Strongly flocculated system

When attractive forces dominate, the suspension becomes flocculated because of the existence of a stable minimum potential, as shown in Figure 7c. The range (acting distance) of the attractive forces determines the degree of the flocculation. For weakly flocculated system, the attractive forces act in a longer range than those of a strongly flocculated system. 

Concentrated suspensions commonly display viscoelatic behavior. The viscoelastic properties can be measured by oscillatory tests (26). Comparing with steady shear measurements, oscillatory measurements are made under small deformations, at which the suspension structure is only slightly perturbed. Hence, oscillatory measurements are suitable for correlating rheological behavior with structural data and interparticle potentials, even for strongly flocculated systems that show irreversible changes when subjected to large deformations. 

Yielding is more difficult to measure and to model. For strongly flocculated systems, Buscall et al. (1987) measured the yield stress under compression and found a concentration law quite similar to that for the shear modulus. This relation differed, however, from that for the yield stress in shear. Patel and Russel (1988) predicted nearly identical power law indices for modulus and shear yield stress. This prediction has been confirmed experimentally (Figure 10.7.2), albeit for reversibly flocculated systems. The theory is based on the classical yielding criteria. Reversible systems do not follow these criteria as yielding becomes a kinetic phenomenon. The yield stress then depends on shear history (Mewis and Meire, 1984). 

The restoring force for a dispersion to return to a random, isotropic situation at rest is either Brownian (thermal fluctuations) or osmotic. The former is most important for submicrometer particles and the latter for larger particles. Changing the flow conditions changes the structure, and this leads to thixotropic effects, which are especially strong in flocculated systems. 

Flocculated Systems. The viscoelastic responses of flocculated systems are strongly dependent on the suspension structure. The suspension starts to show an elastic response at a critical solid volume fraction of 0ct = 0.05 — 0.07, at which the particles form a continuous three-dimensional network (211-213). The magnitude of the elastic response for flocculated suspensions above 0ct depends on several parameters, such as the suspension structure, interparticle attraction forces and particle size, and shape and volume fraction. Buscall et al. (10) found that the volume fraction dependence of the storage modulus follows a power-law behavior. 

On the other hand, flocculated systems, which may be in the flocculated state either by the addition of electrolyte or by the use of a dispersion medium unsuited to create stability (e.g., quartz in benzene), show a very marked plasticity. Against weak stresses they offer a relatively strong resistance and... 

IPEC formation between (strong) polyelectrolytes results in highly aggregated and/or macroscopic flocculated systems. Nevertheless, the aggregation can be stopped at a colloidal level in extremely dilute solutions, and a polydisperse colloidally stable system of nearly spherical particles can usually be achieved [47]. 

Strongly flocculated suspensions usually show much less thixotropy than weaMy flocculated systems. Again, one must be careful in drawing definite conclusions without other independent techniques (e.g. microscopy). 

The effect of shear rate on viscosity in this equation is very strong, and demonstrates the typical situation with flocculated systems where severe shear thickening is seen, giving what looks like a yield stress. 

In dynamic measurements one carries two separate experiments. Firstly, the viscoelastic parameters are measured as a function of strain amplitude, at constant frequency, in order to establish the linear viscoelastic region, where G, G and G" are independent of the strain amplitude. This is illustrated in Fig. 1.13, which shows the variation of G, G and G" with y . It can be seen that the viscoelastic parameters remedn constant up to a critical strain value, Yc, above which, G and G start to decrease and G" starts to increase with a further increase in the strain amplitude. Most cosmetic emulsions produce a linear viscoelastic response up to appreciable strains (> 10%), indicative of structure build-up in the system ( gel formation). If the system shows a short linear region (i.e., a low y, ), it indicates lack of a coherent gel structure (in many cases this is indicative of strong flocculation in the system). 

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