A light look at foam.

A light look at foam

Anyone who has gazed into a foam-capped mug of beer has probably noticed that one can see through the beer but not through the foam. The foam's dense packing of tiny bubbles scatters light so effectively that very little passes directly through, giving the froth its familiar white color.

Douglas J. Durian and his collaborators at the Exxon Research and Engineering Co. in Annandale, N.J., have now exploited this effect to probe the structure and dynamics of three-dimensional foams. By analyzing in detail how a foam scatters light, they have obtained the first direct, noninvasive measure of the average size of bubbles deep inside a foam. They have also managed to track the rearrangement of the bubbles during a process known as "coarsening," in which large bubbles grow at the expense of smaller bubbles, which shrink.

Applying the light-scattering technique to commercial shaving cream, the Exxon team discovered that during the coarsening process, groups of neighboring bubbles usually remain locked in place even while the bubbles slowly change in size. However, these size changes introduce stresses that deform the bubbles. The stresses finally become large enough to induce a sudden rearrangement in which many neighboring bubbles quickly shift their positions and then settle into a new, quiescent state.

Having demonstrated that their technique can provide useful information about foam behavior, the researchers plan to try it on other foams to help determine what factors contribute to a foam's stability. They also want to study how bubbles move past each other when foams flow along a surface or through a pipe. Because foams play important roles in a variety of industrial processes, from beer brewing to oil drilling, the work may eventually yield better methods of tailoring foams for specific applications, the researchers say.