SECONDARY INSTABILITY IN SURFACE-TENSION-DRIVEN BENARD CONVECTION

We report accurate experimental investigations of surface-tension-driv en Benard convection extending up to epsilon = (Delta T - Delta T-c)/D elta T-c = 4.5 into the supercritical range. The measurements are perf ormed in a circular box with an aspect ratio of 23. At epsilon(s)-2.35 +/-0.40 hexagonal cells, perfectly organized at the onset of the prima ry instability, lose stability and are gradually replaced by square ce lls. The transition process is mediated by pentagons.