Operability limits for non-Newtonian liquids in dual-layer slot coating processes using the viscocapillary model

The operability windows and coating bead dynamics for the dual-layer slot coating process with non-Newtonian coating liquids were investigated using the viscocapillary model, a two-dimensional (2D) CFD simulation, and experiments. The simplified viscocapillary model derived from the 2D equation of motion with the shear-thinning Carreau liquids bounds the uniform coating regime without defects such as leaking, bead break-up, and mid-gap invasion in dual-layer coating systems. The coating bead flows from experiments and finite-volume-based 2D simulations with the volume-of-fluid scheme demonstrate the performance of the viscocapillary model in reliably tracking the upstream meniscus position of the bottom layer in the coating bead region. The onset of leaking, bead break-up, and mid-gap invasion defects is strongly affected by the rheological properties of Carreau-type coating liquids and the flow ratio between the top and bottom layers. In particular, the relative thickness of the shear-thinning bottom layer at the onset of mid-gap invasion is smaller than the well-known 1/3 value in the Newtonian case, depending on the degree of shear thinning.