When the
liquid forms an interface with a
gas phase, a
molecule on the border has quite different
physical properties due to the unbalance of attracting forces by the neighboring molecules. At the
equilibrium state of the liquid, interior molecules are under the balanced forces with uniformly distributed adjacent molecules. However, relatively fewer number of molecules in the gas phase above the interface than
condensed liquid phase makes overall sum of
forces applied to the surface molecule direct inside of the liquid and thus surface molecules tend to minimize their own surface area. Such an inequality of molecular forces induces continuous movement of molecules from the inside to the surface, which means the surface molecules has extra
energy, which is called
surface free energy or
potential energy, and such an energy acting on reduced unit area is defined as
surface tension. This is a frame work to interpret relevant phenomena which occurs surface or interface of materials and a number of methods to measure the
surface tension has been developed. Among the various ways to determine surface tension,
Du Noüy ring method and
Wilhelmy slide method are based on the separation of a
solid object from the liquid surface, and
Pendant drop method and
Sessile drop or bubble method depend on the
deformation of the spherical shape of a liquid drop. The most typical impurity to induce
dynamic surface tension
measurement is a surfactant molecule which has both of
hydrophilic segment, generally called “head group” and
hydrophobic segment, generally called “tail group” in a same molecule. Due to the characteristic
molecular structure,
surfactants migrate to the liquid surface bordering gas phase until an external force disperse the accumulated molecules from the interface or surface is fully occupied and thus cannot accommodate extra molecules. During this process, surface tension decrease as function of time and finally approach the equilibrium surface tension (σequilibrium). Such a process is illustrated in figure 1. (Image was reproduced from reference) == Maximum bubble pressure method ==