The Marangoni Effect

Often a typical shape of wine on the rim of a wine glass is observed, called tears of wine [51,52], To understand this phenomenon we take wine as a mixture of water and ethanol. A wine drop is thicker at the bottom than at the top because of its weight (Fig. 3.8). Part of the liquid evaporates. Due to its lower vapor pressure ethanol evaporates faster than water. The rate of evaporation is roughly proportional to the surface area. Since the drop is thinner at the 

When the solute is enriched at the interface, the surface tension decreases upon addition of a solute. When a solute avoids the interface, the surface tension increases when adding the substance. 

3 Carlo Marangoni, 1840-1925. Italian physicist, professor at a Lyceum in Pavia. 

Calculate the surface entropy and the inner surface energy at 25°C. 

Soap bubbles To stabilize a bubble, surfactants are usually added to water. Assume we add a surfactant to a concentration of 2 mM. At this concentration we have a positive surface excess. As an average, each surfactant molecule occupies a surface area of 0.7 nm2. Estimate the change in pressure inside a soap bubble with a radius of 1 cm compared to a hypothetical bubble formed from pure water. 

Surface tension depends on both the temperature and the concentration in the case of solutions. A temperature (or concentration) gradient leads to a surface tension gradient regions of high 7 (7+) pull on regions of low 7 (7-), and the liquid is set in motion. This effect has a great many consequences, some of which we shall now illustrate. 

It also explains why little Marangoni-driven boats can be made to move, by rubbing one end of a matchstick in soap and placing it on the surface of some clean water (without surfactants). The matchstick travels in the direction of its soapless end. 

Droplets Which Seek the Cold. If we place a drop on a plate which is hot at one end and cold at the other, it will rush towards the cold end, provided that it does not get pinned onto a surface defect. The part of the droplet exposed to the cold has a greater surface tension, and this induces a flow within it that shifts its centre of mass towards the colder region. 

Droplets Which Make Fingers (Fig. 1.34). Marangoni instabilities have also been observed in the spreading of a droplet of polymer solution, in the antagonist case where the solvent wets the substrate (S 0) but the polymer does not S 0) (Fig. 1.34. Instead of observing a wetting transition at a volume fraction 0w such as S 4 ) = 0, we observe a leak out transition at a composition t w- The solvent is frustrated, because it loves both the 

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The Marangoni effect has been observed on the rapid compression of a monolayer [54] and on application of an electric held, as in Ref. [55] it occurs on evaporation [56]. 

Tills phenomenon, frequently referred to as the Marangoni Effect, explains some of the anomalously high mass transfer rates reported in the literature. 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

All of these disturbances cause a many-fold increase in the rate of transfer of solute across the interface. If a chemical or thermal difference along an interface causes an interfacial tension gradient, violent flow in the direction of low a will result. This action is usually termed the Marangoni effect. 

U. Influence of the Marangoni Effect on the Mass Transfer BETWEEN Two PHASES... 

In many of these experiments, interfacial turbulence was the obvious visible cause of the unusual features of the rate of mass transfer. There are, however, experimental results in which no interfacial activity was observed. Brian et al. [108] have drawn attention to the severe disagreement existing between the penetration theory and data for the absorption of carbon dioxide in monoethanolamine. They have performed experiments on the absorption of C02 with simultaneous desorption of propylene in a short, wetted wall column. The desorption of propylene without absorption of C02 agrees closely with the predictions of the penetration theory. If, however, both processes take place simultaneously, the rate of desorption is greatly increased. This enhancement must be linked to a hydrodynamic effect induced by the absorption of C02 and the only one which can occur appears to be the interfacial turbulence caused by the Marangoni effect. No interfacial activity was observed because of the small scale and small intensity of the induced turbulence. 

The Marangoni effect generates a stress along the interface. Therefore the equilibrium of forces in the x direction leads to... 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

While interfacial contaminants tend to reduce the mass transfer coefficients by causing the droplets to be stagnant rather than circulating, another surface effect may enhance mass transfer. This is the Marangoni effect, whereby local variations in interfacial tension due to the mass transfer process itself can create rapid motions (interfacial turbulence) at the interface. 

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. 

Surface tension effects have frequently been used to explain observed composition effects (164,187,189) or discrepancies between theory and experiment (146). Zuiderweg (146) and Dribika and Biddulph (169) argue that the Marangoni effect (Sec. 6.4.4) stabilizes the froth and therefore enhances efficiency. The enhancement is related to... 

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