Detachment of a drop from an internal wall in a pulsed liquid-liquid column

In this paper, we present an experimental and numerical study of the critic al detachment force of a drop wetting a disc of a liquid-liquid extraction pulsed column. The critical pulsation frequency necessary to dislodge the d rop from the wall has been determined with different drop diameters, two di fferent liquid phase systems (one at low and one at high interfacial tensio n). The corresponding hydrodynamic critical force exerted on the drop has b een estimated by means of numerical simulation, using a CFD computational c ode to describe the pulsed flow in the column. Due to the flow nonuniformit y, for a given pulsation frequency, the hydrodynamic force is a function of the drop location upon the disc. Results have been compared to the retenti ve adhesion force of the drops sticking on the disc which has been estimate d on the base of measurements performed in a previous work (Masbernat, Petr issans and Gourdon (1997). Recents progres en Genie des Procedes 11(55), 51 -56) using the inclined plate method. It is shown that when the drop is dis lodged, the calculated hydrodynamic force is equal to the adhesion force an d corresponds to the maximum of the pulsation radial velocity (parallel to the disc) calculated at the location of the drop mass centre. These results show that it is possible to correctly estimate the detachment of drops on walls even in complex flows, provided the drop-wall contact angle hysteresi s is known (and uniform) in the column. It is also shown that the critical detachment force is a necessary condition for the drop to be removed from t he plate but not sufficient. (C) 2000 Elsevier Science Ltd. All rights rese rved.