Filaments formed in the hexagonal columnar liquid crystal phase of star-shaped oligobenzoates

We investigate the stability and structure of freely suspended filaments of columnar liquid crystal phases. In the hexagonal columnar phase, filaments with aspect ratios of the order of 20:1 and more can be formed. They can be cooled into a low-temperature body-centred orthorhombic columnar phase without breaking. The slow optic axis is perpendicular to the filament axis and the materials are negatively birefringent in the high-temperature phase. At the transition into the low-temperature phase, the sign of the birefringence is inverted. Within the lifetime of the filaments, three regimes can be distinguished. First, the filaments thin exponentially over a period of about 2 hours. Thereby, the axial alignment improves and the filaments adopt a cylindrical geometry. These cylindrical filaments can persist quasi-stationary for several hours. Fluctuations finally lead to slight inhomogeneities in the filament diameter. This causes Laplace pressure gradients and induces an axial redistribution of material, and the filaments unavoidably rupture within about half an hour.