Du Noüy ring method

The du Noüy ring method is one technique by which the surface tension of a liquid can be measured. The method involves slowly lifting a ring, often made of platinum, from the surface of a liquid. The force, F , required to raise the ring from the liquid's surface is measured and related to the liquid's surface tension, γ :

F = 2 π ⋅ ( r i + r a ) ⋅ γ

where r i is the radius of the inner ring of the liquid film pulled and r a is the radius of the outer ring of the liquid film.

This technique was proposed by the French physicist Pierre Lecomte du Noüy (1883–1947) in a paper published in 1925.