The behaviour of a liquid–liquid interface and drop-interface coalescence under the influence of an electric field

The study of drop-interface coalescence provides an insight for the enhancement of the separation rate of a liquid–liquid dispersion in a settler. This is very important as liquid–liquid separation is usually the bottleneck of any industrial process. Moreover, most of the settlers in the chemical industries are huge, leading to high capital and operating costs. The current understanding of drop-interface coalescence is described in the first part of the paper. Experiments reported in this paper show that the measured electric current through water–sunflower oil and water–n-heptane systems, induced by an applied potential difference, increases linearly until a particular value (i.e. the turning point), beyond which the measured current has been observed to increase very rapidly. For various thicknesses of the water layer and the organic phase layer, the electric current, corresponding to the turning points in the voltage–current characteristic curves, is between 3.7 and 18 nA. Furthermore, it is observed that the turning point of the voltage–current characteristic curve for a liquid–liquid system is caused by the formation of a cone at the interface. Above this turning point, the intensification of the local electric field above the tip of the cone is believed to be responsible for the very rapid increase in the measured electric current. Furthermore, the semi-vertical angle of the induced cones are measured to be very close to the value given by Taylor [Proc. Royal Soc. A 280 (1964) 383]. The measurement of the electric current can be used to monitor and control the behaviour of a liquid–liquid interface, thus providing an optimum condition for instantaneous and single-staged drop-interface coalescence. This, consequently, will produce a higher separation rate of the dispersed drops from the continuous phase, thus reducing the residence time of the liquid–liquid dispersion in the settler. As a result, the size of the settler can be reduced, therefore, lowering the capital and operating costs.