Result of Surface Forces on Behaviour in Air and Water

From the equations for van der Waals forces [Equation (5.8)] and EDL repulsion [Equation (5.11)] one can see that the magnitude of surface forces increases linearly with particle size. Body forces which depend on the mass of the particle, however, increase with the cube of the particle size (since the mass is related to the volume multiplied by the density and volume depends upon the cube of the particle size). It is the relative values of body forces and interparticle surface forces that are important. Although, small particles have very small interparticle surface forces compared with big particles, relative to the body forces, the interparticle surface forces are large for small particles. This effect occurs because of the much stronger dependence of body forces on size than the surface forces. 

The result is that fine powders in air are cohesive due to van der Waals and capillary attraction. Attraction between particles results in cohesive behaviour of 

One difference between dry fine powders and colloids in liquids is that the low viscosity of air (and other gases) make hydrodynamic drag forces minimal for dry powders in many instances, except when the particles have very low density (such as dust and smoke) or the gas velocity is very high. However, fine particles in liquids are strongly influenced by hydrodynamic drag forces as described in Chapter 2 because the viscosity of liquids is much greater than that of gases. 

Low zeta potential (near lEP) High salt (coagulation) Bridging polymers 

High zeta potential (away from lEP) Low salt 

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