Pinning and mode-locking of reaction fronts by vortices

We present results of experiments on the behavior of reaction fronts in the presence of vortex-dominated flows. The flow is either a single vortex or a chain of vortices in an annular configuration, and the reaction is the excitable Belousov–Zhabotinsky chemical reaction. If the vortex chain oscillates periodically in the lateral direction, the reaction front often mode-locks to the oscillations, propagating an integer number of wavelengths of the flow (two vortices) in an integer number of drive periods. In the presence of a uniform “wind”, the front often freezes, remaining pinned to the leading vortex and neither propagating forward against the wind nor being blown backward by it. Studies with an individual vortex verify the ability of a moving vortex to pin and drag a reaction front. We use this pinning behavior to explain the mode-locking for the oscillating case.

► Experiments demonstrate that vortices play a critical role in the propagation of reaction fronts in a fluid flow.
► If a front is moving in a vortex chain that is oscillating periodically, the front mode-locks to the oscillation frequency.
► A front is often pinned (frozen in place) to a vortex chain in the presence of an imposed wind.
► Individual vortices moving through a flow often pin a drag a moving reaction front.