Surface Tension is an effect within the surface layer of a liquid that results in a behavior analogous to an elastic sheet. The effect of surface tension permits insects to walk on water and for drops of water to bead up. When a liquid interfaces with a solid to produce this effect, it is referred to as surface tension. When this effect is found between two liquid phases, it's referred to as interfacial tension. In surface science, both surface tension and interfacial tension are measured in milli-Newtons per meter (mN/m) and is often represented by σ or γ. (1 mN/m = 1 dyne/cm = 0.001N/m).
Surface tension results from an imbalance of molecular forces in a liquid. At the surface of the liquid, the liquid molecules are attracted to each other and exert a net force pulling themselves together. High values of the surface tension means the molecules tend to interact strongly. Lower values mean the molecules do not interact as strongly.
Water has a high value of surface tension because it has a high degree of hydrogen bonding. Organic molecules with polar groups such as iodide and hydroxyl have a slightly lower surface energy than water. Pure hydrocarbons are even lower, while fluorinated compounds are very low because the fluorine atom won't share electrons very well so only dispersion interactions occur.
The water strider (shown above) relies on the surface tension of water to skate about on the surface of streams and ponds without sinking. The relatively high surface tension of water allows its surface to behave like a film. The surface depresses slightly under the waxy pad-like feet which are covered with tiny hairs and fine nanogrooves. These specialized non-wetting legs and feet along with extremely low body weight facilitate the strider's ability to live on the surface of water without sinking.
ramé-hart DROPimage utilizes the pendant drop and sessile drop methods for measuring surface and interfacial tension. DROPimage Advanced employs a proprietary edge tracing technology to precisely capture and analyze the drop dimensions and profile characteristics in order to accurately calculate the surface tension of a liquid using the Young-Laplace equation.
Interfacial tension is measured when the drop phase and the external phase are immiscible liquids -- for example, an oil drop in water. DROPimage is capable of measuring interfacial tension using hanging pendant drops as well as inverted pendant drops. The video below illustrates the steps involved in creating and running a simple measurement using the Experiment Wizard.
Related to surface tension is surface dilatational elasticity which is measured by the axisymmetric drop and bubble method using periodic oscillatory deformation of pendant and sessile drops and bubbles which are produced by an Oscillator.