Flow Imaging of the Fluid Mechanics of Multilayer Slide Coating. Flow visualisation of layers formation in a 3-layers slide coating die, measurement of their thicknesses and interfacial and free surface flow instabilities

Abstract

Coating onto a moving substrate several films simultaneously on top of each other is a challenging exercise. This is due to the fact, depending on operating conditions (thickness and velocity of individual layers and the physical properties of the coating fluids), flow instabilities may arise at the interfaces between the layers and on the top layer. These instabilities ruin the application of the final multi-layered coating and must be avoided. This research addresses this coating flow situation and seeks to develop guidelines to avoid these instabilities. Following a critical literature survey, this thesis presents a novel experimental method that visualises multi-layered coating flow down an inclined multi-slot die. The visualisation is obtained using a unique configuration including a high-speed camera, telecentric objective lens and illumination. The results show for a single layer, as the die angle and Reynolds number increases, the flow becomes more unstable and for a dual layer flow, as Re increases the peak to peak amplitude and the frequency decreases at the free surface and interface. The latter was unexpected and does not conform with existing literature. The triple layer results show either a monotonically increasing or increasing from first to second layer viscosity stratifications are the most stable flows along with flow heights in the first and second layers of <22% and >18% of the total thickness respectively, which concur with current literature. The visualisation additionally obtained other instabilities including single layer back-wetting and vortices, and multilayer slot invasion with the findings concurring with current literature.