(74c) Visualization of Mass Transfer from an Electrically Charged Pendant Drop

The intensification of liquid-liquid transport processes using externally applied electric fields is well known. Enhancement of mass transfer, for example in solvent extraction processes may be explained by increases in interfacial area due to improved dispersion and drop break-up. The acceleration of drops through the continuous phase may also lead to higher rates of mass transfer due to improved rates of interfacial shear. A third possibility is that interfacial disturbances are promoted by the electrical field which per se improve mass transfer fluxes. Promotion of interfacial disturbance is especially interesting because a number of novel liquid-liquid systems such as those involving homogeneous catalysts, or involving enzymes which are inter-facially active potentially could be intensified by the application of electrical fields. Therefore this paper is concerned with the fundamental interfacial mechanisms behind the observed increase in mass transfer between electrically charged drops and a second continuous liquid phase. There is debate in the literature as to the importance of the effects of interfacial disturbances which may be induced by electrical fields upon observed rate of mass transfer. Marangoni induced instabilities are an example. Finite element modelling of interfacial mass transfer from an electrically charged pendant liquid drop, can predict significant time-dependent convective disturbances at the liquid-liquid interface which would translate into enhanced mass transfer. Experimental study of this for some real liquid-liquid systems is presented. A novel technique for measuring the rate of mass transfer from a pendant electrically charged droplet is described. The methodology includes the simultaneous visual recording of the hydrodynamic disturbances in the vicinity of the liquid-liquid interface. The changes in refractive index which occur during mass transfer and associated hydrodynamic disturbances at the interface using an established methodology. The following experimental variables are included in the study: The electrical charge on the drop; the magnitude and direction of the external electrical field; viscosity and interfacial tension of the system; the presence of surface active agents.