Operating limit of vacuum-assisted slot die coating of Herschel-Bulkley fluids

Shear thinning and yield stress are two important rheological properties in industrially relevant coating liquids, and the Herschel-Bulkley constitutive equation can represent them reasonably. We used Carbopol polymer solutions as a model Herschel-Bulkley fluid to visualize slot-coating flows. Visualization was performed using a custom-made apparatus to detect the location of contact lines. We also developed a simple model based on the viscocapillary model. The experimental results were shown through the dimensionless coating window. The effect of the yield stress was significant in weeping when the coating was thin. In situations where the coating is thin, the effect of shear thinning is remarkable because of the heavily curved shear rate profile. As a result, the dimensionless coating window of the Herschel-Bulkley fluid bends downward to a greater extent than that of the Newtonian fluid.