In
surface science, the du Noüy ring method is a technique for measuring the
surface tension of a
liquid. 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:
where ri is the radius of the inner ring of the liquid film pulled, and ra is the radius of the outer ring of the liquid film.[1]wring is the weight of the ring minus the buoyant force due to the part of the ring below the liquid surface.[2]
When the ring's thickness is much smaller than its diameter, this equation can be simplified to
where R is the average of the inner and outer radius of the ring, i.e. [2][3]
This technique was proposed by the French physicist
Pierre Lecomte du Noüy (1883–1947) in a paper published in 1925.[4]
Close up of the ring drawn out of the liquid
The measurement is performed with a force
tensiometer, which typically uses an
electrobalance to measure the excess force caused by the liquid being pulled up and automatically calculates and displays the surface tension corresponding to the force. Earlier, torsion wire balances were commonly used.
The maximum force is used for the calculations, and empirically determined correction factors are required to remove the effect caused by the finite diameter of the ring:
with f being the correction factor.
The most common correction factors include those by Zuidema and Waters (for liquids with low interfacial tension), Huh and Mason (which covers a wider range than Zuidema–Waters), and Harkins and Jordan (more precise than Huh–Mason, while still covering the most widely used liquids).[5]
The surface tension and correction factors are expressed by
where γ is surface tension, R is the average diameter of the ring, and f is correction factor.