ABSTRACT
Contents A. Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 B. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 C. Young Equation and True Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 D. The Equation of State Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 E. The Acid-Base Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 F. Experimental Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 G. Equilibrium and Adsorbed Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 H. Microscopic and Nanoscopic Contact Angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 I. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 J. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
A. Wetting Science Wetting science, in its more classical definition, studies the way of interaction of a fluid with a solid surface, which determines the specific shape that liquids assume when they are deposited on a solid substrate. T. Young in 1805 [1] stated that a liquid drop placed on a solid surface assumes a geometrical shape that can be described by his famous formula (1), that became the milestone of wetting studies:
cos θ = (γsv − γsl)/γlv, (1) where θ is the static contact angle, γsv and γsl are the solid-vapor and solid-liquid surface energies and γlv is the liquid surface tension (Fig. 1). The Young Equation (YE) therefore describes how, in a true equilibrium condition, the contact angle between a fluid and a solid depends only upon the intrinsic nature of the materials engaged in the wetting experience. No dimension or gravity constraints are apparently present in it. Due to its apparent simplicity YE is still considered as the
Contact Angle Systems
Figure 1. Static Cantact Angle: 1 mm diameter water drop on a polymer solid substrate.
