Marangoni convections

The flow processes are described as Marangoni convections and up to now they were determined by several research centers through numeric simulation works [9]. Due to the... 

Eq. (132) states that the interfacial tension has to be balanced by a pressure difference between the two phases. The terms containing derivatives of crin Eqs. (133) and (134) are non-zero only if there are local variations of the interfacial tension, which might be due to differences in concentration or temperature. The flow induced by such an effect is known as Marangoni convection. 

Flow near the interface that is influenced by gradients of interface tension is called Marangoni convection. It may have further modes [27]. Thus, a low Marangoni convection in an interface, which results from small concentration differences, may be increased to a strong flow in the shape of rolling cells by mass transfer. These rolling cells transport liquid out of the... 

Fig. 9.22 Marangoni convection between two drops. If the mass transfer is directed out of the drops, the transfer component is relatively more enriched in the thin region of the layer. This corresponds to a lower interfacial tension for most of the extraction liquid systems dc dx<0 ). Thus, a flow of liquid near the interface out of the region of lower tension toward the ranges of higher tension results is called Marangoni convection. The coalesence of the two droplets is promoted.

Figure 3.3. Various features of diffusion and convection associated with crystal growth in solution (a) in a beaker and (b) around a crystal. The crystal is denoted by the shaded area. Shown are the diffusion boundary layer (db) the bulk diffusion (D) the convection due to thermal or gravity difference (T) Marangoni convection (M) buoyancy-driven convection (B) laminar flow, turbulent flow (F) Berg effect (be) smooth interface (S) rough interface (R) growth unit (g). The attachment and detachment of the solute (solid line) and the solvent (open line) are illustrated in (b).

The surface tension at the surface increases during evaporation, thus giving rise to the onset of Marangoni convection. 

Fig. 4.22. Experimental proof for the onset of Marangoni convection for the mixture iso-propanol-water exposed to ambient air. Left-hand side water below and propanol on top in the blotting paper stable right-hand side propanol below and water on top in the blotting paper unstable.

Formation of various patterns like the honeycomb structure mentioned above can originate from gradients in surface tension or density due to temperature variations caused by solvent evaporation. Both effects can produce convection instability, namely Marangoni [295] and Rayleigh [296] convection. In thin films Marangoni convection predominates. 

In real systems the overall mass transfer coefficient is often much higher than predicted by the published correlations. One reason is the existence of Marangoni convections that originate from concentration differences caused by the mass transfer. 

Fig. 6.4-11 Eruptive Marangoni convections near the interface generated by an interfacial mass transfer...

One of the reasons that make the prediction of the interfacial area so difficult is, again, the existence of Marangoni convectiorrs. Marangoni convections can either hinder or promote the coalescence of neighboring drops as demonstrated in Fig. 6.4-12. 

Marangoni convections are very important to the operation of extractors. On the one hand they can increase the mass transfer coefficients on the other hand they can reduce the interfacial area. Which mechanism has the larger effect cannot be decided generally, it depends on the system and equipment at hand. 

Directing Convection Marangoni Convection/ Evaporation/Self- assembly Coffee rings, polygonal network structures, fingering instabilities, cracks, chevron patterns, etc. Nanometer to micrometer Marangoni convection and stick-slip motion can determine the final patterns observed [132-134]... 

Thompson, I., Duan, F., Ward, C. Absence of Marangoni convection at Marangoni numbers above 27,000 during water evaporation. Phys. Rev. E 80,056308 (2009)... 

The breath figures technique is one of the most widely employed methods for the fabrication of organized porous polymer films [30, 31] and, as fiuther depicted in detail, in this approach the template consists of an ordered array of water droplets that can be removed by simple evaporation. Indeed, the simultaneous evaporation of a volatile solvent and condensation of water vapor in combination with thermocapillary effects and Marangoni convection allow the formation and precise organization of water droplets at the polymer solution-air interface [30]. This array of water droplets will evaporate upon complete evaporation of the solvent of the polymeric solution, and the surface will reflect its presence in the form of pores. 

For a fixed interfacial area, mass transfer rates may be increased if Marangoni convection is present, as has been well acknowledged for many years (see e.g. Sawistowski, 1971, Berg, 1972 and Perez de Ortiz, 1991). The interaction between gravitational and Marangoni effects in ternary systems has also been found to be beneficial to the mass transfer process (e.g. Berg Morig, 1969). 

With that in mind the author and co-workers have been carrying out experiments with many hquid-liquid systems and in this paper the results obtained in recent years are summarised. Sections 2 and 3 include theory and the definition of interfacial convection, which give the background for the analysis of the experimental results shown later. Section 4 describes the experimental equipment and methods used to obtain the results presented in Section 5, for hquid-hquid partially miscible binary systems with and without surfactants Section 6, for the same systems under microgravity conditions and Section 7, for ternary systems with and without surfactants. Section 8 covers work previously done on stability criteria to predict Marangoni convection in hquid-hquid systems. In Section 9 some of the results presented in the previous sections are discussed. Relevant results obtained in the field of Biotechnology by the author and co-workers are mentioned in Section 10. Conclusions and future work follow in Section 11. 

Lewis Pratt, in 1953, were the first to report that the observed Marangoni convection in their experimental ternary systems was beneficial to hquid-hquid extraction processes because it increased mass transfer rates. The effect of density gradients on interfacial convection was studied by several researchers including Berg Morig (1969), who investigated the interaction between buoyancy and interfacial tension driven effects in ternary systems. The combined interfacial convection was also seen to be beneficial to mass transfer processes. 

If, however, SDS or DTAB is added to the aqueous phase Marangoni convection is seen to have been induced in the case the aniline/water interface, as can be seen in Figure 2, cases lb,c. In the other two systems (cases 2b,c and 3b,c), the addition of SDS or DTAB produces an increase in the convection intensity in comparison to the one observed when the water was clean . On the other hand, the addition of ATLAS G1300 has either no effect (case Id) on the interfacial stabihty or damps (cases 2d and 3d) any convection present in the clean system. 

Therefore, this effect caused by the presence of surfactants has strong implications on mass transfer processes selected surfactants may be added to a particular system to induce Marangoni convection in order that mass transfer is enhanced... 

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