The control of surface tension is paramount for any filtration technique, as the surface tension determines the capillary pressure that is needed to be overcome for the liquid to penetrate through filter channels. Wetting (characterized by the wetting angle) is the other parameter determining the ease of liquid penetration through the filter. Equation (4.1) illustrates this by the metallostatic head required for the melt passage through the filter:

( ) =

σ θ − ° ρ

H ga

4 cos 90 , (4.1)

where σ is the surface tension, ρ is the melt density, a is the mesh size, θ is the contact angle, and g is the gravity acceleration. Similar properties are important for introducing nonmetallic particles into the metal-matrix composites, as will be discussed in Chapter 9.