Spiral-Defect Chaos in Rayleigh-Bénard Convection with Small Prandtl Numbers

Rayleigh-Bénard convection is investigated for Prandtl numbers $\sigma$ from 0.3 to 1.0 by using various pure gases and binary gas mixtures. The onset of spiral-defect chaos (SDC) moves to smaller values of $\epsilon \equiv R/R_c-1\mathrm{}$ ( $R$ is the Rayleigh number and $R_c$ its critical value) as $\sigma$ decreases. The measurements suggest that large-scale mean flows, which are expected to be stronger at smaller $\sigma$, play an important role in the spontaneous generation of SDC. SDC can occur and fully develop in the regime dominated by the Soret effect.