Visualization and flow simulation of a two-layer slot coater

Previous studies of two-layer slot coater have assumed that diffusional and convective mixing of the two miscible liquid layers can be ignored along t he interfacial region. In this work, flow visualizations were used to show that diffusional mixing does occur between layers of miscible liquids, as w ell as convective mixing under certain operating conditions. A series of th ree dies were investigated which differed in the shape of the separating pl ate between the two inlet flows. Using square center die blocks, it was fou nd that the presence of the vortices within the coating bead and the positi on of the separation line depended on the ratio of top layer viscosity to b ottom layer viscosity. A knife-shaped center block caused several transient s in the flow within the gap. A rounded or bullet-shaped center block resul ted in the elimination of the vortex that caused convective mixing of the t wo liquid layers for a viscosity ratio of 10:1. Experiments showed good agr eement with a two-dimensional Newtonian model that calculated variations in liquid properties across the interfacial region by defining viscosity as a function of a tracer species concentration, where the isoconcentration con tours were then used to define the interfacial region. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.