Marangoni effect, solute

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. 

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

In the 1990s, this problem of surface structuration was revisited in the light of recent theoretical hndings obtained on nonlinear surface waves. It could be established that the waves sustained by a Marangoni effect, as observed by Linde and Schwarz and Orell and Westwater, are relevant to a nonhnear theory. They have solitonic properties and the patterns that structure the surface are produced by their colhsion. The description and analysis of these nonlinear waves sustained by a solutal Marangoni effect are the subject of this chapter. 

In the opposite case, when the surfactant is soluble in the continuous phase, the Marangoni effect becomes operative and the rate of film thinning becomes dependent on the surface (Gibbs) elasticity (see Equation 5.282). Moreover, the convection-driven local depletion of the surfactant monolayers in the central area of the film surfaces gives rise to fluxes of bulk and surface diffusion of surfactant molecules. The exact solution of the gives the following... 

The Marangoni effect is signihcant only in dilute solution and within a limited concentration range. The amount of solute adsorbed at a new surface in the absence of stirring or an energy barrier to adsorption is given by (Ward, 1946)... 

Solution concentration gradient Diffusiophoresis Particle growth and dissolution Marangoni effect ... 

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. 

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. 

There are several possible mechanisms to explain the enhancement of absorption during surface renewal. The Marangoni Effect results from the fact that dilute solutions of water C10 %) exhibit abnormalities in regard to surface tension. Jones and Ray [8] have observed that absorption of ions at the surface continues until a specific number of sites are occupied. The concentration of these sites is about 5 per 10 surface molecules. If liquid vapor is continually condensed on the drop surface, new surface for sites is being formed at a rate fixed by condensation. A second mechanism for enhancement, Stefan flow, is a trapping of the gas molecules into the liquid phase by the condensing vapor flux. The Stefan flow flux can be expressed as [9] ... 

The effect on surface tension by surfactant adsorption from the bulk solution (Gibbs effect) and by diffusion along an interface (Marangoni effect) is often referred to as the combined Gibbs-Marangoni effect (Figure 11.7). 

In section 2 we consider a spreading of insoluble surfactants over thin aqueous layer. When a drop of a surfactant solution is deposited on a clean liquid-air interface, then tangential stresses develop on the liquid surface. They are caused by the non-uniform distribution of the surfactant concentration, /", over the liquid surface covered by the surfactant, hence, leading to the creation of a surface stresses and a flow (Marangoni effect) (Levich, 1962) ... 

The Marangoni effect refers to the variation of surface tension of a liquid with temperature (thermocapUlarity) or with the concentration of a surfactant (solutal Marangoni effect). The variation of surface tension in turn leads to convective motion of the fluid Marangoni convection. This motion along the surface of the liquid layer then leads to flow in the bulk and may be used to transport fluids in microfluidic devices. 

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