Three-dimensional visualization of surface waves and flow instability of liquid film flow on a spinning disk

Liquid film flow over a spinning disk is a common phenomenon in the industry, such as cleaning wafers. Despite its generality, previous studies have only revealed a simple shape of surface waves without exact information. Perhaps, the scientific complexity of this flow regarding surface tension, centrifugal, and Coriolis forces are due to the fact that there has been a general lack of direct experimental visualization. In this study, we provide a direct high spatiotemporal resolution visualization of thin film flow on the spinning disk using a fluorescent imaging method. With a good agreement of surface waves from circular to turbulent patterns along with inlet flow rates and spin speeds, we identify the existence of capillary ripples and wave amplitude (i.e., film thickness) that have not been reported in previous studies. In detail, if an inertia effect becomes dominant as flow rate and/or spin speed increases, we observed the shape of the main hump becomes unsymmetric. In this case, capillary ripples were found in front of it, which show a small amplitude and multiple crests. On the other hand, if the capillary effect becomes dominant, the main hump retains a symmetric shape, and a capillary ripple was not shown.