PERIODIC TO QUASI-PERIODIC TRANSITION OF CHEMICAL SPIRAL ROTATION

We report observations of the tip motion of spiral waves in excitable Belousov-Zhabotinskii reagent. An open reactor is used to determine the time-asymptotic dynamics of the spiral tip as a function of a control parameter. We have observed a supercritical transition from simple rotation (one frequency) to compound rotation (two frequencies); the compound motion at onset closely resembles that predicted by two-species reaction-diffusion models. Measurements of the frequencies of the motion indicate that the compound rotation is quasiperiodic over the range studied; there is no evidence of frequency locking. The compound tip motion appears to result from fluctuations in the curvature of the spiral wave near its tip. We describe qualitatively the sequence of events during a single tip orbit and suggest that the refractory tail of the wave plays a dominant role in controlling the tip motion.