A rigid sphere of radius \(a\) rests on a flat rigid surface, and a small amount
of liquid surrounds the point of contact making a concave-planar lens whose
diameter is small compared with \(a\). The angle of contact of the liquid with
each of the solid surfaces is zero, and the tension in the air-liquid surface
is \(\gamma\). Show that there is an adhesive force of magnitude \(4 \pi a
\gamma\) acting on the sphere. (The fact that this adhesive force is
independent of the volume of liquid is noteworthy.)
