ELECTRIC FIELD AND CAPILLARY EFFECTS ON DROP CIRCULATION AND MASS TRANSFER

It has been suggested that the use of electric fields in conjunction with ordinary schemes for liquid extraction or gas absorption by drops or bubbles can produce significant enhancements in mass transfer, and that these are traceable to electrohydrodynamic flows caused by the interaction of the electric field with the charge distribution induced at the drop or bubble interface.;The above analysis is extended to the case of intermediate Reynolds numbers in view of the fact that Reynolds numbers in the range 10-100 are more typical of extraction units. Approximate solutions for fluid motion are obtained and continuous-phase mass transfer rates are evaluated for various values of Reynolds number and viscosity ratio. It is shown that the threshold value of the parameter W still holds.;Stream functions, terminal velocity and interfacial velocities are then computed for spherical drops or bubbles supporting interfacial tension gradients and translating in creeping flow in an imposed electric field. In particular, the case in which a trace of surfactant is present is considered, and it is found that such material may produce quasi-stagnant zones along the interface either near the poles or the equator. Terminal velocity is also affected by both the surface tension gradient and the electric field. Mass transfer rates in such surfactant-containing systems under various electric field intensities are also assessed. It is then found that the threshold of W at unity still prevails. Experiments are performed to substantiate the most important characteristics of the flow structure, viz. the stagnant zones in an electrified, surfactant-containing droplet system.;The transient and quasi-steady heat and mass exchange between a translating drop and its surroundings in a uniform electric field is investigated for the case of creeping flow. The rate of transport to or from such a droplet in a surfactant-free system is shown to be enhanced by the electrohydrodynamic effect only when the absolute value of a parameter W, which characterizes the relative importance of electrical and gravitational effects, is greater than unity.