When a wettable solid substrate (flint glass) is brought into contact with a vessel of water on the surface of which an appropriate surfactant has been spread (valinomycin), a visible film is observed to creep up to the substrate. If the substrate is now bent so that its far end makes contact with a second water vessel (at the same height) on which the same surfactant is spread, a net flow of surfactant is observed to occur under the influence of a surface tension differential between the two vessels. From elementary hydrodynamics the thickness of the film of water accompanying the surfactant flow can be deduced from measurements of the surface tension differential and the rate of transfer of the surfactant. We have developed a theory for the thickness of this entrained film, which turns out is governed by the van der Waals dispersion force. Our treatment allows us to deduce the film thickness from first principles and, by comparison with the experimental results, to determine the retarded Hamaker constant B.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics