DISORDERED WAVES IN A HOMOGENEOUS, MOTIONLESS EXCITABLE MEDIUM

EXCITABLE media are physical, chemical or biological systems in which energy dissipation in disturbed regions is compensated by energy suppl y(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 a lso occur in a homogenous and motionless excitable medium. In all expe rimental observations reported so far, however, hydrodynamic flow(12,1 3) or inhomogeneities(14,15) were necessary to induce disorder. Here w e present experimental evidence for disordered chemical waves in a con vection-free and homogeneous medium, a gel containing a light-sensitiv e BZ reaction mixture. We find instabilities transverse to the wavefro nt ('ripples'), wave breakup leading to aperiodic spiral formation, la byrinthine structures, and erratic motion of non-spiralling wave fragm ents. The formal analogy of the BZ reagent with other excitable media suggests that this disordered behaviour may be generic.