An experimental study of columnar crystals using monodisperse microbubbles

• We study the ordering of monodisperse microbubbles within cylindrical confinement.
• X-ray tomography was used to obtain three-dimensional images of their structures.
• Close structural resemblances were found between foam and hard-sphere packings.
• Previously unseen larger structures were formed of several distinct ordered layers.
• A new model predicted the change of contact number with cylinder diameter.

We investigate the ordered arrangements of monodisperse microbubbles confined within narrow cylinders. These foams were imaged using X-ray tomography, allowing the 3D positions of the bubbles of the foam to be accurately determined. The structure of these foams closely resemble the minimum energy configuration of hard spheres in cylindrical confinement as found in simulations. For larger ratios, λ, of cylinder to bubble diameter two- and three-layered crystals were formed. Each layer of these structures is found to be ordered, with each internal layer resembling structures found at lower λ values. The average number of contacts per bubble is seen to increase with λ.

Reconstruction of the raw three-dimensional tomographic data showing the cylindrical ordering of microbubbles. The red boxes indicate regions of consistent ordering, above which a sharp transition to a different ordered structure is observed. Such columnar crystals have been observed in a wide variety of physical systems. Figure optionsDownload as PowerPoint slide