Abstract
Chemical waves propagate in thin gel layers saturated with Belousov-Zhabotinsky (BZ) reaction solution.
When a layer is in contact with air, weakly interacting waves can propagate along the top and the bottom of
the layer. Experiments on the ferrion-catalyzed BZ reaction with stepwise layers reveal a poorly excitable
sublayer in the middle of the layer. We propose that in a BZ excitable layer open to air two opposite transverse
concentration gradients are established, those of oxygen and bromine. The bromine gradient results in a parallel
gradient of the total concentration of bromo derivatives of malonic acid (BrMAs). The threshold of excitability
increases with the concentrations of oxygen and of BrMAs. As a result, the excitability threshold varies
nonmonotonically, with a maximum in the middle of the layer. If this maximum of the excitability threshold
is high enough, a poorly excitable sublayer appears between the two excitable sublayers at the top and bottom.
The wave propagation and interaction have been simulated using an Oregonator-type model with a nonmonotonic
vertical profile of the stoichiometric factor q that relates Br~ production to ferriin reduction. The simulation
confirms that the observed stratification of layers can be explained by the established mechanism of the BZ
reaction supplemented with molecular diffusion.