Stability of roll and hexagonal patterns in Bénard–Marangoni convection

New experimental results concerning the stability of convective patterns in Bénard–Marangoni convection are presented. The hexagonal pattern usually seems to be the most stable. By means of a new thermal technique a predetermined pattern can be imposed and the evolution of a pattern of rolls thus created is observed. For small depths of the fluid layer, hexagons are very stable but they become less and less so as the depth of the layer increases, whereas the imposed rolls become more and more stable. This is linked to the relative importance of the driving mechanisms—surface tension or buoyancy forces—and may correspond to a change in the order of this nonequilibrium phase transition.