Zoltán Noszticzius

Sustained spiral waves in a continuously fed unstirred chemical reactor

Previous experiments on the self‐organization of spatial patterns in chemical reactors have generally been restricted to closed (batch) reactors. In such reactors the system relaxes irreversibly and uncontrollably towards thermodynamic equilibrium. It is difficult to make comparisons with existing theories which address asymptotic (long time) behavior because of the transient nature of the spatial patterns and the lack of well‐defined control parameters in such experiments. We report a novel disk‐shaped reactor that can be maintained far from thermodynamic equilibrium indefinitely by a continuous feed of reagents. Chemical patterns are formed inside a thin layer of inert gel that suppresses any convective motion. The feed to the gel is uniform and normal to the plane in which pattern can form. The reactor is used to conduct the first quantitative study of transitions between well‐defined states with different patterns.

Universality, multiplicity, and the effect of iron impurities in the Belousov–Zhabotinskii reaction

In experiments on the Belousov–Zhabotinskii reaction in a flow reactor we have observed dynamical behavior that is described well by one‐dimensional maps with a single maximum. A sequence of period doubling bifurcations was observed as a parameter was varied, and beyond the accumulation point for the period doubling sequence there was a sequence of periodic states that has the same symbolic dynamics as the states of the U (universal) sequence of Metropolis, Stein, and Stein (1973). However, in another experiment with malonic acid from a different vendor, we found that some states with particular symbol sequences occurred in three different parameter ranges rather than in one range as in the U sequence. Analysis of the effect of impurities in the reagents showed that some impurities (e.g., Fe3+ and esters of malonic acid) at concentrations of only a few ppm produced dramatic changes in the dynamics; such impurities are contained in commercially available malonic acid. Experiments with purified malonic acid...

Explodator: A new skeleton mechanism for the halate driven chemical oscillators

In the first part of this work, some shortcomings in the present theories of the Belousov–Zhabotinskii oscillating reaction are discussed. In the second part, a new oscillatory scheme, the limited Explodator, is proposed as an alternative skeleton mechanism. This model contains an always unstable three‐variable Lotka–Volterra core (the ‘‘Explodator’’) and a stabilizing limiting reaction. The new scheme exhibits Hopf bifurcation and limit cycle oscillations. Finally, some possibilities and problems of a generalization are mentioned.

Involutes: the geometry of chemical waves rotating in annular membranes

According to earlier theories certain parts of a chemical wave front propagating in a 2-D excitable medium with a convex obstacle should be involutes of that obstacle. The present paper discusses a special case where self-sustained chemical waves are rotating around a central obstacle in an annular 2-D excitable region. A simple geometrical model of wave propagation based on the Fermat principle (minimum propagation time) is suggested. Applying this model it is shown that the wave fronts in the case of an annular excitable region should be purely involutes of the central obstacle in the asymptotic state. This theory is supported by experiments in a novel membrane reactor where a catalyst of the Belousov-Zhabotinsky reaction is fixed on a porous membrane combined with a gel medium. Involutes of circular and triangular obstacles are observed experimentally. Deviations from the ideal involute geometry are explained by inhomogeneities in the membrane. (c) 1995 American Institute of Physics.

Chemical waves in modified membranes I: developing the technique

Abstract A novel technique to fix BZ catalysts on commercially available membranes is described. Chemical waves can be observed in the membranes for more than 10 hours, when they are placed in a batch reactor. Two dimensional excitable regions of arbitrary shape can be created either by cutting the membranes or by painting these shapes on the membrane surface.

Bifurcation from excitability to limit cycle oscillations at the end of the induction period in the classical Belousov–Zhabotinsky reaction

The appearance of full blown oscillations at the end of the induction period in the classical BZ reaction (substrate: malonic acid) is usually explained by a subcritical Hopf bifurcation. Our perturbation experiments have revealed that the classical BZ reaction is excitable in its induction period and the threshold of excitability is decreasing gradually to zero during that period. Thus the sudden appearance of limit cycle oscillations at the end of the induction period can be explained by a saddle‐node infinite period (SNIPER) bifurcation as well.

Nonlinear effects of electrolyte diodes and transistors in a polymer gel medium

The polarization curve of an acid-base interface in a hydrogel medium has a diode characteristic. Two of each such electrolyte diodes can be combined to give an electrolyte transistor. When a salt is added to the alkaline or to the acidic part of a reverse biased electrolyte diode, the current response is highly nonlinear. If the salt is added to the acidic side, even bistability can be observed. This bistability can generate complex oscillations in a base-acid-base electrolyte transistor. These nonlinear effects are studied experimentally and theoretically. While the nonlinear salt effect can be explained with the Nernst-Planck equations, to understand the bistable behavior further investigations are necessary. (c) 1999 American Institute of Physics.

Turing patterns visualized by index of refraction variations

Gel pattern is visualized by the refractive index variations.The fossil patterns correspond to a spatial variation in the refractive index. (AIP)

Spacial patterns in a uniformly fed membrane reactor

Abstract A novel reactor, consisting of a thin membrane with each side exposed to fluid from a well-stirred flow reservoir, has been developed for the study of sustained chemical spatial patterns. This reactor bears some similarity to another continuously fed unstirred reactor with a uniform feed, the disk-shaped reactor developed by Tam et al. [J. Chem. Phys. 88 (1988) 3395], but the present reactor differs in two important aspects: (1) The effective residence time, which is determined by the thickness of the gel, can be very short (of the order of seconds). (2) The feed is from two reservoirs rather than one; this opens up new regions of chemical composition for study. Examples are presented of patterns obtained for the Belousov-Zhabotinsky reaction in the membrane reactor.

Interrupted separatrix excitability in a chemical system

Models of excitability fall into two broad categories: one with a sharp threshold, the other with a narrow but finite threshold region. In an attempt to clarify the notion, we have studied the application of two models of excitability to experiments on a variant of the Belousov–Zhabotinskii reaction in a well‐stirred reactor. An interrupted separatrix model, which has a sharp threshold, is found to describe our experimental results better than a simple nullcline model, which has a finite threshold region.