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, ${\displaystyle F}$, required to raise the ring from the liquid's surface is measured and related to the liquid's surface tension, ${\displaystyle \gamma }$:

${\displaystyle F=2\pi \cdot (r_{i}+r_{a})\cdot \gamma }$

where ${\displaystyle r_{i}}$ is the radius of the inner ring of the liquid film pulled and ${\displaystyle r_{a}}$ is the radius of the outer ring of the liquid film.

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