Disordered waves in a homogeneous, motionless excitable medium

Abstract

EXCITABLE media are physical, chemical or biological systems in which energy dissipation in disturbed regions is compensated by energy supply^1–4, so that the media can support waves without attenuation. Well-known examples are nervous tissue, heart muscle, retinae, aggregating amoebae and the autocatalytic Belousov–Zhabotinsky (BZ) reaction. Most familiar in such systems are periodic target and spiral waves, but a number of simulations^5–11 have suggested that disordered waves can also occur in a homogeneous and motionless excitable medium. In all experimental observations reported so far, however, hydrodynamic flow^12,13 or inhomogeneities^14–15 were necessary to induce disorder. Here we present experimental evidence for disordered chemical waves in a convection-free and homogeneous medium, a gel containing a light-sensitive BZ reaction mixture. We find instabilities transverse to the wavefront ('ripples'), wave breakup leading to aperiodic spiral formation, labyrinthine structures, and erratic motion of non-spiralling wave fragments. The formal analogy of the BZ reagent with other excitable media suggests that this disordered behaviour may be generic.