Linear stability analysis of a vertical liquid film over a moving substrate

Abstract

The stability of liquid film flows are important in many industrial applications. In the dip-coating process, a liquid film is formed over a substrate extracted at a constant speed from a liquid bath. We studied the linear stability of this film considering different thicknesses $\hat{h}$ for four liquids, spanning a large range of Kapitza numbers ($\rm Ka$). By solving the Orr-Sommerfeld eigenvalue problem with the Chebyshev-Tau spectral method, we calculated the neutral curves, investigated the instability mechanism and computed the absolute/convective threshold. The instability mechanism was studied through the analysis of vorticity distribution and the kinetic energy balance of the perturbations. It was found that liquids with low $\rm Ka$ (e.g. corn oil, $\text{Ka}$ = 4) have a smaller area of stability than a liquid at high $\rm Ka$ (e.g. Liquid Zinc, $\rm Ka$ = 11525). Surface tension has both a stabilizing and a destabilizing effect, especially for large $\rm Ka$. For long waves, it curves the vorticity lines near the substrate, reducing the flow under the crests. For short waves, it fosters vorticity production at the interface and creates a region of intense vorticity near the substrate. In addition, we discovered that the surface tension contributes to both the production and dissipation of perturbation's energy depending on the $\rm Ka$ number. In terms of absolute/convective threshold, we found a window of absolute instability in the $\text{Re}-\hat{h}$ space, showing that the Landau-Levich-Derjaguin solution ($\hat{h}=0.945 \text{Re}^{1/9}\text{Ka}^{-1/6}$) is always convectively unstable. Moreover, we show that for $\text{Ka}<17$, the Derjaguin's solution ($\hat{h}=1$) is always convectively unstable.