Mechanism of Spiral Wave Unpinning in the Belousov-Zhabotinsky Reaction with a DC Electric Field

We study the mechanism of spiral wave unpinning in the Belousov-Zhabotinsky (BZ) reaction with a DC electric field. The unpinning is characterized by the phase of the spiral tip around the obstacle boundary at the time of unpinning. We systematically measure the unpinning phase as a function of the chirality of spiral rotation, the initial phase of the spiral, the size of the pinning obstacle, the direction, and the strength of the applied electric field. In both BZ experiments and simulations using the Oregonator model, we observe that the spiral wave always unpins at a fixed position with respect to the applied field. The wave unpins when the electric field component in the direction of the tip velocity of the spiral waves becomes equal to a threshold field strength. From these observations, we deduce a relation between the phase of unpinning, the size of the pinning obstacle, the strength, and the direction of the electric field, and it agrees with our observations. We conclude from our observations that a retarding 'electric force' on the chemical wave is responsible for the unpinning in the BZ medium. Our results indicate that the 'electric force' is more effective in unpinning when the wave moves away from the anode than when it is moving toward it.