Transitions between patterns in thermal convection

We present experimental studies of the transitions between conduction, hexagons, and rolls in non-Boussinesq convection of gaseous CO₂ in a cylindrical cell of radius-to-height ratio 86. Except for the transition from conduction to hexagons, transitions occur when the two states involved have nearly the same value of a generalized potential rather than at the stability limits. Conduction gives way to hexagons via the propagation of a front connecting the two states, while the transitions between hexagons and rolls are facilitated at the cell walls which appear to nucleate the minority state.