Forced phase diffusion in a convection experiment

In a thermal-convection box the boundaries parallel to the convection rolls are replaced by ramps, i.e., smooth variations in the Rayleigh number from supercritical to subcritical values. Such a geometry selects a small wavelength band. The wavelength selected by the left and right ramps are not necessarily the same---resulting in a wavelength gradient. This gradient leads to forced phase diffusion: The convection pattern drifts from the short-wavelength end to the ramp selecting the longer wavelength. An experimental observation of this phenomenon is presented.