Aggregation of particles

The world aggregation is used for all the ways in which coilloidal particles are linked together. Thus aggregation includes the following  

Gelling, where the particles are linked together in branched chains that fill the whole volume oF sol so that there is no increase in the concentration of silica in 

0 about five e of silica to At high pH, imes thicker r, the coun-om viscosity nection with article sizes, le layer was 

1 of colloidal to eliminate rent particle larticles and olume. 

Coagulation, where the particles come together into relatively close-packed clumps in which the silica is more concentrated than in the original sol, so the coagulum settles as a relatively dense precipitate. 

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Drying and other chemical processes can have significant effects on this stmcture, there being loss of hydrate water as weU as physically adsorbed water, and coUapse of the stmcture to form more stable aggregations of particles (29,30). 

Aggregation of particles may occur, in general, due to Brownian motion, buoyancy-induced motion (creaming), and relative motion between particles due to an applied flow. Flow-induced aggregation dominates in polymer processing applications because of the high viscosities of polymer melts. Controlled studies—the conterpart of the fragmentation studies described in the previous section—may be carried out in simple flows, such as in the shear field produced in a cone and plate device (Chimmili, 1996). The number of such studies appears to be small. 

Fig. 41. Typical 2D fractal structure obtained by aggregation of particles in the journal bearing flow. Fractal dimension of the cluster is 1.54 (Hansen and Ottino, 1996b).

Thus, ultrasound and surface active agents together help in reducing the aggregation of particles because of the fact that the bonds between them are extended due to cavitation. Additives inhibit the agglomeration during nucleation process by reducing the surface tension. Ultrasound and additives both reduce population of local nuclei hence reduction in particle size [43]. 

These differences in the effect of polymers on various flocculation responses have important theoretical and practical implications and can be explained in terms of various characteristics of floes and floc-aggregates. Polymer adsorption or attachment of particles to polymer can occur in any number of configurations, and as a result the aggregation of particles also can take place in many ways, leading to different floe and suspension structures which will respond differently to different tests. 

Even in applications other than dewatering, permeability methods are quite often used to assess the performance of polymeric flocculants, since, in principle, they can give a very sensitive indication of the state of aggregation of particles and are useful in locating optimum polymer concentrations. 

In the pharmaceutical industry, surface area is becoming more important in the characterization of materials during development, formulation, and manufacturing. The surface area of a solid material provides information about the void spaces on the surfaces of individual particles or aggregates of particles [5], This becomes important because factors such as chemical activity, adsorption, dissolution, and bioavailability of the drug may depend on the surface on the solid [3,5]. Handling properties of materials, such as flowability of a powder, can also be related to particle size and surface area [4],... 

Figure 9.25 Models of granules of monodisperse particles characteristic psds (pore size distributions) are given below (a) uniform packing (b) bidisperse packing of aggregates of particles of similar sizes (c) same as (b) but the size of aggregates vary in a wide range.

Pores may be present as structural features (e. g. between domains) or as a result of aggregation of particles. They may also be the result of partial dehydroxylation (oxide hydroxides) or dissolution. Although the shapes of pores can be quite variable, there are some definite, basic forms. The commonest of these are 1) slit shaped, the walls of which may or may not be parallel 2) ink bottle which are closed upon all sides but one from which a narrow neck opens and 3) cylindrical. Upon partial dissolution, pores bounded by well-defined crystal planes (e. g. 102 in goethite) develop (Chap. 12). 

Due to aggregation of particles, ferrihydrite is microporous, i. e. the porosity is interparticular. Ferrihydrite precipitated at pH 8 from Fe " solution displayed a type IV isotherm with type E hysteresis (Crosby et al., 1983). The freshly precipitated material contained ink bottle pores 2-5 nm in diameter. Larger pores (ca. 20 nm) developed over an 11 day period. Between 83 and 95% of the total pore volume of a 2-line ferrihydrite was found to be due to micropores (Weidler, 1995). 

Adsorption of phosphate is initially rapid and is followed by a slow stage (hours to days) that is more pronounced for less crystalline samples of Fe oxides (Barrow et ak, 1981 Torrent et ak, 1990 Nilsson et ak, 1992). The slow stage has been attributed to diffusion into micropores or grooves (Torrent, 1991 Strauss et ak, 1997) and into aggregates of particles (Anderson et ak, 1985 Willet et ak, 1988). Evidence for slow dif-... 

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