Floes single particle

It should be noted that single particles are not often found in estuaries, and that most particles generally sink as floes (see following section), nevertheless, Stokes law as a general background on the basic concept of particle settlement has been included. 

Wessel and Ball " and Kanai et al. studied in detail the effects of shear rate on the fractal structure of flocculated emulsion drops. They showed that the size of the floes usually decreases with the increase of the shear stress often the floes are split to single particles at high shear rates. As a result, the viscosity decreases rapidly with the increase of the shear rate. 

The formation of floes due to bridging flocculation has a dramatic effect on sedimentation rates, sediment volumes and on the ease of filtration. Effective flocculation may occur over a narrow range of polymer concentration because too little polymer will not permit floe formation, while too much polymer adsorption will eliminate the fraction of free particle surface needed for the bridging action (i.e. the polymer molecules will adsorb onto single particles in preference to bridging several particles). It has been proposed that the optimum degree of bridging flocculation may occur when particle surfaces are half covered with adsorbed polymer. 

Then the gradual floe destruction produces a shear-thinning behavior, until no Hoes remain but single particles, and the flow becomes Newtonian. In fact, the motion of the droplets under oscillatory strains has been observed by means of diffusing wave spectroscopy (31). Near the yield point, only a small fraction of drops flow while the rest remain as solid blocks. 

The use of binary collisions also fails when the distance over which two floes (or single particles) first feel the presence of one another due to their interaction potential is much greater than the average center-to-center distance among floes. For then the term cp, previously taken as the interaction potential between two particles, has to be modified to account for the presence of other particles, even for a two-particle collision. Such effects are observed in nonaqueous media (Albers and Overbeek, 1959) and can occur in concentrated systems as well. 

A general model of floe structure has been proposerd by Sutherland (289), who assumed that coagulation occurs with addition of single particles to doublets, trip lets, and larger aggregates and also that aggregates can collide and become joined. The theory of flocculate density and ease of filtration has been developed by Smellie and LaMer (290). 

Class IL Settling of a dilute suspension of flocculant particles. Randomly moving particles collide and form aggregates (floes) which behave as larger particles and have increased settling velocities compared to a single particle. 

The choice of method from available resources depends largely upon the properties of the material to be analyzed, the basic significance or physical wearing of the measurement, and the purpose for which the information is required. For example, failure to disperse the particles as discrete entities is the biggest single problem in all size analysis methods that depend on individual particulate behavior. With microscopic techniques particles must be dispersed on the slide to permit observation of individual particles, and in sedimentation techniques the material must be suspended in the fluid so that the particles behave as individuals and not as floes. 

Solids contact processes combine chemical mixing, flocculation and clarification in a single unit designed so that a large volume of previously formed floe is retained in the system. The floe volume may be as much as 100 times to in a "flow-through" system. This greatly increases the rate of agglomeration from particle contacts and may also speed up chemical destabilization reactions. 

As mentioned above, two-component flocculants often present advantages over a single-component flocculant, such as better control of flocculation kinetics and improved floe strength. Most dual-component flocculants consist of two polyelectrolytes, two pol5miers, or a polyelectrolyte and a nanocolloid. Usually, one of the components adsorbs on the surface of the particles to be flocculated and the second component bridges these polymer-coated particles. Therefore, this combination of patching and bridging is believed to be responsible for excellent results, as described for instance for retention systems (Fig. 5) [10]. 

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