Capillary contributions to the dynamics of discrete slugs in microchannels
Abstract
The dynamics of discrete slugs made of perfectly or partially wetting liquids is studied experimentally in microchannels with rectangular cross-sections. Two contributions to the applied pressure drop govern the observed steady motions during which the length and the speed of a slug remain constant. The first contribution is the viscous dissipation in the liquid column between the menisci of the slug and is approximated by the Hagen-Poiseuille's law. The second contribution arises at the vicinity of the menisci and is deduced from the experimental data. Distinct regimes are observed for this contribution depending on the wetting properties of the liquid and on the capillary number Ca. In the dynamic regime, a typical visco-capillary scaling in Ca (2/3) is found for the wetting liquid whereas the contribution of the menisci in partial wetting tends towards the previous scaling as the capillary number increases. A quasi-static regime is observed in partial wetting at low driving pressures and the capillary number increases exponentially. This observation suggests the influence of friction on the motion of the contact line and leads us to discuss its effects in the dynamic regime. In particular, experimental results are compared to an empirical prediction involving the previous mechanism as well as dynamic contact angles.