Concentrated suspensions high shear limit

Beyond the percolation limit, the bridging network is more concentrated. Below the critical volume fraction, no continuously bridging networks are formed and the viscosity is low. As shown in Figure 12.7, this bridging network breaks up as the shear rate increases, giving different viscosities at different shear rates. As a result, this gives low and high shear limit viscosities observed at steady state for concentrated poljmier solutions and concentrated particulate suspensions (discussed later). 

SENGUN, M.Z. c PROBSTEIN, R.F. 1989a. High-shear-limit viscosity and the maximum packing fraction in concentrated monomodal suspensions. PhysicoChem. Hydrodynamics 11, 229-241. 

Now the technique provides the basis for simulating concentrated suspensions at conditions extending from the diffusion-dominated equilibrium state to highly nonequilibrium states produced by shear or external forces. The results to date, e.g., for structure and viscosity, are promising but limited to a relatively small number of particles in two dimensions by the demands of the hydrodynamic calculation. Nonetheless, at least one simplified analytical approximation has emerged [44], As supercomputers increase in power and availability, many important problems—addressing non-Newtonian rheology, consolidation via sedimentation and filtration, phase transitions, and flocculation—should yield to the approach. 

The increase in gel strength with increase in bentonite concentration above the gel point is consistent with the increase in yield value and modulus. On the other hand, the limited creep measurements carried out on the present suspension showed a high residual viscosity Oq of the order of 9000 Nm s when the bentonite concentration was 45g dm. As recently pointed out by Buscall et al (27) the settling rate in concentrated suspensions depends on 0. With a model system of polystyrene latex (of radius 1.55 vim and density 1.05 g cm ) which was thickened with ethyl hydroxy ethyl cellulose, a zero shear viscosity of lONm was considered to be sufficient to reduce settling of the suspension with = 0.05. The present pesticide system thickened with bentonite gave values that are fairly high and therefore no settling was observed. 

One can readily see that for n > 1, equation (1.13) predicts increasing viscosity with increasing shear rate. The dilatant behaviour may be observed in moderately concentrated suspensions at high shear rates, and yet, the same suspension may exhibit pseudoplastic behaviour at lower shear rates, as shown in Figure 1.9 it is not yet possible to ascertain whether these materials also display limiting apparent viscosities. 

A very large number of equations, many of them empirical, have been proposed to relate the viscosity with the concentration of particles in suspensions. Fig. 9.23 shows the application of a typical empirical equation for suspensions on which measurements are made at sufficiently high shear rates that the system has become Newtonian. The limiting viscosity at these high shear rates reflects only hydrodynamic interactions. All other interactions, of course, lead to increased viscosity and it is never possible to discount multiple sources of interactions... 

We stated above that for most applications the slurry behaves as a fluid of modified density, and that viscosity effects can usually be neglected. However, there is a limit to the permissible solids concentration. Relatively coarse solids can simply not be carried in suspension above a certain solids concentration for the particular slurry, say 40 per cent by mass. In the case of very fine solids, a thixotropic mixture is reached at higher solids concentration, in which the solid particles stay in suspension but the slurry viscosity is markedly increased. In fact the viscosity no longer exhibits a Newtonian relationship of direct proportionality between shear stress and velocity gradient. This is an area to keep away from in normal plant design, but for special applications like mine backfill plants, when high solids concentration is critical, it may be desirable to commission a special design based on materials... 

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