COMPETITION BETWEEN SPIRAL-DEFECT CHAOS AND ROLLS IN RAYLEIGH-BENARD CONVECTION

We present experimental results for pattern formation in the Rayleigh- Benard convection of a fluid with a Prandtl number sigma similar or eq ual to 4. We find that the spiral-defect-chaos (SDC) attractor, which exists for sigma similar or equal to 1, has become unstable. Gradually increasing the temperature difference Delta T from below to well abov e its critical value Delta T-c no longer leads to SDC. A sudden jump o f Delta T from below to above Delta T-c causes convection to grow from fluctuations and does yield SDC. However, the SDC is a transient; it coarsens and forms a single cell-filling spiral which then drifts towa rd the cell wall and disappears.