Nonbound dislocations in hexagonal patterns: pentagon lines in surface-tension-driven Bénard convection

We report on a novel class of defects in a hexagonal pattern which we call pentalines. They are built up of two nonbound dislocations and are orientated parallel to the roll axis of the mode free of a dislocation. A pentaline has its origin in a transformation of the penta-hepta defect (PHD), taking place at higher supercriticality. The underlying mechanism consists in a combination of glide and climb motion of the original dislocations bound to the PHD. We demonstrate that the pentalines play an important role within the transition from hexagonal towards square convection cells, observed in surface-tension-driven Bénard convection.