SUPER-SPIRAL STRUCTURES IN AN EXCITABLE MEDIUM

ROTATING spiral waves have been observed in various excitable media, including heart muscle 1, retinae 2, cultures of the slime mould Dyctiostelium discoideum 3,4 and chemical oscillators such as the Belousov-Zhabotinsky (BZ) reaction 5-7. Under certain conditions the spiral wave does not exhibit simple periodic rotation, but quasiperiodic 8 (or 'compound' 9) rotation, in which the spiral's origin (the tip) meanders 10. Recent calculations 11 have shown that highly meandering tip motion can impose superstructures on spiral waves. Here we reproduce these patterns experimentally, using the BZ reaction as the excitable medium. We induce high tip meander by applying pulses of electrical current locally at the tip 12. Image processing of the patterns reveals a spiral wave of larger wavelength superimposed on the original wave, an effect that can be described in terms of a Doppler shift in the original spiral.