Theory

Abstract

The mechanical and thermodynamic behavior of systems composed of two or more phases may be influenced by thin interface regions between any two phases. The interfaces possible can be characterized by the three states of matter solid, liquid and gas. The interface between a liquid and its own vapor is called a surface. If sufficiently mobile the interface will assume an equilibrium shape. Most common examples are drops, meniscusses, liquid sheets surrounded by vapor, air or by another liquid as well as soap bubbles formed by thin films. The most obvious phenomenon associated with the interface is that of surface tension. From a macroscopic point of view the interface between a liquid and its vapor appears sharply defined with a discontinuous change of density and the other thermodynamic properties. Therefore the interface is often considered as if it were a thin, uniformly stretched skin or membrane. Surface tension a is defined as the force acting per unit length across a line on this fictitious membrane. Under the operation of this force the fluid behaves as if enclosed in an envelope of constant tension.