The two main routes by which foams collapse have been unified into a single, simplified theory that spotlights the link between them, researchers report. Beermakers, who already add nitrogen to their brews to enhance the head that drinkers crave, and other foam users such as fire fighters and mineral processors, should now find themselves able to engineer a better froth.
Foams fizzle out for two reasons. The liquid surrounding the gas bubbles can simply drain away, pulled by gravity but resisted by the liquid's inherent love of creeping into small spaces. Alternatively, gas can leak from small bubbles to larger ones, ultimately creating fewer, bigger bubbles. Traditionally foam researchers have treated these two processes, foam drainage and bubble coarsening, independently of one another.
But that's not good enough, according to Harvard University engineer Howard Stone. The processes are interlinked, and both depend on bubble size: Bubbles get bigger during coarsening, changing the dynamics of drainage. To account for this relation, Stone's team devised a model of foam collapse that combines a souped-up expression for liquid drainage with another for bubble coarsening due to gas leakage. The researchers compared their predictions to the behavior of soapy foams made with two different gases, one leaky and one not, and the model held up, they report in the 14 May issue of Physical Review Letters.
Fellow foam physicist Douglas Durian of the University of California, Los Angeles, says Stone's examples of beers fizzed with different gases (see illustration) are "a very, very convincing, qualitative demonstration" of the drainage-coarsening link. The story is far from over, however. Stone says that the current model doesn't take into account the effects of bursting bubbles. Meanwhile, Durian challenges Stone's use of theoretical ideas that might work for foams trapped in a two-dimensional flat sheet but which he says are suspect for foams in a three-dimensional tankard. Foam scientists will be staring into their beers for a while yet.