Burning invariant manifolds and reaction front barriers in three-dimensional vortex flows

We describe experiments on the effects of three-dimensional fluid advection on the motion of the excitable, Ruthenium-catalyzed Belousov-Zhabotinsky chemical reaction. The flow is a superposition of horizontal and vertical vortices produced by magnetohydrodynamic forcing and measured with particle image velocimetry. We visualize the propagating fronts in three dimensions with a scanning, laser-induced fluorescence technique that benefits from the fluorescence of the reduced Ru indicator. The experiments reveal a combination of tube- and sheet-like barriers that block the propagating reaction fronts. We study the dependence of the structure of these barriers on the front propagation speed (normalized by a characteristic flow velocity). The locations and blocking properties of these barriers are interpreted with a six-dimensional {\it burning invariant manifold}\footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} theory that follows the evolution of front elements in the flow.