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 weep-ing 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 win-dow of the Herschel-Bulkley fluid bends downward to a greater extent than that of the Newtonian fluid.(c) 2023 Elsevier Ltd. All rights reserved.