It has been observed that the process of mass transfer across the interface of certain systems produces spontaneous interfacial convection. When these interfacial flows are driven by local changes in interfacial tension the phenomenon is called the Marangoni effect. Marangoni phenomena cover different degrees of interfacial turbulence, from microscale structured convection that requires special optical systems to be detected (roll cells) to violent eruptions that can be observed without any optical help. This interfacial convection has multiple effects in liquid-liquid mass transfer processes, the most relevant of which are: (1) increase in mass transfer coefficients over predicted values; (2) interfacial deformations that alter the interfacial area; (3) changes in the coalescence rate due to either an increase or a decrease in the rate of drainage of film trapped between colliding drops; and (4) changes in conditions for drop breakage due to their effect on the stability of the ligaments present in deformed drops. The effects on drop coalescence and breakage affect drop size distribution in liquid-liquid dispersions and therefore the performance of liquid-liquid contactors. Substantial research has been devoted to the study of the conditions that lead to the Marangoni phenomena and to the quantification of their effects on mass transfer rate and dispersion characteristics.
The mechanism of Marangoni convection can be explained using Figure 1. Figure 1a shows a schematic representation of the transfer of a solute S from phase A into phase B at steady state. The transfer may be accompanied by heat effects, in which case, temperature profiles are also present. While interfacial tension over the whole interface is uniform, the interface is quiescent. However, mechanical disturbances in the system may bring an element of fluid from the bulk of one of the phases to the interface, producing a local change in concentration and temperature (Figure 1b). The resulting local change in interfacial tension will generate radially spreading flow from points of low interfacial tension. Depending on the physical properties of the system and the magnitude of the disturbance these movements may either die out or be sustained. In the first case the system will return to its initial state, while in the other it will become unstable. In some simple cases conditions required for the system to become interfacially convective can be predicted qualitatively. However, it is from mathematical analysis that the stability criteria have been established for different types of mass transfer mechanisms and interfacial geometry. The references listed below contain extensive and comprehensive reviews of the subject and its effect on contacting equipment performance.
Berg, J. C. (1972) Interfacial phenomena in fluid phase separation processes. Recent Developments in Separation Science, Vol. II. CRC Press. Cleveland. Ohio.
Sawistowski, H. (1971) Interfacial phenomena. Recent Advances in Liquid-Liquid Extraction (Ed. C. Hanson). Pergamon Press. Oxford.
Ortiz, E. S. P. de (1992) Marangoni phenomena. Science and Practice of Liquid-Liquid Extraction. Vol. 1 (Ed. J. D. Thornton). Clarendon Press. Oxford.