Transition between curved and angular textures in binary fluid convection

We report a transition from a curved texture to an angular texture in Rayleigh-B\'enard convection in a mixture of ethanol and water. The transition occurs as the Rayleigh number is reduced from a high value and approaches the traveling-wave branch and is observed at negative values of the separation ratio in fluid mixtures, but not in a pure fluid. We show that this transition may be understood in terms of a generalization of the Swift-Hohenberg free energy functional. A large increase in wave-number rigidity is observed as the traveling-wave transition is approached. {} 1996 The American Physical Society.