Bubbles Marangoni effects

Cellular foam occurs at low vapor velocities in small columns, where the wall provides foam stabilization. It occurs with some systems or tray designs but not with others and is promoted by surface tension effects such as the Marangoni effect (99). Cellular foam is uncommon in industrial columns. The foam that causes problems in industrial installations is mobile foam, where the bubbles are in turbulent motion. Mobile foam is associated with the froth and emulsion regimes. Cellular foam is encountered in bench-scale and pilot-scale columns. If cellular foam occurs in the test unit, caution is required when scaling up the results. 

Electrophoresis of bubbles and drops is a story on its own. As long ago as 1861 Quincke ) observed the electrophoresis of small air bubbles in water. Such a motion is possible only when there is a double layer at the Interface, containing free ions. It is extremely difficult to keep oil-water or air-water Interfaces rigorously free from adsorbed ionic species. When these are present, especially for surfactants, Marangoni effects make the surface virtually inexten-slble then the drops or bubbles behave as solid spheres. Electrophoretic studies... 

The second function of the surfactant is to lower the surface tension of the system, thereby forming finer bubbles. The third function is to prevent the cell wall from becoming thin and unstable during the period of growth. This is called the Marangoni effect (3, 4). 

The second cause of coalescence is the film rupture between bubbles. This can easily be very significant. Physically, the film rupture stems from a depletion of surfactant at the film surface when the surface is stretched. The film stability is commonly ascribed in large measure to the so-called Marangoni effect and Gibbs effect. The Marangoni effect involves the inability of surfactant molecules to diffuse instantaneously to any locally stretched area in the film surface. The resulting lag permits the stretched surface to be momentarily depleted of surfactant. The Gibbs effect involves... 

The presence of surfactant adsorption monolayers decreases the mobility of the droplet (bubble) surfaces. This is due to the Marangoni effect (see Equation 5.282). From a general viewpoint, we may expect that the interfacial mobility will decrease with the increase of surfactant concentration until eventually the interfaces become immobile at high surfactant concentrations (see Section 5.5.2, above) therefore, a pronounced effect of surfactant concentration on the velocity of film drainage should be expected. This effect really exists (see Equation 5.286, below), but in the case of emulsions it is present only when the surfactant is predominantly soluble in the continuous phase. 

A more common source of Marangoni effects in systems of interest to chemical engineers is surfactants, as discussed in Chap. 2. This is particularly pertinent to the motion of gas bubbles (or drops) in water, or in any liquid that has a large surface tension (the surface tension of a pure air-water interface is approximately 70 dyn/cm). Experiments on the motion of gas bubbles in water at low Reynolds numbers show the perplexing result illustrated in Fig. 7-18. For bubbles larger than about 1 mm millimeter in diameter, the translation velocity is approximately equal to the predicted value for a spherical bubble with zero shear stress at the interface, that is,... 

This is in qualitative accord with the experimental observation that the bubble reverts to noslip behavior for small radii below some critical threshold value. This effect of nonuniform surfactant concentrations is one of the most easily observed manifestations of Marangoni effects in the motion of bubbles or drops. 

In practical systems, the motion of bubbles or droplets in surfactant solutions is strongly retarded by gradients of the surface tension. The schematic in Fig. 3.11. clearly demonstrates the retardation effect on rising bubbles or sinking drops. In surfactant solutions gravitation and the Marangoni effect move in the opposite direction. 

If a solid particle crosses the diffusion layer of a bubble or drop it also includes the long range interaction caused by a local disturbance of the adsorption layer. This leads to Marangoni effects and influences the film drainage between particle and bubble or drop. Local desorption of surfactant from one surface and its adsorption on the other also causes interaction. 

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. 

The mass-transfer-induced Marangoni effect This effect stabilizes the film when liquid surface tension increases due to mass transfer (i.e., the less-volatile component has a higher surface tension than the more-volatile component). The liquid film just between two adjacent vapor bubbles is closer to equilibrium than the liquid at some distance from these vapor bubbles, and therefore has a higher surface tension than the bulk of the liquid (Fig. 14.96). The surface-tension gradient sucks liquid into the film between the two bubbles, thus counteracting drainage. 

Ratulowski J., Chang H. C., Marangoni effects of trace impurities on the motion of long gas bubbles in capillaries, J. Fluid Mech., 1990, Vol. 210, p. 303-328. 

Stebe K. S., Bartes-Biesel D., Marangoni effects of adsorption-desorption controlled surfactants on the leading end of an infinitely long bubble in capillary, J. Fluid Mech., 1995, Vol. 286, p. 25-48. 

Lin et al. [16] conducted an experimental study on the bubble formatiOTi oti a line shaped polysilicon micro-resistor, which was immersed in the sub-cooled liquids such as Fluorinert fluids (inert and dielectric fluids from 3M Company), water, and methanol. Three different types of input currents were applied after initial nucleation. First, a bubble grew and departed when the input current was ccmstant or increased. Second, a bubble collapsed when the current was turned off abruptly. Third, the size of the bubble decreased and stayed on the top of the heater when the current was reduced gradually. Some important bubble formation phenomena such as Marangoni effects on a microscale, controllability of the size of the microbubbles, and bubble nucleation hysteresis were reported. 

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