A novel mechanism for in situ nucleation of spirals controlled by the interplay between phase fronts and reaction-diffusion waves in an oscillatory medium

Understanding the transition from planar fronts, trigger waves, or solitary waves to spirals in excitable media has attracted increasing interest in the past few decades, mainly because of its relevance for biological and medical applications. In this paper, we introduce a new mechanism for the formation of rotating spiral waves based on the collision between a phase reduction front and trigger waves propagating in the oscillatory Belousov-Zhabotinsky (BZ) medium. The phase reduction front is triggered by imposing a heterogeneous spatial gradient of a chemical agent S able to temporarily sequester an inhibitory species of the oscillatory mechanism. This determines an initial oxidized transient over the whole reactor and, successively, a phase desynchronization by which the system recovers the excitable condition. The resulting reduction front can induce and control the transition from phase to trigger waves, whose properties depend upon the concentration profile of S along the spatial coordinate. By means of a numerical approach, we show that smooth gradients of the species S favor the formation of stacking waves with short characteristic wavelengths and the adaptation of the velocity of this cluster of pulses to that of the leading reduction front. Front-back annihilation between the reduction front and an incoming pulse may occur when the concentration profile of S has a steep gradient along the spatial domain. Experimental evidence of this mechanism as a possible source for spiral formation is shown in a quasi-two-dimensional geometry by using the BZ oscillator including the zwitterionic surfactant tetradecyl dimethylammonium oxide (C14DMAO). By segregating the inhibitor Br2, the micelles cause the onset of an oxidized induction period which vanishes after a few minutes through the propagation of a reduction phase front originated from an anisotropic spatial distribution of the surfactant C14DMAO. The reduction front gives a complex interplay with following target waves, including front-back collision. The spontaneous break of symmetry from target to spirals is controlled here by tuning the concentration of the surfactant or the alternative inhibitor species Br-.