Wetting of powder aggregates and agglomerates

Wetting of powders by liquids is very important in their dispersion, for example in the preparation of concentrated suspensions. The particles in a dry powder form either aggregates or agglomerates. This is illustrated in Fig. 4.3. 

Dispersion wetting W is given by the product of the external area of the powder A and the difference between Ysl and ygy  

Thus wetting of the external surface of the powder depends on the liquid surface tension and contact euigle. If 0 90 cos 0 is positive and the work of dispersion is negative, i.e. wetting is spontaneous. The most important parameter that determines wetting of the powder is the dynamic surface tension, Ydynamic O- - the value at short 

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 hne capillaries. To force a liquid through a capillary with radius r, a pressure p is required that is given by 

Ysl has to be made as small as possible rapid surfactant adsorption to the solid surface, i.e. low 0. When 0 = 0, p oc Ylv Thus for penetration into pores one requires a high Ylv Thus, wetting of the external surface requires low contact angle 0 and low surface tension Ylv Wetting of the internal surface (i.e. penetration through pores) requires low 0 but high Ylv These two conditions are incompatible and a compromise has to be made Ysv Ysl must be kept at a maximum. Ylv should be kept as low as possible but not too low. 

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