Adhesion due to electrostatic forces

In the case of electrostatic forces one must distinguish between an excess charge and the electrical double layer (equilibrium). 

By inserting equation (19) into the basic formula, equation (6), and assuming that ke tt, the maximal tensile strength ate of agglomerates due to excess charges is 

Because of the field character of this binding mechanism, the tensile strength is independent of the particle size. Also, the strength due to excess charges is very small and the charges tend to equalize with time. Therefore, this mechanism is most often only significant for the initial formation of agglomerates. 

Much more important are adhesion forces which are due to electrical double layers. This phenomenon can develop if the particles touch each other and is permanent. According to Krupp the attraction pressure due to electrical double layers between two semi-infinite bodies is in the order of Pei = 1 to lO N/m (10 to lO dyn/cm ). In comparison, the van der Waals attraction pressure between two semi-infinite bodies is Pvdw = 2xl0 to 3xl0 N/m (2 X 10 to 3 X 10 dyn/cm ). 

It may seem as if the two mechanisms exclude each other. However, since Pvdw decreases with /a and P remains almost constant, even over macroscopic distances, the electrical double layer will contribute to the adhesion of particles, particularly if the contact surfaces are rough. 

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