Soap films are quite familiar objects that can be produced by simply pulling a frame out of a soapy water vessel. Carefully looking at this film reveals a horizontal succession of colored fringes. Those interferences fringes attest to both the existence of a nonuniform thickness profile in the soap film. Mechanically speaking, this implies that the surface tension in the film is non constant in order to counterbalance the weight of the film. This assertion was first proposed by Gibbs in 1878  and pursued by Mysels  and co-workers. However, no experimental evidence for this surface tension gradient has been provided so far.
In a recent paper , we have experimentally investigated the local surface tension in soap films by use of soft deformable objects, giving an experimental proof of early Gibbs’idea.
 J. W. Gibbs, The Collected Works of J. Willard Gibbs (Longmans Green, New York, 1931).
 K. Mysels, K. Shinoda, and S. Frankel, Soap Films (Pergamon, New York, 1959).
 N. Adami and H. Caps, Surface tension profiles in vertical soap films, Phys. Rev. E 91, 013007 (2015) – PDF