Ostwald ripening foams

Another mechanism of foam instability is due to Ostwald ripening (disproportionation), the driving force for which process is the difference in Laplace pressure between the small and larger foam bubbles. The smaller bubbles have a higher... 

Laplace pressure than the larger bubbles, but as the gas solubility increases with pressure the gas molecules will difTuse from the smaller to the larger bubbles. This process only occurs with spherical foam bubbles, and may be opposed by the Gibbs elasticity effect. Alternatively, rigid films produced using polymers may resist Ostwald ripening as a result of their high surface viscosity. 

In the case of a polyhedral foam with planar hquid lamella, the pressure difference between the bubbles is not large, and consequently Ostwald ripening is not the mechanism for foam instability in this case. With a polyhedral foam, the main driving force for foam collapse is the surface forces that act across the liquid lamella. 

It may even be questioned whether there is a close correlation at all between film stability and the lifetime of a foam. Much of the research on film rupture concerns quite large films, several mm in radius, whereas those in food systems are rarely above 50 pm. Most of the films studied were stabilized by small-molecule surfactants that are never or rarely used in foods. In the author s opinion these studies, however interesting they may be in general, are hardly relevant for foods. Moreover, Ostwald ripening tends to be the dominant instability in most food foams. 

Ostwald ripening is almost the only change in dispersity that proceeds faster for smaller particles, the excess solubility is roughly inversely proportional to particle diameter, and there is generally no free energy barrier. Since the average particle size increases, the process will proceed ever slower. Except in foams, the process is generally not important if the particle radii are over, say, 10 pm. 

Surface Dilational Properties. The calculations just given suggest that all foam will rapidly disappear, but several foams can be fairly persistent, and some can hardly be destroyed. Stabilization to Ostwald ripening (or to disproportionation as foam researchers usually call it) is thus possible. 

Coalescenceis especially typical in concentrated emulsions. In such systems coalescence mainly determines the lifetime of emulsions prior to phase separation. In finely dispersed emulsions, both dilute and concentrated, the average size of drops may noticeably increase due to Ostwald ripening. At the same level of dispersion Ostwald ripening of emulsion droplets is a slower process than mass transfer of bubbles in foams [60]. This is due to a rather low interfacial energy, and consequently, low difference in chemical potentials of substance in droplets of different size, as well as due to a lower mutual solubility of liquids as compared to the solubility of gases in liquids. 

The aggregative foam stability is manifested in changes of the foam dispersity. There are two main causes of aggregative foam stability disturbances a) coalescence of foam cells due to the rupture of bilateral films separating them b) disappearance of foam cells due to gas penetration from small cells into larger ones (Ostwald ripening). 

One of the processes by which an emulsion, like a foam, destroys itself is by Ostwald ripening the diffusion of liquid from small to large droplets. Calculate the time required for a benzene droplet to disappear when it is positioned near much larger droplets at a distance comparable to its radius. Assume droplet radii of 100 and 1000 nm. The solubility of benzene in water may be taken as 0.2% (vol/vol) the diffusion constant of benzene in water D = 10 cm s the interfacial tension of water-benzene s = 25 mN m" and the molar volume of benzene Vm = 100 cm. ... 

The bipolar pulsed pair stimulated echo (BPPSTE) pulse sequence was used to measure the water self-diffusion coefficients these were used as indicator of the Ostwald ripening extent in precursor emulsions leading to poly(divinyl-benzene) emulsion-derived solid foams based on a three component amphiphile system. 

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