Point of Zero Charge Adhesion Dominates

To adjust the height of the energy barrier in an aqueous suspension, it is simplest to alter pH by adding acid or alkali. The double layer of charge around each particle then changes systematically as shown in Fig. 10.11 for aluminum oxide particles, with no energy barrier at the point of zero charge. 

This deceptively simple equation in which was the permittivity of the liquid, (, the permittivity of free space, the potential on the sphere surface (i.e. the zeta potential) and q the viscosity) was derived by equating the electrical 

At the point of zero charge, there is no repulsive electrical force on the particles and so the full adhesion between the grains is developed. If this adhesion is strong, then each Brownian collision between singlet particles will produce a doublet. This was the case considered by Smoluchowski in 1917 and extended by Fuchs in 1934. The theory was based on the idea that colloidal particles behave like molecules which can react to form abimolecular compound. Thus the rate of appearance of doublets dAf /df is proportional to the square of singlet concentration N per unit volume by the law of mass action, and is limited by the Brownian diffusion coefficient to give 

Doublets grew within a few minutes, but then triplets started to appear, followed by quadruplets, and so on. The doublets then began to diminish. The results could be described by a simple theory originally produced by Smolu- 

Shearing such dispersions can speed up the aggregation process by allowing more collisions to occur. This effect is scaled by the Peclet number 

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