The polyacrylamide-methylene blue-oxygen system reveals a variety of complex concentration patterns. Chevron and white-eye patterns develop from simple hexagons in a slow, continuous transition process. A third type of structure (honeycombs) forms from either white-eye patterns or hexagons. This transition occurs rapidly along propagating fronts. Two-dimensional Fourier analyses reveal that the complex patterns have a simple structure and that the wave vectors of the initial hexagons are conserved in all transitions. Quantitative data on surface undulations of the gel are presented. The surface structure resembles the structure of the observed methylene blue patterns. A schematic phase diagram is presented that characterizes the pattern geometry in terms of reaction time and initial sulfide concentration.
We report experimental results on the formation of macroscopic concentration patterns in the polyacrylamide-methylene blue-oxygen-sulfide (PA-MBO) reaction. The patterns have the geometry of stripes or hexagons. The latter transform to more complicated structures within the gelled medium. For the case of hexagons, the pattern wavelength (here 1-5 mm) shows a nearly proportional dependence on the height of the reaction layer. The system generates colloidal particles (average diameter 30 µm) that we use to monitor the physical changes within the medium. Our results reveal three distinct phases of particle movement. The first one is caused by hydrodynamic flows within the pre-gel solution, whereas the third, and possibly the second, phase arises from localized volume changes within the gel matrix.
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