Chapter 10 - Chemical waves and stationary patterns

Abstract

When phenanthroline is added to the Belousov–Zhabotinskii (B–Z) system, in a test tube, traveling red and blue bands are observed displaying spatio-temporal oscillations. This feature denotes a specific type of time order and space order. In the system, export of entropy through diffusion occurs and a specific type of dissipative structure is obtained. Both traveling waves and stationary structures are formed depending on the experimental conditions. This chapter discusses experimental studies, mechanisms and mathematical modeling of such structures. It also discusses turning patterns, mosaic structure, precipitation patterns, and their mechanism in detail. Essentially non-linear kinetics involving reaction–diffusion is the basis of mathematical modeling. During one-dimensional study of uncatalyzed oscillatory reaction systems, specific types of structures called mosaic structures are observed in phenol–bromate–H2SO4 and ferroin–bromate–H2SO4 systems. Analytical studies showed that the structure in the latter case is simply an agglomeration of fine grains of solid solutions of the reaction products, tri-bromophenol and p-benzoquinone formed during periodic nucleation. Mosaic structures were not formed in a tube which had been freshly cleaned by steaming, indicating the occurrence of heterogeneous nucleation. Interdiffusion of one electrolyte into another reacting electrolyte may lead to a rhythmic deposition of precipitate, which is known commonly as Liesegang phenomenon. Liesegang ring-type patterns are observed both in living and non-living systems including bacterial and fungal growth. When a solution of a large concentration of K2Cr2O7 is allowed to diffuse into a less concentrated solution of lead nitrate containing a lyophilic gel medium, periodic deposition of precipitate appears after several hours.