Plate Coating: Influenceof Concentrated Surfactantson the Film Thickness.

We present a large range of experimental data concerningthe influenceof surfactants on the well-known Landau-Levich–Derjaguin experimentwhere a liquid film is generated by pulling a plate out of a bath.The thickness hof the film was measured as a functionof the pulling velocity Vfor different kinds ofsurfactants (C12E6, which is a nonionic surfactant,and DeTAB and DTAB, which are ionic) and at various concentrationsnear and above the critical micellar concentration (cmc). We reportthe thickening factor α = h/hLLD, where hLLDis the filmthickness obtained without a surfactant effect, i.e., as for a purefluid but with the same viscosity and surface tension as the surfactantsolution, over a wide range of capillary numbers (Ca = ηV/γ, with η being the surfactant solution viscosityand γ its surface tension) and identify three regimes: (i) atsmall Ca α is large due to confinement and surface elasticity(or Marangoni) effects, (ii) for increasing Ca there is an intermediateregime where α decreases as Ca increases, and (iii) at larger(but still small) Ca α is slightly higher than unity due tosurface viscosity effects. In the case of nonionic surfactants, thesecond regime begins at a fixed Ca, independent of the surfactantconcentration, while for ionic surfactants the transition dependson the concentration, which we suggest is probably due to the existenceof an electrostatic barrier to surface adsorption. Control of thephysical chemistry at the interface allowed us to elucidate the natureof the three regimes in terms of surface rheological properties. [ABSTRACT FROM AUTHOR]