Oscillatory Reaction-diffusion Systems Research Articles

Excited state of spiral waves in oscillatory reaction-diffusion systems caused by a pulse.

Previous studies claim that the dynamic behaviors of spiral waves are uniquely determined by the nature of the medium, which can be determined by control parameters. In this article, the authors break from the previous view and present an alternate stable state of spiral waves, named the excited state. The authors find that two states of the spiral wave switch to each other after a one-off pulse is applied to the medium. The dynamic behaviors of the two states are quite different, specifically, the spiral tip trajectory of the original spiral, which is named the ground-state spiral as observed in the previous studies, is a point, while the spiral tip trajectory of the excited-state spiral is a circle. Moreover, the authors study the trajectories of the spiral tip of spiral waves in both states after the pulse is applied and find two types of trajectories, a spiral trajectory and a spiral-inward-petal trajectory. The frequency of the spiral wave in the excited state is less than that in the ground state. The findings enrich the dynamics of pattern formation.

Noise-induced standing waves in oscillatory systems with time-delayed feedback.

In oscillatory reaction-diffusion systems, time-delay feedback can lead to the instability of uniform oscillations with respect to formation of standing waves. Here, we investigate how the presence of additive, Gaussian white noise can induce the appearance of standing waves. Combining analytical solutions of the model with spatiotemporal simulations, we find that noise can promote standing waves in regimes where the deterministic uniform oscillatory modes are stabilized. As the deterministic phase boundary is approached, the spatiotemporal correlations become stronger, such that even small noise can induce standing waves in this parameter regime. With larger noise strengths, standing waves could be induced at finite distances from the (deterministic) phase boundary. The overall dynamics is defined through the interplay of noisy forcing with the inherent reaction-diffusion dynamics.

A Local, Self-Organizing Reaction-Diffusion Model Can Explain Somite Patterning in Embryos

During somitogenesis in embryos, a posteriorly moving differentiation front arrests the oscillations of "segmentation clock" genes, leaving behind a frozen, periodic pattern of expression stripes. Both mathematical theories and experimental observations have invoked a "clock and wavefront" model to explain this phenomenon, in which long-range molecular gradients control the movement of the front and therefore the placement of the stripes in the embryo. Here, we develop a fundamentally different model-a progressive oscillatory reaction-diffusion (PORD) system driven by short-range interactions. In this model, posterior movement of the front is a local, emergent phenomenon that, in contrast to the clock and wavefront model, is not controlled by global positional information. The PORD model explains important features of somitogenesis, such assize regulation, that previous reaction-diffusion models could not explain. Moreover, the PORD and clock and wavefront models make different predictions about the results of FGF-inhibition and tissue-cutting experiments, and we demonstrate that the results of these experiments favor the PORD model.

Inwardly rotating spirals in nonuniform excitable media

Inwardly rotating spirals (IRSs) have attracted great attention since their observation in an oscillatory reaction-diffusion system. However, IRSs have not yet been reported in planar excitable media. In the present work we investigate rotating waves in a nonuniform excitable medium, consisting of an inner disk part surrounded by an outer ring part with different excitabilities, by numerical simulations of a simple FitzHugh-Nagumo model. Depending on the excitability of the medium as well as the inhomogeneity, we find the occurrence of IRSs, of which the excitation propagates inwardly to the geometrical spiral tip.

Inducing and modulating spiral waves by delayed feedback in a uniform oscillatory reaction–diffusion system

Spiral waves are induced by delayed feedback in a reaction–diffusion system spontaneously exhibiting a uniform bulk oscillation. Feedback can modulate the spiral wavelength and tip trajectory. For a relatively large feedback intensity ( g > 0.45), the spiral wavelength increases monotonously with an increase in delay time. However, when g < 0.45, the wavelength depends non-monotonously on delay time. Spiral waves are rigid for a moderate delay time, and their tip trajectory radius increases with increasing delay time. When delay time exceeds some critical values, spiral waves begin to meander. The traveling wave dispersion relation of the delayed system is computed to account for the spiral wavelength variation with delay time.

Noise-induced suppression of periodic travelling waves in oscillatory reaction–diffusion systems

Ecological field data suggest that some species show periodic changes in abundance over time and in a specific spatial direction. Periodic travelling waves as solutions to reaction–diffusion equations have helped to identify possible scenarios, by which such spatio-temporal patterns may arise. In this paper, such solutions are tested for their robustness against an irregular temporal forcing, since most natural populations can be expected to be subject to erratic fluctuations imposed by the environment. It is found that small environmental noise is able to suppress periodic travelling waves in stochastic variants of oscillatory reaction–diffusion systems. Irregular spatio-temporal oscillations, however, appear to be more robust and persist under the same stochastic forcing.

Perturbation analysis of the Kuramoto phase-diffusion equation subject to quenched frequency disorder

The Kuramoto phase-diffusion equation is a nonlinear partial differential equation which describes the spatiotemporal evolution of a phase variable in an oscillatory reaction-diffusion system. Synchronization manifests itself in a stationary phase gradient where all phases throughout a system evolve with the same velocity, the synchronization frequency. The formation of concentric waves can be explained by local impurities of higher frequency which can entrain their surroundings. Concentric waves in synchronization also occur in heterogeneous systems, where the local frequencies are distributed randomly. We present a perturbation analysis of the synchronization frequency where the perturbation is given by the heterogeneity of natural frequencies in the system. The nonlinearity in the form of dispersion leads to an overall acceleration of the oscillation for which the expected value can be calculated from the second-order perturbation terms. We apply the theory to simple topologies, like a line or sphere, and deduce the dependence of the synchronization frequency on the size and the dimension of the oscillatory medium. We show that our theory can be extended to include rotating waves in a medium with periodic boundary conditions. By changing a system parameter, the synchronized state may become quasidegenerate. We demonstrate how perturbation theory fails at such a critical point.

Absolute Stability of Wavetrains Can Explain Spatiotemporal Dynamics in Reaction-Diffusion Systems of Lambda-Omega Type

The lambda-omega class of reaction-diffusion equations has received considerable attention because they are more amenable to mathematical investigation than other oscillatory reaction-diffusion systems and include the normal form of any reaction-diffusion system with scalar diffusion close to a standard supercritical Hopf bifurcation. Despite this, detailed studies of the dynamics predicted by numerical simulations have mostly been restricted to regions of parameter space in which stable wavetrains (periodic traveling waves) are selected by the initial or boundary conditions; we use the term “stability” to denote spectral stability on the real line. Here we consider the emergent spatiotemporal dynamics on large bounded domains, with Dirichlet conditions at one boundary and Neumann conditions at the other. Previous studies have established a parameter threshold below which stable wavetrains are generated by the Dirichlet boundary condition. We use numerical continuation techniques to analyze the spectral stability of wavetrain solutions, and we identify a second stability threshold, above which the selected wavetrain is absolutely unstable. In addition, we prove that the onset of absolute stability always occurs through a complex conjugate pair of branch points in the absolute spectrum, which greatly simplifies the detection of this threshold. In the parameter region in which the spectra of the selected waves indicate instability but absolute stability, our numerical simulations predict so-called “source-sink” dynamics: bands of visibly regular periodic traveling waves that are separated by localized defects. Beyond the absolute stability threshold our simulations predict irregular spatiotemporal behavior.

A comparison of periodic travelling wave generation by Robin and Dirichlet boundary conditions in oscillatory reaction-diffusion equations

Periodic travelling waves are an important solution form in oscillatory reaction-diffusion equations. I have shown previously that such waves arise naturally near a boundary at which a Dirichlet condition is applied. This result has applications in ecology, providing a potential explanation for the periodic waves seen in a number of natural populations. However, in ecological applications the Dirichlet boundary condition typically arises as a simple approximation to a more realistic Robin condition. In this paper, I consider the generation of periodic travelling waves by Robin boundary conditions and how the wave amplitude compares with that arising from Dirichlet conditions. I study a 'λ-ω' system of equations, which is the normal form of an oscillatory reaction-diffusion system with scalar diffusion matrix close to a Hopf bifurcation. I consider a Robin boundary condition close to the Dirichlet limit, with proximity measured by a small parameter e, and I study the equations as a perturbation problem in this small parameter. I show that the perturbation is singular and that although the solution itself changes at 0(e), the amplitude of the periodic travelling wave which this solution approaches far from the boundary is unchanged at both O(∈) and O(∈ 2 ). This provides strong justification for the use of the Dirichlet approximation to the Robin condition when studying periodic travelling wave generation in equations of λ-ω type. Finally, I discuss the ecological applications of the results.

Pattern formation in 4:1 resonance of the periodically forced CO oxidation on Pt(110)

Periodically forced oscillatory reaction-diffusion systems may show complex spatiotemporal patterns. At high-frequency resonant forcing, multiple-phase patterns can be found. In the present work, the dynamics of turbulent CO oxidation on Pt(110), forced with the fourth harmonic of the system's natural frequency, is investigated. Experiments result in subharmonic entrainment, where the system locks to a quarter of the forcing frequency. Cluster patterns are observed, where different parts of the pattern show a defined phase difference. The experimental results are compared with numerical simulations using the realistic Krischer-Eiswirth-Ertl model for catalytic CO oxidation. Using the fourth harmonic of an uncoupled surface element's natural frequency, we find 3:1 entrainment with three-phase cluster patterns in a wide parameter range of forcing amplitudes and frequency detuning. Numerical analysis of the spatially extended, turbulent system reveals a remarkable upshift of the mean oscillation frequency compared to homogeneous oscillations. Using the fourth harmonic of the most prominent frequency found in the turbulent system results in four-phase patterns with partial or full 4:1 entrainment, depending on the forcing parameters chosen.

Bifurcation analysis of bistability between spatially uniform and non-uniform electrochemical oscillations

Experimental and theoretical studies for bistability between spatially uniform and non-uniform electrochemical oscillations are performed, by taking a current oscillation of N-NDR type, appearing in H 2O 2 reduction on platinum (Pt) ring electrodes, as a model system. Nonlinear bifurcation analysis of the model equations revealed that the bistability occurs in regions where both spatially uniform and non-uniform oscillations are stable. The strategy for understanding of the bistability can be applied generally to oscillatory reaction-diffusion systems.

Fronts between Rhythms: Spatiotemporal Dynamics of Extended Polyrhythmic Media

We study the propagation of fronts in extended oscillatory reaction-diffusion systems that contain several coexisting limit cycles. In contrast with the variational behavior, fronts between regions oscillating in two different limit cycles are found to propagate not necessarily towards the region of the less stable limit cycle, but towards the regions of the largest amplitudes, provided that the frequency mismatch between the cycles is sufficiently large. In other words, the smaller oscillations can always be made to control the whole system.

Control of turbulence in oscillatory reaction-diffusion systems through a combination of global and local feedback

Global time-delay autosynchronization is known to control spatiotemporal turbulence in oscillatory reaction-diffusion systems. Here, we investigate the complex Ginzburg-Landau equation in the regime of spatiotemporal turbulence and study numerically how local or a combination of global and local time-delay autosynchronization can be used to suppress turbulence by inducing uniform oscillations. Numerical simulations show that while a purely local control is unsuitable to produce uniform oscillations, a mixed local and global control can be efficient and also able to create other patterns such as standing waves, amplitude death, or traveling waves.

Traveling and standing patterns induced by delay feedback in uniform oscillatory reaction–diffusion system

The influence of delay feedback (DF) on pattern formation in reaction–diffusion system is investigated in the parametric domain where the undelayed system exhibits uniform oscillations. Depending on the feedback parameters, both traveling patterns and standing patterns are observed. The linear stability analysis implies the traveling patterns arise from an oscillatory instability at a finite wavenumber (wave instability), whereas the standing patterns are formed due to another type of instability – the instability of homogeneous limit cycle with respect to spatial perturbations. It is demonstrated that, rather than stabilizing the system, the DF can unusually induce two kinds of instability.

The effects of unequal diffusion coefficients on periodic travelling waves in oscillatory reaction–diffusion systems

Many oscillatory biological systems show periodic travelling waves. These are often modelled using coupled reaction–diffusion equations. However, the effects of different movement rates (diffusion coefficients) of the interacting components on the predictions of these equations are largely unknown. Here we investigate the ways in which varying the diffusion coefficients in such equations alters the wave speed, time period, wavelength, amplitude and stability of periodic wave solutions. We focus on two sets of kinetics that are commonly used in ecological applications: lambda–omega equations, which are the normal form of an oscillatory coupled reaction–diffusion system close to a supercritical Hopf bifurcation, and a standard predator–prey model. Our results show that changing the ratio of the diffusion coefficients can significantly alter the shape of the one-parameter family of periodic travelling wave solutions. The position of the boundary between stable and unstable waves also depends on the ratio of the diffusion coefficients: in all cases, stability changes through an Eckhaus (‘sideband’) instability. These effects are always symmetrical in the two diffusion coefficients for the lambda–omega equations, but are asymmetric in the predator–prey equations, especially when the limit cycle of the kinetics is of large amplitude. In particular, there are two separate regions of stable waves in the travelling wave family for some parameter values in the predator–prey scenario. Our results also show the existence of a one-parameter family of travelling waves, but not necessarily a Hopf bifurcation, for all values of the diffusion coefficients. Simulations of the full partial differential equations reveals that varying the ratio of the diffusion coefficients can significantly change the properties of periodic travelling waves that arise from particular wave generation mechanisms, and our analysis of the travelling wave families assists in the understanding of these effects.

Dispersion relation in oscillatory reaction-diffusion systems with self-consistent flow in true slime mold

In the large amoeboid organism Physarum, biochemical oscillators are spatially distributed throughout the organism and their collective motion exhibits phase waves, which carry physiological signals. The basic nature of this wave behaviour is not well-understood because, to date, an important effect has been neglected, namely, the shuttle streaming of protoplasm which accompanies the biochemical rhythms. Here we study the effects of self-consistent flow on the wave behaviour of oscillatory reaction-diffusion models proposed for the Physarum plasmodium, by means of numerical simulation for the dispersion relation and weakly nonlinear analysis for derivation of the phase equation. We conclude that the flow term is able to increase the speed of phase waves (similar to elongation of wave length). We compare the theoretical consequences with real waves observed in the organism and also point out the physiological roles of these effects on control mechanisms of intracellular communication.

Inwardly rotating spiral wave breakup in oscillatory reaction-diffusion media

The breakup of inwardly rotating spiral waves has been investigated in an oscillatory reaction-diffusion system near a Hopf bifurcation point. The breakup first occurred at the region far away from the core area, then gradually involved the whole medium by increasing the diffusion coefficient ratio between the two components of the oscillator system. With the approximation of the Complex-Ginzburg-Landau equation (CGLE), the criteria for the occurrence of the inwardly rotating spiral wave are examined theoretically. The analysis of the stability in the corresponding CGLE revealed that the breakup of the inward spiral wave was related to the Eckhaus instability.

Target patterns in two-dimensional heterogeneous oscillatory reaction–diffusion systems

Properties of target patterns created by pacemakers, representing local regions with the modified oscillation frequency, are studied for two-dimensional oscillatory reaction–diffusion systems described by the complex Ginzburg–Landau equation. An approximate analytical solution, based on the phase dynamics approximation, is constructed for a circular core and compared with numerical results for circular and square cores. The dependence of the wavenumber and frequency of generated waves on the size and frequency shift of the pacemaker is discussed. Instabilities of target patterns, involving repeated creations of ring-shaped amplitude defects, are further considered.

Turing Instability Leads Oscillatory Systems to Spatiotemporal Chaos

We present that Turing instability can lead oscillatory reaction-diffusion (RD) systems to spatiotemporal chaos instead of spatially periodic steady states. Similar onset of spatiotemporal chaos was discovered in an equation describing seismic waves. We demonstrate that the seismic equation can be derived from a certain class of oscillatory RD systems in the neighborhood of a codimension-two Turing-Benjamin-Feir point. Also, we show numerical studies of reduced equations and discuss robustness of this spatiotemporal chaos.

Front explosion in a periodically forced surface reaction

Resonantly forced oscillatory reaction-diffusion systems can exhibit fronts with complicated interfacial structure separating phase-locked homogeneous states. For values of the forcing amplitude below a critical value the front "explodes" and the width of the interfacial zone grows without bound. Such front explosion phenomena are investigated for a realistic model of catalytic CO oxidation on a Pt(110) surface in the 2:1 and 3:1 resonantly forced regimes. In the 2:1 regime, the fronts are stationary and the front explosion leads to a defect-mediated turbulent state. In the 3:1 resonantly forced system, the fronts propagate. The front velocity tends to zero as the front explosion point is reached and the final asymptotic state is a 2:1 resonantly locked labyrinthine pattern. The front dynamics described here should be observable in experiment since the model has been shown to capture essential features of the CO oxidation reaction.