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Operating natural or artificial chemical systems requires nonequilibrium conditions under which temporal and spatial control of the process is realizable. Open reaction–diffusion systems provide a general way to create such conditions. A key issue is the proper design of reactors in which the nonequilibrium conditions can be maintained. A hydrogel with flow-through channels is a simple, flexible, and easy-to-make device in which chemical reactions are performed in the diffusion field of localized separated sources of reactants. Two reactants separated in two channels create a bipolar antagonistic diffusion field, where the reaction intermediates firmly separate in space. Numerical simulations and corresponding experiments are performed to present this inhomogeneous diffusion field-induced chemical separation in sequential reactions. A remarkable result of this bipolar spatial control is localized wave phenomena in a nonlinear activatory–inhibitory reaction. These findings may help design functioning artificial nonequilibrium systems with the desired spatial separation of chemicals. [ABSTRACT FROM AUTHOR]

Precipitation processes are essentialin many natural systems,especially in biomineralization and in geological pattern formation.We observe temporal oscillations in the total mass of the precipitate,the formation of propagating and annihilating waves, and morphologicalinstabilities in a thin precipitation layer in a two-side-fed gelreactor containing the AlCl3/NaOH reaction–diffusionsystem. Contrary to the standard Liesegang patterns, these structuresform in the lateral direction at the meeting of the counterpropagatingdiffusion fronts of the electrolytes. The two main ingredients ofthe system are the amphoteric precipitate and the cross gradient ofthe chemicals due to the fixed boundary conditions. Simulations witha four-variable precipitation/redissolution model qualitatively reproducethe oscillations in the total mass of the precipitate and point outthe stratified three-dimensional structure of the precipitate. [ABSTRACT FROM AUTHOR]