Experimental studies of reaction front barriers in a three-dimensional nested vortex flow*

We present experiments that study the behavior of reaction fronts propagating in three-dimensional, laminar fluid flows. The primary flow is a chain of nested horizontal and vertical vortices, a flow that has been shown to produce chaotic mixing even if time-independent\footnote{M.A. Fogleman, M.J. Fawcett and T.H. Solomon, Phys. Rev. E {\bf 63}, 020101(R) (2001).} The fronts are produced by the excitable, Ruthenium-catalyzed Belousov-Zhabotinsky chemical reaction. When illuminated with a near-UV laser beam, the Ru indicator fluoresces everywhere except where there is a reaction front. By scanning the laser beam and imaging from above, we are able to do a full 3D-visualization of the reaction front propagating through the flow. The fronts are observed to encounter tube- and sheet-like barriers, whose properties we measure experimentally. We interpret the results by generalizing a recent theory of ``burning invariant manifolds'' \footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} which have been shown previously to act as one-way barriers for reaction fronts propagating in two-dimensional fluid flows.