Wetting imperfect solid surfaces

In many practical circumstances, suspended and dissolved impurities, as well as imperfectly wetted solid boundaries, provide potential sites for the formation of a new phase. This process is known as heterogeneous nucleation. In the absence of impurities or solid surfaces, small embryos of the new phase are formed within the bulk metastable phase. This process is known as homogeneous nucleation. In this article we discuss homogeneous, not heterogeneous nucleation. For a discussion of the latter phenomenon, the interested reader is referred to [5]. 

The addition of fines to a powder system allows a larger top size of particle to be agglomerated due to the attendant increase in cohesive forces caused by a decrease in surface mean particle diameter and increase in agglomerate density. Thus, although eqn. (1) indicates a top size of about 150 pm for monosized particles with aqueous binders, the top size of feed for industrial disc pelletizers is usually higher at 30 to 50 mesh (300 to 600 /zm) with the provision that at least 25% should be finer than 200 mesh (75 /zm) [7]. Other liquids with surface tensions lower than that of water, or liquid/ solid systems in which the particle surface is imperfectly wetted, require finer particle sizes to make successful balling possible. 

Occasionally, when imperfectly wetted solid particles are present, stabilizing of the foam may be caused by the solid particles holding entrapped gas between their hydrophobic surfaces and hindering coalescence of bubbles. 

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