Control of chemical chaos through medium viscosity in a batch ferroin-catalysed Belousov-Zhabotinsky reaction.

In this paper we show that the active interplay of nonlinear kinetics and transport phenomena in a chemical oscillator can be exploited to induce and control chaos. To this aim we use as a model system the ferroin-catalysed Belousov-Zhabotinsky (BZ) oscillating reaction, which is known to evolve to characteristic chaotic transient dynamics when carried out under batch and unstirred conditions. In particular, chemical chaos was found to appear and disappear by following a Ruelle-Takens-Newhouse (RTN) scenario. Here we use medium viscosity as a bifurcation parameter to tune the reaction-diffusion-convection (RDC) interplay and force the reaction in a specific sequence of dynamical regimes: either (i) periodic → quasi-periodic → chaotic or (ii) periodic → quasi-periodic or (iii) only periodic. The medium viscosity can be set by adding different amounts of surfactant (sodium dodecyl sulphate), known to have a little impact on the reaction mechanism, above its critical micelle concentration. Experimental results are supported by means of numerical simulations of a RDC model, which combines self-sustained oscillations to the related chemically-induced buoyancy convection.

Specific anion effects on urease activity: A Hofmeister study.

The effects of a range of electrolytes on the hydrolysis of urea by the enzyme urease is explored. The autocatalytic behavior of urease in unbuffered solutions and its pH clock reactions are studied. The concentration dependence of the experimental variables is analyzed in terms of specific ion-enzyme interactions and hydration. The results offer insights into the molecular mechanisms of the enzyme, and on the nature of its interactions with the electrolytes. We found that urease can tolerate mild electrolytes in its environment, while it is strongly inhibited by both strong kosmotropic and strong chaotropic anions. This study may cast light on an alternative therapy for Helicobacter pylori infections and contribute to the design of innovative materials and provide new approaches for the modulation of the enzymatic activity.

Macroscopic dynamics as reporter of mesoscopic organization: the Belousov-Zhabotinsky reaction in aqueous layers of DPPC lamellar phases.

The propagation of traveling chemical waves in the excitable Belousov-Zhabotinsky (BZ) system when performed in the presence of 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl choline (DPPC) bilayers responds sensitively to the phospholipid content. The characteristic features of wave propagation, such as spiral pitch, rotation period, and size of the spiral core region, show two regions of different behavior, one below and the other above a DPPC content of 12.5% (w/w) thus suggesting a transition in the organization of the lipid domains at a DPPC content of ∼12.5% (w/w). This transition is supported by small-angle X-ray scattering data, which show pronounced changes in the coherence lengths of the lyotropic smectic domains. Thus, the dynamics of the chemical system occurring at a macroscopic length scale reflects the organization of the water/lipid domains which extend over mesoscopic lengths. These findings indicate that in the BZ/DPPC system, there is an interaction between processes that occurs at length scales differing by as much as 3 orders of magnitude.

Role of the reagents consumption in the chaotic dynamics of the Belousov-Zhabotinsky oscillator in closed unstirred reactors.

Chemical oscillations generated by the Belousov-Zhabotinsky reaction in batch unstirred reactors, show a characteristic chaotic transient in their dynamical regime, which is generally found between two periodic regions. Chemical chaos starts and finishes by following a direct and an inverse Ruelle-Takens-Newhouse scenario, respectively. In previous works we showed, both experimentally and theoretically, that the complex oscillations are generated by the coupling among the nonlinear kinetics and the transport phenomena, the latter due to concentration and density gradients. In particular, convection was found to play a fundamental role. In this paper, we develop a reaction-diffusion-convection model to explore the influence of the reagents consumption (BrO in particular) in the inverse transition from chaos to periodicity. We demonstrated that, on the route towards thermodynamic equilibrium, the reagents concentration directly modulates the strength of the coupling between chemical kinetics and mass transport phenomena. An effective sequential decoupling (reaction-diffusion-convection --> reaction-diffusion --> reaction) takes place upon the reagents consumption and this is at the basis of the transition from chaos to periodicity.

Hofmeister Effect in Self-Organized Chemical Systems.

We studied the effect of spectator ions in the prototype of far-from-equilibrium self-organized chemical systems, the Belousov-Zhabotinsky (BZ) reaction. In particular, we investigated the specific ion effect of alkali metal cations, connoted for their kosmotropic and chaotropic properties. By means of combined experimental and numerical approaches, we could show a neat and robust evidence for the Hofmeister effect in this system. Spectator cations induce a marked increment of the induction period that preludes regular oscillations and decrease the oscillation amplitude following the sequence Li+ < Na+ ≪ K+ ∼ Cs+. These ions affect the system kinetics by interfering in the interaction between the oxidized form of the catalyst and the organic substrate, responsible for resetting the BZ system to pre-autocatalytic (reduced) conditions. The specific ion effect on these key reactive steps is systematically characterized and correlated with different parameters which describe the interaction of the cations with the solvent.

Bifurcations in spiral tip dynamics induced by natural convection in the Belousov-Zhabotinsky reaction.

The transition to spatial-temporal complexity exhibited by spiral waves under the effect of gravitational field in the Belousov-Zhabotinsky reaction is numerically studied on the basis of spiral tip dynamics. Successive transformations in tip trajectories are characterized as a function of the hydrodynamical parameter and attributed to a Ruelle-Takens-Newhouse scenario to chaos. The analysis describes the emergence of complexity in terms of the interplay between the evolution of the velocity field and concentration waves. In particular, (i) by mapping the tip motion in relation to some hydrodynamical pseudopotentials, the general mechanism by which the velocity field affects the tip trajectory is pointed out, and, (ii) by comparing the dynamical evolutions of local and mean properties associated with the inhomogeneous structures and to the velocity field, a surprising correlation is found. The results suggest that the reaction-diffusion-convection (RDC) coupling addresses the system to some general regimes, whose nature is imposed by the hydrodynamical contribution. More generally, RDC coupling would be formalized as the phenomenon that governs the system and drives it to chaos.

Dynamics of pattern formation in biomimetic systems.

This paper is an attempt to conceptualize pattern formation in self-organizing systems and, in particular, to understand how structures, oscillations or waves arise in a steady and homogenous environment, a phenomenon called symmetry breaking. The route followed to develop these ideas was to couple chemical oscillations produced by Belousov-Zhabotinsky reaction with confined reaction environments, the latter being an essential requirement for any process of Life. Special focus was placed on systems showing organic or lipidic compartments, which represent more reliable biomimetic matrices.

The effect of reactor geometry on frontal polymerization spin modes.

Using reactors of different sizes and geometries the dynamics of the frontal polymerization of 1,6-hexanediol diacrylate (HDDA) and pentaerythritol tetraacrylate (PETAC), with ammonium persulfate as the initiator were studied. For this system, the frontal polymerization exhibits complex behavior that depends on the ratio of the monomers. For a particular range of monomers concentration, the polymerization front becomes nonplanar, and spin modes appear. By varying the reactor diameter, we experimentally confirmed the expected shift of the system to a greater number of "hot spots" for larger diameters. For square test tubes a "zig-zag" mode was observed for the first time in frontal polymerization. We confirmed the viscosity-dependence of the spin mode instabilities. We also observed novel modes in cylinder-inside-cylinder reactors. Lastly, using a conical reactor with a continuously varying diameter, we observed what may be evidence for bistability depending on the direction of propagation. We discuss these finding in terms of the standard linear stability analysis for propagating fronts. (c) 2002 American Institute of Physics.

Ruelle-Takens-Newhouse scenario in reaction-diffusion-convection system.

Direct numerical simulations of the transition process from periodic to chaotic dynamics are presented for two variable Oregonator-diffusion model coupled with convection. Numerical solutions to the corresponding reaction-diffusion-convection system of equations show that natural convection can change in a qualitative way, the evolution of concentration distribution, as compared with convectionless conditions. The numerical experiments reveal distinct bifurcations as the Grashof number is increased. A transition to chaos similar to Ruelle-Takens-Newhouse scenario is observed. Numerical results are in agreement with the experiments.

An integrated framework for regional studies: emergy based spatial analysis of the Province of Cagliari.

This research proposes an integrated framework to investigate human-dominated systems and provide a basic approach to urban and regional studies in which the multiple interactions between economic and ecological processes are considered as a whole. Humans generate patterns of land use, infrastructures and other settings and redistribute ecosystem functions as flows of energy and matter for self-maintenance. To understand these emerging interactions between humans and ecological processes, human activities (e.g. transformation processes, land conversions, use of resources) and biophysical agents such as geomorphology, climate and natural cycles need to be considered. Emergy Analysis (spelled with an "m") is then used as an environmental accounting method to evaluate different categories of resource use with reference to their environmental cost. A case study of the Province of Cagliari (in the island of Sardinia, Italy) is reported and the procedure for allocating emergy flows, assigning them to districts and managing point data is discussed. Outcomes plotted on a map showed non-homogeneous spatial distribution of emergy flows throughout the region, suggesting the way ecosystem functions are affected and restructured by the human economy.

Chemical waves.

In our paper we try to describe the basic concepts of chemical waves and spatial pattern formation in a simple way. We pay particular attention to self-organisation phenomena in extended excitable systems. These result in the appearance of travelling waves, spiral waves, target patterns, Turing structures or more complicated structures called scroll waves, which are three-dimensional systems. We describe the most famous oscillating reaction, the Belousov-Zhabotinsky (BZ) reaction, in greater detail. This is because it is of great interest in both physical chemistry and in studies on the evolution and sustenance of self-organising biological systems.

Chemical waves and pattern formation in the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/water lamellar system.

The present work deals with the spatially extended oscillatory Belousov Zhabotinsky reaction-diffusion system carried out in an anisotropic environment of phosphatidylcholines/water binary system, which presents layered aqueous domains separated by lipid bilayers. We report the occurrence of stable Turing patterns, spiral waves, and other exotic structures in phospholipids bilayers that are generally used as a models for cell plasma membranes.