The fall of viscous slugs in vertical capillary tubes is described. Deviati
ons toward Poiseuille law are analyzed by taking into account the dissipati
on in menisci, together with the existence of a film behind the slug. Slugs
are found to fall slower in dry tubes than in prewetted ones, which is qua
ntitatively discussed in term of viscous friction. A criterion for the mini
mal length of the slug obeying the Poiseuille solution is finally derived.
(C) 2001 Academic Press.