Du Noüy ring method
In surface science, the du Noüy ring method is a technique for measuring the surface tension of a liquid. This technique was proposed by Pierre Lecomte du Noüy in 1925. 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.
Description
The method involves slowly lifting a ring, often made of platinum, from the surface of a liquid. The force,, required to raise the ring from the liquid's surface is measured and related to the liquid's surface tension :where is the radius of the inner ring of the liquid film pulled, and is the radius of the outer ring of the liquid film. is the weight of the ring minus the buoyant force due to the part of the ring below the liquid surface.
When the ring's thickness is much smaller than its diameter, this equation can be simplified to
where is the average of the inner and outer radius of the ring, i.e.
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 being the correction factor.
Correction factors
The most common correction factors include Zuidema–Waters correction factors, Huh–Mason correction factors, and Harkins–Jordan correction factors.The surface tension and correction factors are expressed by
where is surface tension, is the average radius of the ring, and is correction factor.
Zuidema–Waters correction factors
H. H. Zuidema and George W. Waters introduced the following correction factor in 1961:where