Reentrant hexagons in non-Boussinesq Rayleigh-Bénard convection: effect of compressibility.

We present experimental studies of a new pattern sequence observed in non-Boussinesq convection in a compressible fluid near its gas-liquid critical point (CP). Besides the known hysteretic transitions among conduction state, hexagons, and rolls, another hysteretic transition from rolls to hexagons at higher values of the control parameter is found. This reentrance phenomenon is observed in a rather narrow range of about 60- to 100-microm cell heights and is attributed to large compressibility of a fluid near the CP.