Dynamical thickening transition in plate coating with concentrated surfactant solutions

TL;DR

This study investigates how surfactants influence film thickness in the Landau-Levich-Derjaguin experiment, revealing a dynamic transition linked to surface rheology and surfactant type, with implications for coating processes.

Contribution

The paper provides experimental insights into the surfactant-dependent dynamic thickening transition in film coating, connecting surface rheology to the observed phenomena.

Findings

Two regimes of constant thickening identified at different capillary numbers.

Dynamic transition occurs at a fixed Ca for non-ionic surfactants, but varies with concentration for ionic surfactants.

Surface rheology controls the nature of the dynamic transition.

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 $\alpha=h/h_{LLD}$, where hLLD is obtained for a pure liquid, in a wide range of capillary ($Ca=\eta V/\gamma$), two regimes of constant thickening can be identified: at small capillary number, $\alpha$ is large due to a confinement and surface elasticity (or Marangoni) effects and at large Ca, $\alpha$ is slightly higher than unity, due to surface viscous effects. At intermediate Ca, $\alpha$ 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.