The format that a liquid drop assumes in contact with a solid surface depends on the angle that the drop of liquid creates with the solid surface (Figure 2.3.7). The liquid will completely wet the surface when Θ=0° because the newly surface has surface energy γ smaller than the original solid-gas surface (that is, \(\gamma _ {SV} > \gamma _ {SL} +<\gamma _ {LV}\)). At the other extreme, the complete lack of wetting occurs when Θ=180°. Partial wetting happens in the interval between 0°<Θ<90°. Figure 2.3.7 presents photos of drops of water and mercury on a stainless steel surface and on a cross section of a surface made of treated cow bone. The water wets most of the solids while Hg wets very little.
The wetting of a solid by a liquid happens when an interaction between the molecules of liquid with the solid interface is greater than the interaction of between the molecules. In general, the energies γ \(\gamma_{Solid-Liquid(SL)}\) are smaller than the corresponding \(\gamma_{Solid-Vapour(SV)}\)and \(\gamma_{Liquid-Vapour(LV)}\) because in a Solid-Liquid interface (SL) almost all the atomic and molecular bonds are complete.
This compatibility exists when compounds or bonds are formed on the interface. Liquid copper (Cu) wets solid nickel (Ni) because it forms a solid solution with copper. Lead (Pb) liquid, on the other hand, is immiscible in liquid and solid iron (Fe). The introduction of a third metallic element like tin (Sn) or antimony (Sb), that form composites with Fe, promote wetting.
Wetting (or nonwetting) is severely altered when adsorbed contaminants are present (See Topic 2.3.9) on the solid’s surface. In fact, one of the largest difficulties in the measurement of the angle of contact is guaranteeing a clean and homogeneous surface. Solid surface are different than liquid surfaces in their high ddegree of heterogeneity even after the most careful of polishing procedures. The value of the angle will also depend on whether the liquid is spreading and wetting a dry retracting from an already wetted surface. The degree of vibration to which it is subjected will affect the measurement of the contact angle. Figure 2.3.8 shows a tool for measure such contact angles (tensiometer).
