Floe packings dispersions

With increasing interparticle collisions the probability of formation of floes from dispersed (nonflocculated) particles increases. Thus the horizontal axis can also be interpreted to mean a change from weakly flocculated particles on the left to increasingly flocculated particles toward the right. An outcome is that, irrespective of the degree of interparticle interaction, at low values of cp the viscosity rises slowly, but tends to increase rapidly when particle packing becomes dense.For randomly packed spheres this change occurs at about 95 = 0.60. A simple viscoplastic model is the Herschel-Bulkley equation... 

The lower the value of D the more open the packing. Suppose we now imagine a dispersion which has been driven to instability by, say, changing temperature. We can visualise sites for the nucleation of floes occurring randomly throughout the whole volume of the dispersion. The total number of primary particles remains unchanged so we can determine the volume fraction of floes ... 

Sediment Volume. If the dispersion is unstable, the sediment bed will be quite deep and sedimenting particles will stick together where they first strike the sediment bed, thus forming an open structure with considerable occluded liquid. If the dispersion is stable to reagglomeration, the particles will move freely past one another to avoid contact as long as possible. The result is a thin sediment bed with maximum solids packing and minimum occluded liquid (12). Since dispersed particles setde more slowly than floes, centrifugation maybe needed to force sedimentation of small particles within a reasonable analysis time. 

Flocculation comes from the Latin word flocculate meaning loose and woolly. Flocculated systems result in rapid rate of settling because each individual unit is composed of many particles and is therefore larger. However, due to the loose packing of floes they are easily dispersible on shaking. Deflocculated systems on the other hand are made up of smaller particles whose settling rate is slower, but the settled particles tend to form an irreversible compact and are difficult to redisperse. This phenomenon is called caking. For coarse suspensions, a deflocculated suspension will have better uniformity of dose but poorer stability... 

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