Formation Of Spatial Structures Research Articles

Materials surface modification using quasi-stationary plasma accelerators

Quasi-stationary plasma accelerators of a new generation were studied for the capability to modify surface properties of materials widely applied in engineering (carbon steels) and in microelectronics (silicon). The action of high-energy plasma flow was shown to result in hardening the samples from carbon steels, the depth (∼0.1–0.3 mm) and microhardness (more than 10 000 MPa) of the modified surface layer being sufficient for practical purposes. Exposure of silicon wafer to medium-power compression plasma flow results in the formation of periodic sub-micron spatial structures on its surface. The main factors ensuring efficient modification of various materials under the compression plasma flow action are shown to be the rapid heating of surface due to kinetic energy thermalization of compression plasma flow during its deceleration, keeping temperature and pressure at necessary levels until the completion of physico-chemical transformations in the surface layer, and the fast crystallization of the molten layer in the presence of magnetic fields induced by the currents ‘swept-away’ from the discharge device with the compression flow.

Materials surface modification using quasi-stationary plasma accelerators

Quasi-stationary plasma accelerators of a new generation were studied for the capability to modify surface properties of materials widely applied in engineering (carbon steels) and in microelectronics (silicon). The action of high-energy plasma flow was shown to result in hardening the samples from carbon steels, the depth (∼0.1–0.3 mm) and microhardness (more than 10 000 MPa) of the modified surface layer being sufficient for practical purposes. Exposure of silicon wafer to medium-power compression plasma flow results in the formation of periodic sub-micron spatial structures on its surface. The main factors ensuring efficient modification of various materials under the compression plasma flow action are shown to be the rapid heating of surface due to kinetic energy thermalization of compression plasma flow during its deceleration, keeping temperature and pressure at necessary levels until the completion of physico-chemical transformations in the surface layer, and the fast crystallization of the molten layer in the presence of magnetic fields induced by the currents ‘swept-away’ from the discharge device with the compression flow.

Periodic local disturbance in host–parasitoid metapopulations: host suppression and parasitoid persistence

Within many agricultural systems, insect pests and their natural enemies are forced to persist as a metapopulation, continuously recolonizing patches following disturbance through harvesting or spraying with insecticides. Despite the need to understand factors influencing biocontrol success, few theoretical studies of host–parasitoid interactions have examined the potential impact of local disturbance within a metapopulation framework. Here, we add periodic local mortality to series of classical host–parasitoid models to examine its effect on host suppression and parasitoid persistence. Using a deterministic lattice model, we show that despite the wide range of complex dynamics generated at the patch level, the region wide pattern of disturbance is the key factor influencing host suppression. The level of host suppression achieved can be understood in terms of both the strength of density dependent parasitism, and the relative amounts of host and parasitoid mixing amongst patches of different ages. Local dispersal among patches is sufficient to ensure coexistence of the host and parasitoid, though persistence is not necessarily associated with the formation of self-organized spatial structures reported in previous studies. Finally, a stochastic version of the model is developed, in order to highlight how the effects of demographic stochasticity may influence biocontrol success in highly disturbed agricultural systems.

Manipulation and removal of defects in spontaneous optical patterns.

Defects play an important role in a number of fields dealing with ordered structures. We demonstrate theoretically and experimentally the possibility of an active manipulation of defects in terms of an externally induced motion. We focus on the spontaneous formation of two-dimensional spatial structures in a nonlinear-optical system, a liquid crystal light valve under single optical feedback. For a particular parameter setting, a spontaneously formed hexagonal intensity pattern contains several dislocation-type defects. A scheme based on Fourier filtering allows us to restore spatial order in a selectable part of the pattern. Starting without control, the controlled area is progressively expanded, such that defects are swept out of the pattern.

MODELS OF FREE COOLING GRANULAR GASES

We consider the free evolution of systems of granular particles whose dynamics is characterized by a collision rule which preserves the total momentum, but dissipates the kinetic energy. Starting from an inelastic version of a minimal model proposed by Ulam for a gas of Maxwell molecules, we introduce a new lattice model aimed at investigating the role of dynamical correlations and the onset of spatial order induced by the inelasticity of the interactions. We study, in one- and two-dimensional cases, the velocity distribution, the decay of the energy, the formation of spatial structures and topological defects. Finally, we relate our findings to other models known in other fields.

Pattern formation and spatial self-entrainment in bistable chemical systems.

We describe the formation of spatial structures generated by diffusive instabilities in bistable systems. The coupling between the different spatial modes emanating from the two homogeneous steady states can then give rise to self-parametric instabilities favoring the occurrence of resonant rhombic or quasiperiodic structures such as superlattices or quasicrystalline patterns.

Cluster Formation of Colloids in Nematics

Colloidal suspension show interesting phenomena which are mediated by the solvent. Recently, colloids in nematic liquid crystal phase used as anisotropic solvent have been investigated experimentally and have inspired theoretical work[1-3]. In relation to these work, effective pair potential between spherical colloidal particles suspended in nematic liquid crystals has been suggested[4,5]. The interparticle interaction can cause nontrivial collective behavior that results in formation of new spatial structures in colloidal particle systems. We determine these structures by particle dynamics simulations using these effective pair-interaction.

Spatial structures of continuous microwave discharge

The paper deals with the results of investigations of spatial structures of continuous microwave discharge in a quasi-optical resonator. The results are given of experimental observations and easurements of the parameters of plasma in discharges of different forms, and the reasons are analyzed for the formation of spatial discharge structures. It is demonstrated that, as a result of the plasma-resonance amplification of the field, the discharge makes a transition to the contracted state with a size that is much less than the microwave-frequency wavelength and with an electron concentration in excess of the critical. It is found that the stratification of the contracted state across the electric field vector, which arises in some gases, is caused by the development of thermoelectric-current instability that was not previously observed in microwave discharges.

Super-réseaux et quasi-cristaux dans les systèmes bistables

We describe the formation of spatial structures generated by diffusive instabilities in bistable systems. It is shown that the coupling between the different spatial modes emanating from the two homogeneous states can give rise to self-parametric instabilities favoring the occurrence of resonant quasiperiodic structures such as superlattices or quasipatterns.

Plasma spots in a gas discharge system: birth, scattering and formation of molecules

Experiments on a planar gas discharge device, where formation of spatial structures is of the Turing origin, demonstrate that particle-like states of a pattern can scatter at each other, form bound (“molecular”) states that propagate over the active area of the device, and generate additional quasi-particles in the course of collisions.

Optical pattern formation in alkali metal vapors: Mechanisms, phenomena and use

The phenomena in optical pattern formation are reviewed using a single mirror feedback experiment as an example. The nonlinear medium is sodium metal vapor. We discuss the mechanism responsible for the spontaneous formation of spatial structures and the selection between simple periodic patterns as hexagons and squares. Introducing a polarization changing element in the feedback loop, quasiperiodic patterns with a twelve-fold rotational symmetry are obtained and their properties are studied by filtering methods in Fourier space. Finally, we discuss the formation of localized states as spatial solitons due to a self-induced lensing effect. Their interaction is studied and the formation of clusters provides a link between localized states and extended patterns. Genuine optical features, such as the polarization degrees of freedom, the experimental accessibility of the Fourier space, and the possibility to control the optical susceptibility of atomic vapors by means of an external magnetic field, provide very powerful tools for investigating the principles of pattern formation.

Formation of laser-induced localized spatial structures in semiconductors in the presence of optical bistability

The article analyzes the formation of localized spatial structures in an optically bistable resonance-free system with a nonlinear temperature dependence of the relaxation time of free charge carries. The light-induced electrical field does not introduce qualitative changes in their characteristics.

Structure formation in a zonal barotropic current: a treatment via the centre manifold reduction

Formation of spatial structures in zonal barotropic currents is a classical problem of dynamic meteorology. It is of fundamental importance in the forecasting of lowfrequency variability of climate...

Dynamics of spatial structures in arrays of nonlocally coupled bistable units

We present the results of investigation of a dynamical system consisting of nonlocally coupled bistable units. The dynamics of spatial structures in an array with a gradually increasing coupling coefficient is studied. The formation of spatial structures in the case of strong nonlocal coupling between units is examined.

Optical Diffraction-Free Patterns Induced by a Discrete Translational Transport

We report the formation of spatial structures induced by a spatial offset in a two-dimensional nonlinear optical interferometer in which diffractive effects are absent. The observed patterns arise from the destabilization of wide, continuous bands in the Fourier spectrum. The selection of these bands is shown to be determined by the interplay between the amount of spatial offset and the strength of the diffusion in the nonlinear optical material.

New experimental data on the possibility of influencing fluctuational particle fluxes in a L-2M stellarator edge plasma

The results of an experiment show that in the edge plasma of a L-2M stellarator the radial electric field is one of the determining factors influencing the formation of spatial structures and fluctuational particle fluxes. A point where the poloidal velocity of the fluctuations reverses is observed in an experiment with a high neutral-gas flux from the chamber walls.

Unusual electromechanical effects in glycine

Unusual piezoresponse signals have been observed in glycine aminoacid powder at a frequency near 10 MHz, which exhibited a regular pattern in time determined by a periodic phase variation in the elastic vibrations of individual powder particles. This phenomenon results from the formation of spatial structures in glycine powder under the action of a strong rf field.

Statistical description of model systems of interacting particles and phase transitions accompanied by cluster formation

We develop an approach to the statistical description of a system of interacting particles in order to describe spatially inhomogeneous structures. A criterion is proposed for selecting system states whose contributions in the partition function are dominant. A nonperturbative calculation of the partition function is demonstrated. The known results for various systems (hard sphere model, gravitating gas, etc.) are reproduced. Spatially inhomogeneous system states are considered. The conditions for the phase transition accompanied with cluster formation are found for model systems. Cluster size distribution and cluster interaction residual energy are estimated. The formation of new spatial structures in a cluster system is considered.

On spatial structures formation in dusty plasmas

A dynamical model is proposed, which treats the formation of spatial structures as the result of a dissipative instability of charged particle distribution in three-component nonideal dusty plasma. Instability arises due to the correlation interaction between dust particles taken into account in the framework of the diffusion approach. The expressions for the characteristic spatial scale and the effective dust diffusion coefficient are found.

Localized structures in optical parametric oscillation

Formation of one-dimensional spatial structures in degenerate optical parametric oscillation due to diffraction effects is discussed and their analogy with parametrically excited localized structures studied in hydrodynamics is illustrated. It is shown that, when the emergence of roll paterns is inhibited by cavity constraints, localized structures in the form of bright or dark solitary waves for the signal field may be generated for appropriate detunings of the pump field from a cavity resonance. The dark waves exist when bifurcation is supercritical and are stable close to threshold, whereas bright waves spontaneously arise in the subcritical case for large pump detunings and below threshold for oscillation due to a modulational instability of the homogeneous state.