ABSTRACT
The contact angle is defined as the angle of liquid at the three-phase line. Figure 27.1 depicts contact angles involved in sessile droplet and sessile bulb. Figure 27.2 depicts force balance at the three-phase line for a sessile droplet and for a sessile bubble. For a sessile bubble, Young’s equation still holds. However, the location and the direction of gsl and gsv are different from the case for a sessile droplet. Figure 27.2b depicts a cross-sectional view of an air bubble on a solid surface immersed in liquid water (sessile bubble). The angle , which is generally recognized as contact angle in the case of a sessile droplet, is the supplementary angle of the contact angle y in the case of a sessile bubble. Thus, a surface that gives a large contact angle of water droplet gives a small when a bubble develops on the surface immersed in water, i.e., an air bubble spreads on a hydrophobic surface.
