Let σ be the surface tension ...... I got to know that the excess pressure for a liquid-gas interface with radii of curvature (see http://en.wikipedia.org/wiki/Surface_tension#Surface_curvature_and_pressure .....the part on Surface curvature and pressure and Young-Laplace equation). is given by ΔP= σ(1/R1 + 1/R2) I have 3 cases as shown in my attached figure... in there cases, how do we know which radii/lengths to plug into the above formula? how do we/on what basis do we make the selection..? I'm guessing that we just look at the liquid-gas interface and see what radii are on the opposite sides....but that doesn't work for the rectangular plate.... also, especially in regard to the liquid droplet, if we take the inner radius to be one radius, on the other side of the boundary of the droplet, we have an infinite radius!! Please help!!