2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/153227For Rayleigh-Benard convection in a square cell with a fluid of Prandtl number one, we report experimental results on the transition between a stationary pattern of ideal straight rolls (ISR) and the spatiotemporal chaotic state of spiral defect chaos (SDC). In contrast to experiments in circular geometries, we found ISR states below a particular value of the control parameter and SDC states above this value. By characterizing the pattern with a global measure, the pattern entropy, we found that the transition from SDC to ISR showed similarities to phase transitions in equilibrium finite-size systems.4 pages, including 6 inlined figures; submitted to Phys. Rev. Lett; See http://milou.msc.cornell.edu for related moviesPattern Formation and Solitonstext