Why the Du Noüy ring method does not depend on the material the ring is made of? Du Noüy ring method is a way to measure surface tension.
Why the measured force,
$$F= 2\pi \cdot (r_i + r_a) \cdot \gamma$$
does not depend on the material the ring itself is made of? Wouldn't a hydrophobic material and a hydrophilic material result in different measured forces?
In other words, if I try to measure the surface tension of water using a polished metal ring and then cover that same metal ring with a hydrophobic layer, will I end up getting the same measurement?
(source)
 A: It seems to me it might depend on the material the ring made of.  After a bit of poking around Amazon book previews I managed to find page 19 of "Physics and Chemistry of Interfaces" by Butt, Hans-Jürgen; Graf, Karlheinz; Kappl, Michael (2003). This is the source cited by the Wikipedia article you got the equation from.
The next sentence, immediately following the equation reads:

A necessary condition is that the ring surface must be completely wetted.  A platinum wire is often used; it can be annealed for cleaning before the measurement.

http://www.amazon.com/Physics-Chemistry-Interfaces-Hans-J-uuml/dp/3527412166
A: You can think of the force as arising due to the energy associated with the air-fluid interface. The force is a change in energy per unit distance. The surface contact of the fluid with the ring does have an energy associated with it, but this energy is constant as the ring is lifted out of the water, and thus does not contribute to the surface tension.
This assumption of constant energy between the ring and fluid wouldn't be valid for a hydrophobic surface (no fluid would stick to the ring as it's lifted out), but it doesn't matter how hydrophilic the ring is, as long as the fluid adheres to it well.
