ABSTRACT
Contact angle is the angle at the three-phase (gas/liquid/surface) line of a sessile droplet or a sessile bubble. Contact angle is related to but not a direct measure of wettability of the surface by the liquid as seen in the case of water sessile droplet contact angle of a gelatin hydrogel (Fig. 23.1). Wettability is the ease with which an air/solid interface can be converted to a liquid/solid interface. The interfacial tension of a liquid/solid system is a direct measure of wettability; however, it is not a readily measurable quantity. The interfacial energy or interfacial tension of a solid with ambient air is generally referred to as the surface energy of the solid, and the smaller the difference between the surface energy of a solid and that of a liquid, the higher is the wettability of the surface by the liquid. In a strict sense, a ‘‘surface’’ is a hypothetical concept. The surface of a solid exists only as an interface as described in the previous chapters. When the contacting medium is ambient air or vacuum, such an interface is generally termed a surface. The difference of surface energies, therefore, does not represent the interfacial energy as described in Chapter 25.
