Blade-coating of yield stress fluids

Abstract

Blade-coating of pastes (yield stress fluids) consists of coating the fluid by pushing and spreading it over a solid substrate. It is a widely used process in civil engineering (mortars, cement pastes, rendering, paints) and in cosmetics (coating of gels or creams). Here we study this process experimentally by displacing horizontally a vertical plate partially immersed in the initially uniform layer of model yield stress fluid (Carbopol gel). We look at the impact of the initial fluid thickness, the scratched layer thickness, the fluid yield stress, and the blade velocity. The analysis is completed by rough PIV measurements. We show that the displacement of the blade at a constant distance from the solid plane induces, behind the blade, the formation of a uniform layer of thickness equal to 1.09 times this distance. In front of the blade the fluid forms a heap of growing volume. It appears that this heap advances over the initial uniform layer which induces a shear in a band between two almost undeformed blocks. It is shown that the shape of this heap results from the momentum balance on the fluid volume submitted to gravity and shear stress (along the shear-band). Nevertheless the force to apply on the blade is larger than the stress along the band times the band surface by a factor larger than 1.5, which might result from normal force due to heap weight and elastic deformation in the upstream static layer.