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1.
    
Breakup of spiral wave and consequent patterns induced by a strong polarized advective field are studied numerically. The dependences of the critical amplitude of the polarized advective field on its polarized modes and the frequency of the spiral wave are obtained. The effects of the circularly polarized advective field (CPAF, one of the polarized modes) are investigated especially. Under the modulation of a suitable CPAF, a new spiral may be reorganized after breakup, with rotating direction determined by the applied external CPAF. Moreover, hexagonal patterns are observed when the frequency of the CPAF is equal to that of the spiral and the amplitude is suitable. Our finding shows that the symmetry and chirality of the applied external field are important to the organization and selection of new-formed patterns.https://doi.org/10.1209/0295-5075/84/34002  相似文献   

2.
    
A thin liquid film covered with an insoluble surfactant in the vicinity of a first-order phase transition is discussed. Within the lubrication approximation we derive two coupled equations to describe the height profile of the film and the surfactant density. Thermodynamics of the surfactant is incorporated via a Cahn-Hilliard–type free-energy functional which can be chosen to describe a transition between two stable phases of different surfactant density. Within this model, a linear stability analysis of stationary homogeneous solutions is performed, and drop formation in a film covered with surfactant in the lower density phase is investigated numerically in one and two spatial dimensions.https://doi.org/10.1209/0295-5075/86/66003  相似文献   

3.
    
We study the non-equilibrium pattern formation that emerges when magnetically repelling colloids, trapped by optical tweezers, are abruptly released, forming colloidal explosions. For multiple colloids in a single trap we observe a pattern of expanding concentric rings. For colloids individually trapped in a line, we observe explosions with a zigzag pattern that persists even when magnetic interactions are much weaker than those that break the linear symmetry in equilibrium. Theory and computer simulations quantitatively describe these phenomena both in and out of equilibrium. An analysis of the mode spectrum allows us to accurately quantify the non-harmonic nature of the optical traps. Colloidal explosions provide a new way to generate well-characterized non-equilibrium behaviour in colloidal systems.https://doi.org/10.1209/0295-5075/94/48008  相似文献   

4.
    
Based on Dynamical Density Functional Theory (DDFT) we investigate a binary mixture of interacting Brownian particles driven over a substrate via a one-dimensional ratchet potential. The particles are modeled as soft spheres where one component carries a classical Heisenberg spin. In the absence of a substrate field, the system undergoes a first-order fluid-fluid demixing transition driven by the spin-spin interaction. We demonstrate that the interplay between the intrinsic spinodal decomposition and time-dependent external forces leads to a novel dynamical instability where stripes against the symmetry of the external potential form. This structural transition is observed for a broad range of parameters related to the ratchet potential. Moreover, we find intriguing effects for the particle transport.https://doi.org/10.1209/0295-5075/106/56004  相似文献   

5.
We demonstrate with the example of Cahn-Hilliard dynamics that the macroscopic kinetics of first-order phase transitions exhibits an infinite number of constants of motion. Moreover, this result holds in any space dimension for a broad class of nonequilibrium processes whose macroscopic behavior is governed by equations of the form /t = W(), where is an order parameter,W is an arbitrary function of , and is a linear Hermitian operator. We speculate on the implications of this result.  相似文献   

6.
    
We study flocking of self-propelled, interacting microorganisms with finite sizes and mass immersed in a turbulent flow. In the presence of the competing interactions of self-propulsion and the carrier turbulent flow, as is typical in nature, we show that including the effect of inertia is essential for the stability of flocks. We examine the problem from the point of view of global as well as local order and the statistics of the velocity of the microorganisms as a function of the inertia, the interaction radius, the level of self-propulsion as well as noise.https://doi.org/10.1209/0295-5075/112/24005  相似文献   

7.
    
Heap formations in a layered bi-disperse granular bed under vertical vibration with strength Γ have been studied experimentally. With the volume ratio (χ) between the two different types of grains and Γ as control parameters, different heap patterns can be created and details of the heap formation can be investigated. In particular, a periodic state in which the heap is being created and destroyed more or less periodically has been realized. This periodic heaping state is found to be related to two instabilities controlled by χ and Γ; namely the instability of a horizontal granular interface and the instability of a heap in a layered granular system. This latter instability would occur only in a layered system. A nonlinear phenomenological model is developed to understand these two instabilities and the predictions of the model are found to be in good agreement with experiments.https://doi.org/10.1209/0295-5075/100/44002  相似文献   

8.
This paper proposes a novel method to obtain frequency modulation (FM) signals from a single fringe pattern for phase retrieval. First, a 1D discrete Meyer wavelet is employed to decompose the pattern image signal row by row and the soft-thresholding approach is applied to remove noise. The low frequency coefficients of the wavelet decomposition are then set to 0, and the signal is reconstructed. Moreover, the optimal wavelet decomposition level is adaptively determined using a cost function-based method. The reconstructed signal, which no longer contains a background component, is normalized using a nonlinear and piecewise normalization method. The proposed method is faster and more accurate than some other phase retrieval approaches, which is illustrated with two test cases.  相似文献   

9.
    
We investigate the dynamical coupling between the motion and the deformation of a single self-propelled domain based on two different model systems in two dimensions. One is represented by the set of ordinary differential equations for the center of gravity and two tensor variables characterizing deformations. The other is an active cell model which has an internal mechanism of motility and is represented by the partial differential equation for deformations. Numerical simulations show a rich variety of dynamics, some of which are common to the two model systems. Possible experimental realization of the predictions is discussed.https://doi.org/10.1209/0295-5075/91/20001  相似文献   

10.
    
The physics of the breakdown of a green wave (GW) in a city is revealed. We have found that there are two regions of flow rates in the GW within which the GW breakdown is possible. In the region of larger flow rates bounded by the maximum capacity and threshold flow rate, a time-delayed spontaneous GW breakdown occurs with some probability during a given observation time. In the region of smaller flow rates bounded by the threshold flow rate and minimum capacity, only an induced GW breakdown is possible. The GW breakdown can be simulated with traffic flow models in the context of either three-phase or two-phase theories. However, in three-phase models the spontaneous GW breakdown is initiated by the emergence of a moving synchronized flow pattern (MSP); the MSP results from an initial speed disturbance at the beginning of the GW. In contrast, in two-phase models no MSPs can occur, therefore, considerably heavier conditions for both spontaneous and induced GW breakdowns are found.https://doi.org/10.1209/0295-5075/102/28010  相似文献   

11.
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Ying-Yuan Deng 《中国物理 B》2021,30(8):88101-088101
Grain boundary directed spinodal decomposition has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The morphological selection mechanism of spinodal decomposition at grain boundaries is a major challenge to reveal, and remains elusive so far. In this work, the effect of grain boundaries on spinodal decomposition is investigated by using the phase-field model. The simulation results indicate that the spinodal morphology at the grain boundary is anisotropic bicontinuous microstructures different from the isotropic continuous microstructures of spinodal decomposition in the bulk phase. Moreover, at grain boundaries with higher energy, the decomposed phases are alternating α/β layers that are parallel to the grain boundary. On the contrary, alternating α/β layers are perpendicular to the grain boundary.  相似文献   

12.
    
The coiling of a thin filament of viscous fluid falling onto a surface is a common and easily reproducible hydrodynamic instability. Here we report for the first time that this instability can generate regular spiral patterns, in which air bubbles are trapped in the coil and then advected horizontally by the fluid spreading on the surface. We present a simple model that explains how these beautiful patterns are formed, and how the number of spiral branches and their curvature depends on the coiling frequency, the frequency of rotation of the coiling center, the total flow rate, and the thickness of the spreading fluid film.https://doi.org/10.1209/0295-5075/81/38004  相似文献   

13.
    
Complex shapes can occur through successions of instabilities, like in the growth of chemical gardens, where a semi-permeable membrane precipitates at the interface between a sodium silicate solution and a metal salt. Instead of letting the osmotic pressure during the metal salt dissolution lead the dynamics of the growth, we inject a ferric sulfate solution into a sodium silicate solution, both of controlled concentration, controlling also the other hydrodynamical parameters. Although qualitatively distinct regimes can be obtained, we focus here on a previously unobserved regime where the reactive interface grows in tubular fingering patterns with ocean-ridge-like dynamics: the tubes grow evenly on both sides of a central fracture where the silica deposits continuously. Our experiments show that the whole dynamics is intrinsically related to the precipitation occurring at the interface: the tubes elongation rate remains constant even when the injection rate is varying, but strongly depends on the limiting concentration in injected solution, thus on the reaction rate.https://doi.org/10.1209/0295-5075/89/44004  相似文献   

14.
    
We propose a new approach for the denoising of a phase fringe pattern recorded in an optical interferometric setup. The phase fringe pattern which is generally corrupted by high amount of speckle noise is first converted into an exponential phase field. This phase field is divided into a number of overlapping patches. Owing to the small size of each patch, the presence of a simple structure of the interference phase is assumed in it. Accordingly, the singular value decomposition (SVD) of the patch allows us to separate the signal and noise components effectively. The patch is reconstructed only with the signal component. In order to further improve the robustness of the proposed method, an enhanced data matrix is generated using the patch and the SVD of this enhanced matrix is computed. The matrix enhancement results in an increased dimension of the noise subspace which thus accommodates more amount of noise component. Reassignment of the filtered pixels of the preceding patch in the current patch improves the noise filtering accuracy. The fringe denoising capability in function of the noise level and the patch size is studied. Simulation and experimental results are provided to demonstrate the practical applicability of the proposed method.  相似文献   

15.
    
Suspensions of small anisotropic particles, “rheoscopic fluids”, are used for flow visualisation. By illuminating the fluid with light of three different colours, it is possible to determine Poincaré indices for vector fields formed by the longest axis of the particles. Because this vector field is non-oriented, half-integer Poincaré indices are possible, and are observed experimentally. An exact solution for the direction vector appears to preclude the existence of topological singularities. However, we show that upon averaging over the random initial orientations of particles, singularities with half-integer Poincaré index appear. We describe their normal forms.https://doi.org/10.1209/0295-5075/89/34003  相似文献   

16.
    
We investigate the dynamics of a liquid film flowing down a uniformly heated wall. We use a four-field weighted-average model for the film thickness, the surface temperature and the two-dimensional flow rate vector. Time-dependent simulations are rationalized in a phase diagram. For small Reynolds numbers we observe regularly spaced rivulets aligned with the flow that prevent the development of hydrodynamic waves while for large Reynolds numbers, the evolution is similar to that observed in the isothermal case, i.e. wave-dominated. The transition between the two regimes is characterized by an intricate-pattern formation dynamics, including regimes of quasi-2D solitary pulses riding the crests of rivulets.https://doi.org/10.1209/0295-5075/84/64002  相似文献   

17.
    
In their way to/from turbulence, plane wall-bounded flows display an interesting transitional regime where laminar and turbulent oblique bands alternate, the origin of which is still mysterious. In line with Barkley's recent work about the pipe flow transition involving reaction-diffusion concepts, we consider plane Couette flow in the same perspective and transform Waleffe's classical four-variable model of self-sustaining process into a reaction-diffusion model. We show that, upon fulfillment of a condition on the relative diffusivities of its variables, the featureless turbulent regime becomes unstable against patterning as the result of a Turing instability. A reduced two-variable model helps us to delineate the appropriate region of parameter space. An intrinsic status is therefore given to the pattern's wavelength for the first time. Virtues and limitations of the model are discussed, calling for a microscopic support of the phenomenological approach.https://doi.org/10.1209/0295-5075/98/64001  相似文献   

18.
    
This paper examines through numerical simulations the impact of a solutal Marangoni effect on the deposit obtained from a polymer solution. A hydrodynamical model with lubrication approximation is used to describe the liquid phase in a dip-coating–like configuration. The studied case considers evaporation in stagnant air (diffusion-limited evaporation), which results in a coupling between the liquid and gas phases. Viscosity, surface tension, and saturated vapor pressure depend on the solute concentration. In the evaporative regime, when the surface tension increases with the polymer concentration, the Marangoni effect induces a periodic regime. This results in a self-organized periodic patterning of the dried film in certain control parameter ranges. A morphological phase diagram as well as meniscus and dry-deposit shapes are provided as a function of the substrate velocity and bulk solute concentration.https://doi.org/10.1209/0295-5075/103/14001  相似文献   

19.
    
We present the results of an experimental study of pattern formation in horizontally oscillating granular suspensions. Starting from a homogeneous state, the suspension turns into a striped pattern within a specific range of frequencies and amplitudes of oscillation. We observe an initial development of layered structures perpendicular to the vibration direction and a gradual coarsening of the stripes. However, both processes gradually slow down and eventually saturate. The probability distribution of the stripe width P(w) approaches a nonmonotonic steady-state form which can be approximated by a Poisson distribution. We observe similar structures in MD simulations of soft spherical particles coupled to the motion of the surrounding fluid.https://doi.org/10.1209/0295-5075/107/34006  相似文献   

20.
    
Complex ecosystems generally consist of a large number of different species utilizing a large number of different resources. Several of their features cannot be captured by models comprising just a few species and resources. Recently, Tikhonov and Monasson have shown that a high-dimensional version of MacArthur's resource competition model exhibits a phase transition from a “vulnerable” to a “shielded” phase in which the species collectively protect themselves against an inhomogeneous resource influx from the outside. Here we point out that this transition is more general and may be traced back to the existence of non-negative solutions to large systems of random linear equations. Employing Farkas' Lemma we map this problem to the properties of a fractional volume in high dimensions which we determine using methods from the statistical mechanics of disordered systems.https://doi.org/10.1209/0295-5075/124/18004  相似文献   

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