Weakly 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. 

Weakly flocculated systems have been studied by Buscall et al. (165), Heath and Tadros (166), Goodwin et al. (167), Patel and Russel (168, 169), Otsubo (170-172), Buscall et al. (51,173), Woutersen and de Kruif (174), and Nakai et al. (175). The weak flocculation can be obtained by several means, including secondary-minimum flocculation, depletion flocculation, polymer-bridging flocculation, and incipient flocculation. Details of the various mechanisms of interparticle attraction can be found in Russel et al. (27) and Somasundaran and Yu (176). Normally, flocculated systems have a solid volume fraction of no less than 0.2. When 0 < 0.2, an attractive system will settle down quickly. When 0 > 0.2, a flocculated suspension can be maintained easily for a period of time for a steady shear measurement to be completed. 

Figure 27. Low shear viscosity variation with solid volume fraction for weakly flocculated systems.

Figure 28. Bingham yield stress variation with solid volume fraction for weakly flocculated systems of different particle sizes and different layer of thickness (173,).

Weakly flocculated systems are the most common encountered in industry and, as we have seen, the most complex Theologically because of the flow-dependent floe size and long time constants. Such systems continue to be an active area of lesearch. 

One of the characteristics of weak flocculation is that the system is reversible. At low volume fractions the system will form some clusters and some single particles. The clusters can be easily disrupted by gentle shaking. As the concentration is increased the system will reach a percolation threshold . The number of nearest neighbours around any test particle reaches 3 at about (p — 0.25 and the attractive forces between... 

With nonadsorbing polymer, rheological effects of similar magnitude accompany the phase transitions described earlier (Patel and Russel, 1989a,b). Since macroscopic phase separation takes weeks or months, rheological measurements performed within a few days on samples formulated within the two-phase region, with — Q>miJkT 2 - 20, detect a metastable structure that changes little over time. The systems respond as flocculated dispersions, but the microstructure recovers relatively quickly to a reproducible rest state after shear. Hence these weakly flocculated dispersions are quite tractable materials. 

This process refers to aggregation of the droplets (without any change in primary droplet size) into larger units. It is the result of the van der Waals attractions that are universal with all disperse systems. Flocculation occurs when there is insufficient repulsion to keep the droplets apart at distances where the van der Waals attractions are weak. Flocculation may be either strong or weak, depending on the magnitude of the attractive energy involved. 

The small droplet size also prevents any flocculation of the droplets. If weak flocculation can be prevented this enables the system to remain dispersed, with no separation. 

Most disperse systems used in practice are weakly flocculated, and they also contain thickeners or structuring agents to reduce sedimentation and to acquire the correct rheological characteristics for applications, such as handcreams and lotions. The exact values of G and G" required will depend on the system... 

Clearly, depends on the volume fraction of the dispersion, as well as the particle size distribution (which determines the number of contact points in a floe). Therefore, for quantitative comparison between various systems, it must be ensured that the volume fraction of the disperse particles is the same, and that the dispersions have very similar particle size distributions. also depends on the strength of the flocculated structure - that is, the energy of attraction between the droplets - and this in turn depends on whether the flocculation is in the primary or secondary minimum. Flocculation in the primary minimum is associated with a large attractive energy, and this leads to higher values of when compared to values obtained for secondary minimum flocculation (weak flocculation). For a weakly flocculated dispersion, as is the case for the secondary minimum flocculation of an electrostatically stabilised system, the deeper the secondary minimum the higher the value of (at any given volume fraction and particle size distribution of the dispersion). 

Typical potential ener r diagrams that might be expected for simple combinations of depletion stabilization with steric or electrostatic stabilization are displayed in Fig. 17.14. The minimum in the potential enCTgy curve need not be below the zero energy level in order to induce weak flocculation, although it would be expected that this type of behaviour would be rare and depend upon the specific details of the system in question. 

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... 

Hence, as a first approximation, we will assume that type (a) represents stable systems, type (b) flocculated systems, and that the transition between stable and flocculated systems is represented by the potential curve with a weak potential barrier for which the top coincides with the horizontal axis Vr- Va = 0. 

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