Flow characteristics around a plate withdrawn from a bath of yield stress fluid

Dip-coating is a process widely used in industry to coat a fluid on a solid substrate. The general characteristics of dip-coating for simple liquids (Newtonian) are well known but a lot of applications involve complex fluids. Here we focus on the case of a solid plate coated with a yield stress fluid through the immersion followed by the withdrawal of a plate from a bath of such fluid. We carried out a precise analysis of the force applied to the plate during its withdrawal from the bath, and determined the velocity field developed around the plate in the fluid bath with the PIV technique. It appears that, inside the bath, after a transient regime during the plate withdrawal, a linear force regime is set up, which is associated with a stationary uniform flow (liquid region) settled along the plate while the rest of the material is in its solid regime. We show that the thickness of this liquid region increases slowly with the plate velocity and its value is almost the same for immersion and withdrawal, and independent of a possible period of rest preceding plate motion. Finally we show that the thickness of the liquid region is related to the force in the linear regime. (C) 2014 Elsevier B.V. All rights reserved.