Wetting of the bulk powder

Equation (13.12) shows that depends on y v both of which are lowered by the addition of surfactants (wetting agents). If 0 9O°, is negative and dispersion is spontaneous. 

Wetting of the internal surface requires penetration of the liquid into channels between and inside the agglomerates. The process is similar to forcing a liquid through fine capillaries. To force a liquid through a capillary with radius r, a pressure p is required that is given by, 

The above conclusions illustrate the problem of choosing the best dispersing agent for a particular powder. This requires measurement of the above parameters, as well as testing the efficiency of the dispersion process. 

The contact angle of hquids on solid powders can be measured by appHcation of the Rideal-Washburn equation. For horizontal capillaries (gravity neglected), the depth of penetration I in time t is given by the Rideal-Washbum equation [8, 9)  

To enhance the rate of penetration, Y y must be made as high as possible, 6 as low as possible, and p as low as possible. For dispersion of powders into liquids, surfactants should be used that lower 0 but do not reduce Y y too much the viscosity of the liquid should also be kept at a minimum. Thickening agents (such as polymers) should not be added during the dispersion process. It is also necessary to avoid foam formation during the dispersion process. 

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