Stability of liquid film coating a horizontal cylinder: interplay of capillary and gravity forces

Abstract

We study the drainage of a viscous liquid film coating outside a horizontal cylinder. We first study the evolution of the axially invariant draining flow, initiated at rest with uniform film thickness $δ$. Non-linear simulations indicate that for each $δ$, there is a threshold in the Bond number ($Bo$), which compares the gravitational effects with surface tension, above which the draining liquid bulk ruptures. This critical $Bo$ is found to scale inversely with $δ$, defining the existence of a quasi-stationary pendant curtain sustained below the cylinder by surface tension. The interface of the pendant curtain is unconditionally linearly unstable and is prone to Rayleigh-Plateau-like, capillarity-driven, and Rayleigh-Taylor, gravity-driven, instabilities. The linear stability of the quasi-static state along with an energy analysis of the unstable mode illustrates that while the Rayleigh-Taylor instability is always present, capillary effects dominate the instability at small $Bo$, which promotes the formation of pearls enveloping the cylinder. In contrast, at large $Bo$, capillarity acts in a stabilising way and the instability is purely gravity-driven, forming underside modulations. We present the asymptotic energy repartition representing the different physical mechanisms at play in the instability of the saturated curtains for a wide range of $\{Bo,δ\}$. The results of the linear analysis agree with the pre-existing experiments of~\citeauthor{Bruyn1997} and non-linear simulations of~\citeauthor{Weidner1997} on thin film and extend the results for thick films. Additionally, based on the volume made available for droplet growth by the development of the most linearly amplified wavelength, we build a tentative regime diagram predicting the final patterns emerging from the pendant curtain: an array of pearls or pendant drops or three-dimensional droplet pinch-off.