Blade-coating of yield stress fluids

• Published in: Journal of non-Newtonian fluid mechanics Vol. 237; pp. 16 - 25
• Main Authors: Maillard, M., Mézière, C., Moucheront, P., Courrier, C., Coussot, P.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.11.2016; Elsevier BV; Elsevier
• Subjects: Blade coating; Cement paste; Computational fluid dynamics; Cosmetics; Deformation; Elastic deformation; Fluid mechanics; Gels; Gravitation; Liquid-solid equilibrium; Liquid-solid transition; Mechanics; Mortars (material); Paints; Physics; Protective coatings; Shear stress; Thickness; Yield strength; Yield stress; Yield stress fluid; Blade-coating; Liquid-solid transition; Yield stress fluid; liquid-solid transition; yield stress fluid
• ISSN: 0377-0257; 1873-2631; 1873-2631
• DOI: 10.1016/j.jnnfm.2016.10.002

•Blade-coating of yield stress fluids (or pastes) is a widely used process in industry.•Experiments with a model fluid show that the a growing heap of paste is moved over a static layer.•The heap shape is that of a paste sample submitted to gravity and a horizontal yield stress.•The coated layer is larger by about 10% than the gap size (distance between blade tip and bottom plane).•This is likely due to residual elastic deformation in the fluid around the heap. 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. [Display omitted] Lateral aspect of the blade-coating of a yield stress : an almost rigid heap of growing size is moved by the blade over the rest of the fluid which is only deformed, which leaves a uniform coated thickness larger than the gap size.