Soap films diffusion

Curvature relates to the local change in interface area when an interface moves. The energy change per unit volume swept out by the interface is equal to the product of k and the interfacial energy per unit area 7. Normally, for fluids, 7 is independent of the interface inclination h in this case, the interface is isotropic. For example, a soap bubble has isotropic interface tension. If perturbed, a floating individual soap bubble will quickly re-establish its equilibrium form—a sphere of fixed volume. Such a soap bubble will also shrink slowly—the gas will diffuse out of the bubble because of a pressure difference across the soap film (AP = jk = /Rc). Thus,... 

The diffusion of hydrogen through balloon fabrics and soap films has been studied by McLennan and Shaver, Phil. Mag., 1920, [6], 40, 272. 

More Indirectly, Information on the charge- or potential distribution is also obtainable from studies involving two double layers in interaction, as in colloid stability, soap films and wetting films. Overlap Is essentially determined by the diffuse parts, the Stem layers defining the boundary conditions. Most of this will be deferred to Volume IV. 

The modified Wicke - Kallenbach cell developed in our laboratory [26,27], was used for measurement of isobaric counter-current ternary diffusion. Figure 1 shows schematically the diffusion set-up including the modified Wicke-Kallenbach cell.. Gl-4 are gas sources FMC are flow-meter controllers D is the diffusion cell 01-2 are gas outlets VI-3 are valves B is a calibrated glass burette with soap film. The diffusion cell contains a metallic disc with cylindrical holes into which the porous pellets are mounted. Volumes of cell compartments are approximately 150 cm. ... 

Measurement procedure A mixture of gases 1 and 2 flows through the bottom cell and another gas flows through the upper cell compartment (flow-rates of gases in both cells are 150 cm /min). Valves VI and V3 are closed and valve V2 opened at the same time. Movement of the soap film in the burette follows the net diffusion flux. The net volumetric diffusion flux, V, gradually decreased with the increase of the gas 3 concentration in the bottom cell compartment. Net volumetric diffusion flux is determined from the slope at zero time of the V(t) dependence. 

The relative stability of soap films is partially due to their elasticity. To see this, we consider a film with an equilibrium state represented by the surface excess T and the dissolved surfactant concentration c. A local stretching disturbs these parameters. If the disturbance occurs on a short time scale, the soap molecules do not have time to diffuse out from the inner fluid to the surface, so the same number of soap molecules remains both inside and at the surface. As the area increases, Tg decreases, which results in an increase of the surface tension according to Eq. (2.11), thus opposing the stretching. This process is called the Marangoni elasticity and explains the film stability to rapid disturbances. If the time scale of the stretching is... 

As explained by Pugh (1996), when a soap film is stretched, due to an external disturbance, a part of the film will expose a pure water interface before surfactant molecules have time to diffuse from bulk to the surface. The water region will have a much higher... 

After a while the flux of water leaving the smaller droplet equals the flux entering the larger one. Using Pick s law at the interface of each droplet, as Princen (1965) and Kralchevsky, Danov, and Denkov (2002) did for the diffusion of a gas in a soap film, we can express the relation between the gradient of concentration at the surface of each droplet and the radii ... 

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. 

The decrease in the size of soap bubbles caused by air diffusion has been observed by some researchers [480]. Manegold et. al. [480] have determined gas permeability of bubble films with a diameter of 5 cm produced from a 2% Nekal solution and filled with hydrogen and carbon dioxide. These experiments can hardly be used to draw any definite conclusions on the effect of adsorption layer on the rate of diffusion transfer. 

Diffusion of the products of reaction away from the surface is slow enough to be important if there is attraction due either to electrostatic or to adsorption forces. The first observation of this effect seems to be that of Alexander and Rideal (30), who found that in the alkaline hydrolysis of trilaurin the soap produced was liable to remain in the film. This complicated the kinetics of the reaction to such an extent that it was found necessary to work under conditions such that the laurate ions were more rapidly expelled. Without this precaution, the negative potential which built up on the interface considerably retarded the reaction by offering a barrier to the approaching catalytically active hydroxyl ions. 

Depending on the amount (more than 11 wt% is critical) and type of fat (mainly unsaturated fatty acids) in leather, poor adhesive bond can be produced. The fat may diffuse through to the surface of the polyurethane adhesive film (possibly while the adhesive still wet, with the solvent acting as the vehicle) and interfere with its subsequent coalescence with the adhesive or the sole. When polychloroprene adhesive is used, the reaction of the fatty adds with some components in the adhesive formulation (zinc oxide, magnesium oxide) forms insoluhle metal soaps (salts of fatty acids) at the interface that impart antiadherend properties. Polyurethane adhesives are more tolerant to greasy leather than polychloroprene adhesives. Roughening of the leather is always necessary and the addition of 5 wt% isocyanate to the adhesive just before application helps to reach good adhesive joints. 

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