Contact Angle Equation of Young and Dupre

In systems where A G - A G 0, the solid is not completely wetted by the liquid but a sessile drop of L is formed on S. Such a drop on a flat, smooth, and horizontally positioned solid surface is depicted in Rgure 8.1. The shape of the drop or, more precisely, the contact angle 0 with the solid is determined by the balance of interfacial tensions. In equilibrium 

FIGURE 8.1 Interfacial tensions determine the shape of a sessile drop of liquid at an interface. 

Generally, G contains the vapor of L (assuming nonvolatile S). Molecules of L adsorb from the vapor on S causing an interfacial pressure Jt , defined as 

For practical purposes, if 9 exceeds, say, 15° = 0. Most apolar solid surfaces in 

2 SOME COMPLICATIONS IN THE ESTABLISHMENT OF THE CONTACT ANGLE HYSTERESIS, SURFACE HETEROGENEITY, AND ROUGHNESS 

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