Domain walls in vertically vibrated monolayers of cylinders confined in annuli

Liquid-crystalline ordering in vertically vibrated granular monolayers of metallic rods confined in annuli of different sizes is examined. The annuli consist of circular cavities with a central circular obstruction. In the absence of the central obstruction, rods of low aspect ratio exhibit global tetratic order, except for the existence of four small defected regions which restore the tetratic symmetry broken by the circular confinement. However, very different configurations are observed in the annuli, with a complex structure consisting of alternating layered regions separated by tetratic domain walls. We use concepts of equilibrium elastic theory for liquid crystals and topology along with arguments based on dissipation mechanisms to qualitatively explain this behavior. The results show that selective confinement of vertically vibrated monolayers of rods could be used as a tool to study the creation and dynamics of various types of defects in ordered systems.