This type of method has been widely used, and its experimental principle is shown in the figure. A platinum ring is placed on the liquid surface, and when it is lifted from the liquid surface, some liquid will be taken up. When the weight of the lifted liquid is equal to the surface tension of the liquid along the ring, the weight of the liquid is the largest, and when it is lifted again, the surface tension is not enough to support the weight of the liquid, the liquid ring is broken, and the ring is separated from the liquid surface. Therefore, the maximum external pulling force F applied before leaving the liquid should be equal to the weight of the ring plus the downward pulling force produced by the surface tension of the liquid. Because the membrane has two inner and outer faces, the circumference is 4Tr, namely
F=Wg+4πrγ
Then the surface tension γ of the liquid is: γ = (F-Wg)/4πr
Usually the tension is measured by a screw scale. During the measurement, the ring should be kept level on the liquid surface, and special care should be taken when leaving the liquid surface to avoid fluctuations in the liquid surface. Because of the thickness of the film, and because some liquid is brought up when the ring is pulled up, the radius of the pulled up liquid film is different from the radius of the ring, so correction must be made during accurate measurement, otherwise the error will be large.
The ring method has simple procedures and is especially suitable for systems where the surface tension is easily changed. The commercially available Dü Nouy Surface Tensiometer is made with the above principle. The disadvantage is that it is not accurate enough and can only be used with liquids that completely wet the ring. When the test object contains surfactants, the adsorption phenomenon tends to occur on the metal ring to change its wettability characteristics. In such cases, polytetrafluoroethylene or polyethylene rings can be used.
