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The effect of quenched disorder on the propagation of autowaves in excitable media is studied both experimentally and numerically in relation to the light-sensitive Belousov-Zhabotinsky reaction. The spatial disorder is introduced through a random distribution with two different levels of transmittance. In one dimension the (time-averaged) wave speed is smaller than the corresponding to a homogeneous medium with the mean excitability. Contrarily, in two dimensions the velocity increases due to the roughening of the front. Results are interpreted using kinematic and scaling arguments. In particular, for d 2 we verify a theoretical prediction of a power-law dependence for the relative change of the propagation speed on the disorder amplitude. [S0031-9007(98) Propagation of excitable waves in inhomogeneous media has also been studied from different points of view, such as the interaction of waves and inert obstacles (which is of some interest in cardiology, since reentries can be anchored [4][5][6] or planar fronts broken [7][8][9][10]) and the effect of modulations [11,12] or fluctuations [13][14][15]. Besides, the influence of additive and multiplicative fluctuations on symmetric bistable wave motion has been analyzed in detail [16-19].On the other hand, front roughness induced by fluctuations has received recently increasing attention [20,21]. Particularly analyzed have been the quenched versions [22,23] In this Letter we aim at studying, both experimentally and numerically, the effects on the propagation of autowaves originated by introducing time-independent random spatial fluctuations in the medium excitability.Experimentally a photosensitive highly excitable BZ medium was chosen. As shown in Fig. 1, we have considered two distinct configurations, where two parts of the medium, the leftmost homogeneous and the rightmost inhomogeneous, were separated by a vertical, completely unexcitable, strip with higher illumination. In the quasione-dimensional configuration, horizontal stripes of random dichotomic illumination, with the same average light intensity as in the homogeneous part, introduced the disorder. The two-dimensional setup was prepared analogously, this time with randomly distributed squares of two possible light intensities. The typical experiments (see Fig. 1) consisted in generating a planar wave at the bottom of the medium and observing its upwards evolution along the vertical axis. In this way both the shapes and velocities of the two free-end noninteracting FIG. 1. Propagating wave fronts on light-sensitive media consisting of both a left-side homogeneous and a right-side inhomogeneous medium with a brighter strip between them where fronts cannot propagate. (a) Quasi-one-dimensional setup. An initial flat front splits into two that were represented at three different times. The front which propagates through the inhomogeneous part undergoes an appreciable delay with respect to the other one. Stripe width in the direction of propagation l 1.1 cm. (b) Two-dimensional setup with randomly distributed...

The spiral wave behavior was examined in a bounded medium within the framework of the Belousov-Zhabotinsky reaction. When the spiral tip was located far from the center of a circular medium, the spiral wave was observed both to drift along the boundary and to approach it. ͓S1063-651X͑96͒06305-2͔

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Refraction of a train of autowaves on a moving boundary, separating two active media of different excitabilities, is studied using the light-sensitive Belousov-Zhabotinsky reaction. It was found that the frequency of the outgoing waves can be smoothly modulated by changing the velocity of the moving boundary. Results are compared with theoretical predictions showing a perfect agreement. They are valid for both autowaves and classical conservative waves. ͓S1063-651X͑96͒50912-8͔PACS number͑s͒: 03.40.Kf R5922 54 A. P. MUÑUZURI et al.