Local Marangoni number at the onset of hydrothermal waves

We report the results of an experiment in which a layer of fluid, placed in a container open to the air, is subjected to a horizontal temperature difference Delta T. The fluid height h is kept constant and both the horizontal temperature difference and the horizontal dimensions of the fluid layer are changed. In this configuration, when Delta T goes beyond a certain threshold, waves propagating from the cold to the hot side appear (hydrothermal waves) with a determined group velocity. We study the influence of the container geometry on these waves. Close to the threshold, they are spatially localized near the hot side and a local Marangoni number is defined to describe this behavior. If Delta T is further increased, the waves fill the whole fluid layer. We also find an agreement between our experimental results and theoretical works and simulations.