Other Influences on Foam Stability

Foaming capability relates to both foam formation and foam persistence. Surface tension lowering is necessary, but not sufficient. Other important factors include surface elasticity, surface viscosity, and disjoining pressure [303], Considering stabil- 

Some mildly surface-active compounds will stabilize weakly stable, transient foams. Examples include short-chain alcohols, short chain fatty acids, aniline, phenol, and pine oil. These weak frothers tend to produce foam films having stabilities on the order of seconds. More strongly surface-active compounds can stabilize quite strong, meta-stable foams. Examples include long-chain alcohols and fatty acids, and proteins. These strong frothers tend to produce foam films having stabilities on the order of minutes to hours. 

The stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

Micro-scale experiments involve the microscopic observation of flowing foams in etched-glass micromodels. Here the pore dimensions are typically on the order of hundreds of micrometers. Such experiments provide valuable and rapidly obtainable qualitative information about foam behaviour in constrained media under a variety of experimental conditions, including the presence of a residual oil saturation 

Where there exists a bubble size distribution, there will exist pressure gradients between bubbles of different sizes in a foam. Concentration gradients may form across lamellae that promotes gas diffusion between bubbles. This effect will cause larger bubbles to grow at the expense of smaller bubbles, termed disproportionation, and becomes a mechanism for foam degeneration as the liquid films thin 

---