Dynamical thickening transition in plate coating with concentrated surfactant solutions

Abstract

We present a large range of experimental data concerning the influence of surfactants on the well-known Landau-Levich-Derjaguin experiment where a liquid film is generated by pulling a solid plate out of a bath. The thickness h of the film was measured as a function of the pulling velocity V for different kind of surfactant and at various concentrations. Measuring the thickening factor $α=h/h_{LLD}$, where hLLD is obtained for a pure liquid, in a wide range of capillary ($Ca=ηV/γ$), two regimes of constant thickening can be identified: at small capillary number, $α$ is large due to a confinement and surface elasticity (or Marangoni) effects and at large Ca, $α$ is slightly higher than unity, due to surface viscous effects. At intermediate Ca, $α$ decreases as Ca increases along a "dynamic transition". In the case of non-ionic surfactants, the dynamic transition occurs at a fixed Ca, independently of the surfactant concentration, while for ionic surfactants, the dynamic transition depends on the concentration due to the existence of an electrostatic barrier. The control of physico-chemical parameters allowed us to elucidate the nature of the dynamic transition and to relate it to surface rheology.