Absolute and convective instabilities in a liquid film over a substrate moving against gravity

Fabio Pino, Miguel Alfonso Mendez, Benoit Scheid

Published 2023-12-22Version 1

The drag-out problem for small Reynolds numbers (Re) admits the Landau-Levich-Derjaguin (LLD) solution for small capillary numbers (Ca), and Derjaguin's solution for large Ca. We investigate whether these solutions are absolutely or convectively unstable, solving the Orr-Sommerfeld eigenvalue problem. For Derjaguin's solution, we show that the instability is unconditionally convective for Kapitza number (Ka) smaller than 17 and becomes absolute for Ka>0.15 and Re^(1.7) for Re>10. For water (Ka=3400), the LLD solution is always convectively unstable. The absolute instability is observed only when the dip-coated film is additionally fed from above.