Crystalline arrangements of microbubbles in monodisperse foams

Monodisperse liquid foams consisting of equal-sized bubbles with diameters of a few hundred microns exhibit crystalline order, with a predominance of the fcc structure. This is inferred from observations at both bottom (foam–water) and top (foam–air) interfaces, together with supporting ray-tracing simulations. Diffusion of gas from the top layer into the atmosphere results in the formation of an ordered bidisperse top layer within a few minutes. In foam samples with a height of about 100 bubbles we have found a transition from mainly fcc packing at the (wet) bottom to the (bcc) Kelvin structure at the (dry) top. The issue of templating for the ordering in wet foams is addressed in soft sphere computer simulations.