User:Yesteryear/Sandbox
From Wikipedia, the free encyclopedia
(1773-1829) discovered (among many of his notable discoveries) that the meniscus of water in a glass tube made an angle of zero degree withh the glass walls. The systems had to be clean, but the result could be reproduced. Similarly, mercury made an angle of 128°. He called them ' and they are known as equilibrium contact angles' at the present time. He gave a laborious description of how they come about, and it was probably Athanase Louise Victorie Dupré (1808-1869) who gave a quantitative definition of the equilibrium contact angles and the equation is known now as the Young-Dupré equation. If the solid substrate has a horizontal surface on which rests a drop of liquid, then the line common to the solid, liquid and air is called the contact line. Three interfaces meet at the contact line, and there is one surface tension acting on each. Along the horizontal direction γLVcosλ = γSV-γSL
where γ is the surface tension, and the subscripts refer to S(solid), L(liquid) and V(vapor). λ is the contact angle. What happens to the vertical components of these forces? These tend to distort the solid surface but with very little to show unless the solid is soft. The cosine function is limited from -1 to +1, and it is not obvious how this requirement is satisfied in the Young-Dupré equation. In fact, +1 (λ = 0°) hides a whole set of liquids called the wetting liquids. These have an affinity for the solid and in the absence of other forces, would spread on the solid without end. W. A. Zisman, and in a different form William Draper Harkins (1873-1951), gave illustrative examples and empirical means to quantify the difference between the wetting and non-wetting liquids.
showed that instead of making a force balance, the Young-Dupré equation could be also derived by minimizing the total energy. He also showed what can happen if the substrate had a corner at the contact line and introduced the concept of line tension. Line tension affects the contact angle in very small drops. Equilibrium contact angles decrease slowly with increasing temperatures. J.W. Cahn argued that there exists a temperature called the surface critical temperature above which the liquid became wetting. This has not been demonstrated when the substrate is a solid, but is easily shown in a liquid-liquid-vapor system. In case of liquid-solid-air, the surface critical temperature lies below the critical temperature for the liquid-solid-vapor system and below the upper consolute temperature in a liquid-liquid-vapor system.
