Bimodal suspension

Figure 12 shows Chong et al. (28) data for monodisperse and bidisperse (bimodal) suspension systems. In a bidisperse suspension, the volume fraction of small spheres (diameter d) in the mixture is kept constant at 25% of the total solids. The figure shows that the viscosity of a bidisperse suspension is a strong function of the particle size ratio, d/D, where D is the diameter of the large particles. The viscosity decreases substantially by decreasing d/D at a given total solids concentration. The data for the unimodal system fall well above the bimodal suspensions. Also, the effect of particle size distribution decreases at lower values of total solids concentration. 

Figure 13 illustrates another very interesting point. Here the relative viscosity of a bimodal suspension is plotted as a function of volume percent of small spheres in total solids. At any given total solids concentration, the relative viscosity decreases initially with the increase in volume percent of small spheres, and then it increases with further increase in small spheres. The minimum observed in the relative viscosity plots of a bimodal suspension is quite typical. There are no fundamental reasons why a similar behavior would not be true for emulsions. 

Figure 19. Effect of composition of bimodal suspensions of colloidally stable polymer particles on the viscosity measured at a steady shear level of 0.0155 Pa (129).

The relationships between 17 and ( ) have been derived for suspensions of monodispersed hard spheres in Newtonian liquids. However, most real systems are polydispersed in size, and do not necessarily consist of spherical particles. It has been found that here also Simha s Eq 7.24, Mooney s Eq 7.28, or Krieger-Dougherty s Eq 7.8 are useful, provided that the intrinsic viscosity and the maximum packing volume fraction are defined as functions of particle shape and size polydispersity. For example, by allowing ( ) to vary with composition, it was possible to describe the vs. ( ) variation for bimodal suspensions [Chang and Powell, 1994]. Similarly, after values... 

Truly bimodal suspensions of colloidal and noncolloidal particles are of considerable practical interest. For such suspensions at low shear rates, the viscosity is high so that, for example, during storage, settling is reduced. On the other hand, because the mixture is shear thinning, at higher shear rates when the suspension is pumped the viscosity decreases, thereby enabling the mixture to be pumped at a lower pressure drop. 

According to the bimodal concept, the net relative viscosity, 17, bimodal suspension is given by... 

Effect of particle size distribution on the rheology of concentrated bimodal suspensions. J. RhedL, 38 (1), 85-98. 

Figure 6.18(b) Variation of relative viscosity as a function of vol% solid spheres in monomodal and bimodal suspensions, with volume fraction of small spheres being 25%. (Reprinted from Ref. 113 with kind permission from Society of Rheology, USA.)... 

Chang, C. Y. and Powell, R. L. 1993. Dynamic simulation of bimodal suspensions of hydrodynamically interacting spherical particles. J. Fluid Mech. 53,1. 

Structure and Colloidal Properties of Extremely Bimodal Suspensions... 

DYNAMIC MOBILITY OF BIMODAL SUSPENSIONS 5.3.1 General Behavior and Effect of Particle Size... 

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