Antje van der Net
School of Physics
Trinity College Dublin, Ireland
Currently, researchers are working hard on creating foams with tiny, equal-sized bubbles for various industrial applications.We used a technique called microfluidic flow focusing to generate microbubbles of diameters in the range of 200-500 µm. When these bubbles are deposited on a liquid pool, they form a wet microfoam where the bubbles take on nearly spherical shapes. Surprisingly, one finds that the bubbles order in various familiar crystalline structures, such as those found in metals . The reason for the ordering remains unclear, the phenomenon is not observed for equal size hard spheres whose deposition generally gives a disordered Bernal packing.
Three-dimensional crystal structure, spontaneously formed by small bubbles (diameter 200 µm) in a wet foam. The composite figure shows a photograph of the surface of the foam (right), together with a raytracing simulation (left), confirming the packing sequence ABC consistent with an fcc packing.
The internal structure of the bubble crystals can not be seen due to diffusive character of light propagation in a foam. However, from the observed fractal-like optical patterns of the crystals and computer simulations we can deduce ordering up to several layers into the bulk.
By removing water from the microbubble crystals, ordered dry foams can be made. The bubbles become polyhedral and are separated by thin water films. It remains to be seen whether this technique is useful for finding the Weaire-Phelan structure, a computed foam structure which minimizes surface area for monodisperse bubbles, experimentally.
 A. van der Net, W. Drenckhan, D. Weaire, S. Hutzler ; "The Crystal Structure of Bubbles in the Wet Foam Limit.", Soft Matter, 2006, 2, 129-134.