The effects of gravity on the capillary instability in tubes
Résumé
We study the capillary instability of a liquid film (thickness $h_0$) coating a horizontal cylindrical tube (radius $R_0$). We show experimentally that the instability only occurs if $h_0/R_0>0.3(R_0/a)^2$, where $a$ is the capillary length. If this criterion is not fulfilled, the liquid film does not destabilize into an array of drops, owing to the gravitational drainage.