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MARANGONI EFFECTIt 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 coverdifferent 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 thephases 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.
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Figure 1. Mechanism of Marangoni flows.
References
1. Berg, J. C. (1972) Interfacial phenomena in fluid phase
separation processes. Recent Developments
in Separation Science , Vol. II. CRC Press. Cleveland. Ohio.
2. Sawistowski, H. (1971) Interfacial phenomena. Recent Advances
in Liquid-Liquid Extraction (Ed. C.
Hanson). Pergamon Press. Oxford.
3. Ortiz, E. S. P. de (1992) Marangoni phenomena. Science and
Practice of Liquid-Liquid Extraction.Vol.
1 (Ed. J. D. Thornton). Clarendon Press. Oxford.4.
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