Physically one may define the surface (ST) as the measurement of the cohesive (excess) energy present at a gas/liquid interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equally attractive force in all directions. Molecules on the surface of a liquid experience an imbalance of forces. The net effect of this situation is the presence of free energy at the gas/liquid interface (surface). The excess energy is called surface free energy and can be quantified as a measurement of energy/area, or alternatively as a line tension or surface tension, which is quantified as a force/length measurement. The common units for surface tension are dynes/cm or mN/m. These units are equivalent.
Polar liquids, such as water, have strong intermolecular interactions and thus high surface tensions. Any factor which decreases the strength of this interaction, will lower the surface tension. Thus an increase in the temperature of this system will lower surface tension. Any contamination, especially by surfactants, will lower surface tension. It is often necessary to distinguish between the static surface tension and dynamic surface tension. Dynamic processes such as speed of surfactant adsorption to the gas/liquid interface of for example surfactants, can be studied with the maximum bubble tensiometer BPA-800P. The Theta and Sigma series tensiometers provide the means for measuring semi-dynamic and equilibrium surface tensions.
Learn more about surface tension.