Dip-coating of yield stress fluids

International audienceWe review and discuss the characteristics of dip-coating of yield stress fluids on the basis of theoretical considerations, numerical simulations of the flow in the bath and experimental data with different materials. We show that in general, due to the yield stress, viscous dissipations are sufficiently large for capillary effects to be negligible in the process. Dip-coating with yield stress fluids is thus essentially governed by an equilibrium between viscous and gravity effects. In contrast with simple liquids the coated thickness is uniform and remains fixed to the plate. At low velocities it appears to tend to a value significantly smaller than the Derjaguin and Levi prediction [B.V. Derjaguin, S.M. Levi, Film coating theory (The Focal Press, London, 1964)], i.e. critical thickness of stoppage of a free surface flow along a vertical plate. We show that this comes from the fact that in the bath only a relatively small layer of fluid is in its liquid regime along the moving plate while the rest of the material is in a solid regime. From numerical simulations we describe the general trends of this liquid layer and in particular its thickness as a function of the rheological characteristics and plate velocity. We finally propose a model for the dip-coating of yield stress fluid, assuming that the solid volume of fluid finally fixed to the plate results from the mass flux of the liquid layer in the bath minus a mass flux due to some downward flow under gravity in the transition zone. A good agreement between this model and experimental data is found for a yield stress larger than 20 Pa