Falling slugs

The fall of viscous slugs in vertical capillary tubes is described. Deviations toward Poiseuille law are analyzed by taking into account the dissipation 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 quantitatively discussed in term of viscous friction. A criterion for the minimal length of the slug obeying the Poiseuille solution is finally derived. (C) 2001 Academic Press.