Smectic Foams

Abstract

Because of their layered structure, thermotropic smectic mesogens can form stable foams. In this study, two-dimensional foams of 8CB are prepared in the smectic A phase. We determine the structures of the foam cells and study the aging dynamics. Three stages of foam evolution are distinguished. The freshly prepared foam consists of multilayers of small cells. After several hours, a 2D foam with predominantly hexagonal cells develops. It takes several days until the foam reaches an asymptotic structure with a characteristic distribution of n-polygons and self-similar scaling behavior of the coarsening. The structural changes are essentially caused by gas exchange between cells; film rupture can be neglected. We confirm predicted distributions and asymptotic scaling laws quantitatively. In the nematic phase, stable foams could not be produced, but smectic foams survive a transition into the nematic state up to several degrees above the phase transition. The reason for that is obviously smectic ordering at the film surfaces. The nematic foams coarsen much faster than smectic foams; film rupture is the dominant contribution to the aging dynamics. With 5CB, which has no smectic phase, we were not able to prepare foams.