Flocculated dispersions

If there is particle—particle interaction, as is the case for flocculated systems, the viscosity is higher than in the absence of flocculation. Furthermore, a flocculated dispersion is shear thinning and possibly thixotropic because the floccules break down to the individual particles when shear stress is appHed. Considered in terms of the Mooney equation, at low shear rates in a flocculated system some continuous phase is trapped between the particles in the floccules. This effectively increases the internal phase volume and hence the viscosity of the system. Under sufficiently high stress, the floccules break up, reducing the effective internal phase volume and the viscosity. If, as is commonly the case, the extent of floccule separation increases with shearing time, the system is thixotropic as well as shear thinning. 

Stability implies a resistance to change, and may be defined qualitatively in those terms. In the specific case at hand, stability is defined as resistance to molecular or chemical disturbance. This requirement recognizes that a flocculated dispersion may be more stable than a peptized dispersion from the standpoint of its future behavior. A physically stable dispersion is one which will not undergo molecular replacements at the interface between the dispersed solid and the continuous phase. 

Perhaps the best rheological stability criterion is the volume occupied by the sediment per unit volume of original dispersion. A flocculated dispersion settles rapidly to a high sediment volume, while a deflocculated one settles slowly to a low volume, as shown in Figure 8. Of course, the ultimate volume depends on the concentration of the dispersion and in order to give a criterion of more fundamental interest the concept of relative sediment volume RSV may... 

The relative absorbance E3/5 to characterize the variation in the sedimenting flocculated dispersion. 

As discussed above, the contribution from Gg to the attraction will be significantly larger than the van der Waals attraction. Thus, E may be equated to Gg. Equation (4) may be used to calculate Eg from the measured fg values for the flocculating dispersions. The results are summarized in tables I and II. ... 

One of the major difficulties in developing theories of the rheology of coagulated or flocculated dispersions is that the microstructures of the aggregates are nonequilibrium structures under shear. Understandably, the rheology of such dispersions is history dependent, as we have seen above, and requires computer simulations and nonequilibrium statistical mechanics for proper study. 

The flocculation results on the individual mineral suspensions are shown in Figure 2 (A B). These graphs show the effect of polyacrylic acid dispersant before (PAA) Figure 2A, and after xanthation (PAAX) Figure 2B, on the flocculation-dispersion behavior of individual suspensions of coal and pyrite with Purifloc-A22 flocculant. 

What is the practical importance of colloid flocculation-dispersion in nature ... 

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. 

There are three chapters in this volume, two of which address the microscale. Ploehn and Russel address the Interactions Between Colloidal Particles and Soluble Polymers, which is motivated by advances in statistical mechanics and scaling theories, as well as by the importance of numerous polymeric flocculants, dispersants, surfactants, and thickeners. How do polymers thicken ketchup Adler, Nadim, and Brenner address Rheological Models of Suspensions, a closely related subject through fluid mechanics, statistical physics, and continuum theory. Their work is also inspired by industrial processes such as paint, pulp and paper, and concrete and by natural systems such as blood flow and the transportation of sediment in oceans and rivers. Why did doctors in the Middle Ages induce bleeding in their patients in order to thin their blood ... 

Once flocculation has occurred, the properties of the flocculated dispersion depend strongly on the details of the interaction at separations near the primary minimum, including the depth of this primary minimum. This depth is often estimated simply by evaluating... 

Capillary forces during drying minimise the large differences in packing density which exist in wet coatings from stable and coagulated or flocculated dispersions. 

Particle-particle interaction is central to a wide range of engineering applications and processing industries. Examples include coagulation, flocculation, dispersion, emulsification, and froth flotation. In these applications, the particle size is small, and the overall particulate behavior is determined by forces associated with the surface properties rather than those related to mass or volume. The surface properties of a particle in a liquid medium are the result of a complex interaction between molecules, atoms, and ions at the particle surface and in the surrounding liquid. If a number of particles are present, interactions also take place between particles at short separation distances, and it is this interaction that is of most interest as it can determine the overall stability or instability of dispersions and/or suspensions. 

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). 

On the other hand, if the dispersion is flocculated, particles on contact lend to stick in the configuration in which they collide and thus lend to form a much more open structure (Figure 9.8). Of particular practical interest are weakly flocculated dispersions because, although they may sediment or cream, they form relatively open structures that arc readily dispersed by shaking or stirring. 

Increased water solubility in the range of 50-100% hydrolysis, controlling flocculation/ dispersion of clay,... 

Let us consider a dispersed system that has undergone flocculation. If p is the number of panicles in a floe, the effective internal phase content is, for a flocculated dispersion,... 

Figure 32 Ideal represeniatiun of the effect of increasing shear or increasing shearing time on the size of aggregates and viscosity for a flocculated dispersion.

The rheological behavior that goes along with doc destruction is illustrated in Fig, 34, which depicts shear. stress as a function of shear rate for a strongly flocculated dispersion. A simple explanation of the events occurring on a microscopic level can be presumed. When shear is started, flocculates are first distorted without breaking, producing an elastic response. However, once shear overcomes the attraction forces, flocculates start to come apart and a yield stre.ss appears. 

Uses Comonomer (anionic flocculants/dispersants) protective colloid and/ or copolymerizable surfactant in actylic/modified acrylic latexes for use in paints, inks, adhesives, and sealants in food-contact paper/paper-... 

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