topics have concentrated on motion at low Re, while very high Reynolds numbers have concerned aerodynamicists. The gap between these two ranges is wide and uncharted, and we make no attempt to close it here. 

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