NONUNIFORM BROKEN-PARITY WAVES AND THE ECKHAUS INSTABILITY

We report measurements on broken-parity traveling finger patterns whic h form at an oil-air interface driven by the rotation of two acentrica lly mounted horizontal cylinders. In the parameter region studied here , the wavelength, traveling speed, and degree of asymmetry of the trav eling fingers vary along the interface. As our control parameter is in creased, we observe a transition at which the average values of these properties suddenly decrease. The traveling patterns are intermittentl y interrupted by periods of disordered behavior, which become more fre quent near this transition. The variation of wavelength along the patt ern allows us to observe what appears to be the Eckhaus instability: o utside a range of wavelengths, the drifting fingers become unstable, r esulting in the birth of new fingers if the local wavelength is too lo ng, or the disappearance of a finger if it is too short. We measure th e Eckhaus stability boundary for this system.