Surface Tension | KRÜSS Scientific

In the case of liquids with surface-active molecules (surfactants), the surface tension is time-dependent until an equilibrium has been established. At this equilibrium, the measured surface tension is called static surface tension. If, on the other hand, the time-dependent value is measured, this is the dynamic surface tension.

Significance

The surface tension determines the behavior of liquids in a number of processes and phenomena:

• Wetting and wettability: The wetting of a solid by a liquid depends, among other things, on the surface tension. This plays a major role in washing and cleaning, and also in wetting processes such as printing and coating.
• Droplet size: With a spray or an emulsion, the total surface area of the droplets is larger the smaller the individual droplets get. The size of droplets produced therefore depends on the surface tension.
• Dispersions and dispersibility: Boundary surfaces are formed between solid particles and the liquid during dispersion. A high surface tension has a negative effect on the dispersibility, as contact between similar particles is preferred (formation of clots).

Measuring methods

• Ring method according to Du Noüy: The force acting on an optimally wettable ring as a result of the tension of the withdrawn liquid lamella when removing the ring is measured in this method.
• Plate method according to Wilhelmy: The force acting on an optimally wettable plate which is immersed vertically in the liquid is measured in this method.
• Rod method: As the plate method, wherein a cylindrical rod with a smaller wetted length is used for measurement with a smaller liquid volume.
• Bubble pressure method: The maximum internal pressure of a gas bubble which is formed in a liquid by means of a capillary is measured.
• Drop volume method: The volume of a drop of liquid produced at a vertical capillary is measured at the moment of its detachment. This method is mainly used for measuring the interfacial tension.
• Pendant drop method: The shape of a drop suspended from a needle is determined from the surface tension and the weight of the drop. The surface tension can be ascertained from the image of the drop using drop shape analysis.