Marangoni Film Flow Effect

It may not be conunonly known that the tank wall above flie liquid level in a LNG tank is wetted by surface tension driven film flows up flie tank wall. This effect, an example of the so-called Marangoni effect, was first studied at Southampton in 1974 [2]. Have a look to see for yourself Then compare what you see with the tear-drops round the edge of a glass of sherry or port (see Fig. 6.1). 

Due to surface tension differences between methane and ethane, a fihn of ethane-rich liquid is drawn up the wall, losing methane by evaporation, until pure ethane droplets build up at the 185 K (NBP of ethane) temperature level. The ethane droplets run back down the wall, and being more dense than LNG, they collect over a long period of time at the tank bottom as a thin, warm, dense, stratified layer. Above the 185 K level, a film of propane rich liquid is drawn up the warmer levels of the wall and propane droplets appear at the 231 K level (NBP of propane) if there is sufficient concentration in the LNG (Fig. 5.9). 

However, measurements of the mass dynamics of flie surface-tension driven separation process show that the mass flows in flie films are not great enough to be commercially viable for extracting ethane or propane from LNG [3]. 

Another point to watch is that the wetting of walls and cold exit piping, due to flie Marangoni effect, make the monitoring of vapour compositian particularly difficult 

5 Auto-stratification in Both Single Component Liquids and Mixtnres 

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Hu, Y.-C., Zhou, Q., Ye, H.-M., Wang, Y.-F., Cui, L.-S. Peculiar siuface profile of poly (ethylene oxide) film with ring-like nucleation distribution induced by Marangoni flow effect. Colloid Surf. A 428, 39 6 (2013)... 

Generalization of the basic model system (9) by taking into account electrical forces is given (Shkadov and Sisoev, 2000a). Film flows with Marangoni effect are considered in section 6. 

Surfac e tension gradients due to mass or heat transfer along or across an interface (Marangoni effect) either create flows or alter existing ones or even trigger instabilities eventually leading to flow motions, steady or otherwise. For film flow the influence of... 

Marangoni effect. Schematic diagram showing appearance of transparent container wall, with film flows above LNG,... 

The sketch and pertaining explanation are simplified. In practice the dynamics of film thinning also has to be considered. The flow can lead to Marangoni effects emanating from Vy s in the LG Interface. 

When a membrane expands and the concentration of a surfactant at the interface decreases, there exist two mechanisms to restore the surfactant surface concentration. The first mechanism, termed the "Marangoni effect" (16), refers to the fact that the surface flow can drag with it some of the underlying layers, i.e. the surface layer can flow from areas of low surface tension, thus restoring the film thickness. It is also a source of film elasticity or resilience. 

Closely related to the above mechanism is the Gibbs-Marangoni effect [13-17], which is represented schematically in Figure 10.19. The depletion of surfactant in the thin film between approaching drops results in a y-gradient without Hquid flow being involved. This results in an inward flow of liquid that tends to drive the drops apart. 

Film stability. The formation of y-gradients is all that allows stable liquid films to be made. A film of pure water immediately breaks. To be sure, a thin film is never stable in the thermodynamic sense, but its lifetime can be quite long if it contains surfactant. Figure 10.29c illustrates the so-called Gibbs mechanism for film stability. If for some reason a thin spot forms in a film, this implies a local increase in surface area, hence a local decrease in surface load, hence a local increase in surface tension, hence motion of the film surfaces in the direction of the thin spot, hence a Marangoni effect, i.e., flow of liquid toward the thin spot, hence a self-stabilizing mechanism. Actually, a more elaborate treatment of film stability is needed (see Section 13.4.1), but the Gibbs mechanism is essential. 

Thin liquid films in foam and emulsion systems are usually stabilised by soluble surfactants. During the formation of such films the flow-out process of liquid disturbs the surfactant equilibrium state in the bulk and film surfaces. The situation of drainage of a surfactant containing liquid film between two oil droplets is shown in Fig. 3.15. (after Ivanov Dimitrov 1988). Here j" and are the bulk fluxes in the drops and the film, respectively, j and j are the fluxes due to surface diffusion or spreading caused by the Marangoni effect, respectively. 

Figure 5. Gibbs—Marangoni effect in the thin-film drainage process. Surfactant is swept to the Plateau borders by flow in the film and droplet phases, and thereby create surface concentration gradients that engender surface tension gradients.

A different antifoaming mechanism was suggested by Kulkarni et al. (96). They found that surfactants adsorb on the surface of hydrophobic particles during antifoaming, and this adsorption results in deactivation of the particles. On the basis of this observation, they postulated that the adsorption of surfactants onto the hydrophobic particles is so fast that it results in surfactant depletion around the particle in a foam film, and this effect breaks the film. However, no direct proof was presented on this theory. Moreover, depletion of surfactant would cause the film liquid to flow toward the particle because of the increased surface tension (Gibbs— Marangoni effect), and thus cause a stabilizing effect. 

The Marangoni effect A surface-active solute tends to concentrate at the liquid surface. When liquid drains from a film surrounding a bubble, it thins a part of the film. In the thin part, surface eirea increases (Fig. 14.9a). The additional surface is supplied by liquid from the bulk, which is leaner in the surface-active solute. Surface tension at the thinned surface therefore rises. This causes a surface flow from the nonthinned (low-surface-tension) surface to the thinned (high-surface-tension) surface, which counteracts film drainage and restores the film. 

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