Flow of a surfactant across a thin liquid film wetting a solid substrate

Transfer of a monolayer (valinomycin) between two Langmuir troughs via an interconnecting glass ``bridge'' has been observed when there is a difference between the monolayer pressures in the two troughs. Assuming that the transfer occurs over the surface of the thin film of water that wets the ``bridge,'' the transfer rate can be used to estimate the thickness of the liquid film. The thickness thus determined may be used to determine the retarded van der Waals (Hamaker) constant associated with the interaction of the film with the underlying substrate.