Rayleigh-Bénard convection near the gas-liquid critical point

We present experimental results on Rayleigh-B\'enard convection in SF₆ near the gas-liquid critical point. We measured the critical temperature difference for the onset of convection, {Δ}T₀, as a function of the reduced average temperature {τ}=(T{}-Tc)/Tc and found {Δ}T₀=525×{}τ1.89, which is close to the expected power law behavior. The strong temperature dependence of the physical properties is used to scan the Prandtl number in a wide range. A new, many ``target'' pattern state, initiated by a defect instability, was observed.