Interplay between the glass and the gel transition

By changing the control parameters, many physical systems reach a slow dynamics regime followed by an arrested or a quasiarrested state. Examples, among others, are gels and glasses. In this paper, we discuss some experimental and theoretical results in polymer and colloidal systems, where gel and glass transitions interfere, and use models from Mode Coupling Theory(MCT)to illustrate the rich phenomenology observed. The continuous and the discontinuous transition lines, found in the MCT models,are considered suitable to describe respectively the gel and the glass transitions, so we suggest that the interplay between gel and glass may be interpreted in terms of the F_(13) MCT model, clarifying also the origin of logarithmic decays often observed in such systems. In particular, the theoretical predictions of the MCT in the F_(13) model are compared with Molecular Dynamics simulations in model systems for chemical gels and charged colloids.     

基于FRFT域分形特征的压制干扰存在性检测（英）

基于分数阶傅里叶变换（FRFT）域的分形特征研究压制干扰的存在性检测问题。首先,分析了典型的三种压制干扰的分形特征,说明了典型压制干扰信号具有分形特性,并采用盒维数和信息维数定量描述它们的分形特征。然后,发现了压制干扰和高斯白噪声在FRFT域具有不同的分形特征,进而提出了一种检测压制干扰存在性的方法。最后,仿真验证了该方法的有效性和优越性。     

Mechanical, vibrational, and dynamical properties of amorphous systems near jamming

Amorphous systems undergo the jamming transition when the density increases, temperature drops, or external shear stress decreases, as described by the jamming phase diagram which was proposed to unify different processes such as the glass transition, random close packing, and yielding under shear stress. At zero temperature and shear stress, the jamming transition occurs at a critical density at Point J. In this paper, we review recent studies of the material properties of marginally jammed solids and the glassy dynamics in the vicinity of Point J. As the only singular point in the jamming phase diagram, Point J exhibits special criticality in both mechanical and vibrational quantities. Dynamics approaching the glass transition in the vicinity of Point J show critical scalings, suggesting that the molecular glass transition and the colloidal glass transition are equivalent in the hard sphere limit. All these studies shed light on the long-standing puzzles of the glass transition and unusual properties of amorphous solids.     

Energetic instability unjams sand and suspension

Jamming is a phenomenon occurring in systems as diverse as traffic, colloidal suspensions, and granular materials. A theory on the reversible elastic deformation of jammed states is presented. First, an explicit granular stress-strain relation is derived that captures many relevant features of sand, including especially the Coulomb yield surface and a third-order jamming transition. Then this approach is generalized, and employed to consider jammed magnetorheological and electrorheological fluids, again producing results that compare well to experiments and simulations.     

Reactive jammer detection in OFDM with index modulation

Detection of jamming attacks is an important tool to improve the resource efficiency of jammer resilient communication networks. Detecting reactive jammers is especially difficult since the attacker is cognitive and focuses only on the used channels. Orthogonal frequency division multiplexing with index modulation (OFDM-IM) consists of active and passive subcarriers. Only active subcarriers carry modulated signals while passive subcarriers are left unused. In OFDM-IM systems, information bits are also dynamically embedded in the indices of these active subcarriers. As a result, remaining passive subcarriers cause instantaneously changing and unused holes in the spectrum that a reactive jammer cannot escape from attacking. In this paper, we propose an OFDM-IM-based detection scheme to improve the detection performance against reactive jammers. The proposed method exploits the dynamically changing empty OFDM-IM subcarriers to improve detection performance. A detection mechanism that is based on the variance of received signals is considered to identify the jammed subcarriers reliably and with low complexity. We assumed a destructive and elusive reactive jammer model that applies a zero-mean Gaussian jamming signal to the occupied channels. The performance of the variance detector is investigated analytically for OFDM-IM and OFDM-based systems under the given jammer model. The results showed that passive subcarriers of OFDM-IM inherently provide a better detection performance compared to the classical OFDM. Lastly, the analytical results are verified via simulations against both full-band and partial-band reactive jammers. Also, the effect of noise and the jamming power on the detection performance is investigated via extensive simulations.     

Modeling abnormal strain states in ferroelastic systems: the role of point defects

Recent experiments have revealed a rich variety of strain states in doped ferroelastic systems. We study the origin of two abnormal strain states; precursory tweed and strain glass, and their relationship with the well-known austenite and martensite (the para- and ferroelastic states). A Landau free energy model is proposed, which assumes that point defects alter the global thermodynamic stability of martensite and create local lattice distortions that interact with the strain order parameters and break the symmetry of the Landau potential. Phase field simulations based on the model have predicted all the important signatures of a strain glass found in experiment. Moreover, the generic "phase diagram" constructed from the simulation results shows clearly the relationships among all the strain states, which agrees well with experimental measurements.     

Granular fingers on jammed systems: new fluidlike patterns arising in grain-grain invasion experiments

In this Letter we report spontaneous pattern formation in dense granular assemblies confined to a Hele-Shaw cell and quasistatic regime. Varied unexpected patterns, ranging from rounded to fingered, are observed due to the displacement of one granular material by another. Computer simulations reproduce the major features observed in these experiments. Two mechanisms are responsible for the pattern formation: crystallization of the injected grains and plastic deformation of the displaced grains. The experiment suggests analogies with viscous fingering and jamming transition experiments.     

Slow dynamics of embedded fluid in mesoscopic confining systems as probed by NMR relaxometry

Mesoscopic media such as porous materials or colloidal dispersions strongly influence the dynamics of the embedded fluid. In the strong-adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long-time pathology, enlarging the time domain of the embedded-fluid dynamics towards the low-frequency regime. An interesting way to probe such a slow interfacial process is to use the field-cycling NMR relaxometry. This technique is used here to probe the fluid dynamics in two types of interfacial systems: i) a colloidal glass made of thin and flat particles; ii) a fully saturated porous media, the Vycor glass. Experimental results are critically compared to either a simple theoretical model of NMR dispersion involving elementary steps of the fluid dynamics near an interface (loops, trains, tails) or Brownian-dynamics simulations performed inside 3D reconstructions of these confined systems.Received: 1 January 2003, Published online: 14 October 2003PACS: 76.60.Es Relaxation effects - 61.43.Gt Powders, porous materials - 82.70.Dd Colloids     

Nematics with quenched disorder: how long will it take to heal?

Nematics with quenched disorder have been repeatedly predicted to form glass phases. Here we present turbidity experiments and computer simulations aimed at studying glass key features such as dynamics and history dependence in randomly perturbed nematics. Electric field-cooling alignment has been employed to prepare samples in suitably oriented starting states. Remarkable remnant order and slow dynamics are found both by experiment and simulations, indicating that random disorder can, by itself, induce a nematic glass state even without perturber restructuring.     

Computer Simulations of Polymers Close to Solid Interfaces: Some Selected Topics

This paper presents a topical overview of molecular-dynamics and Monte Carlo simulations for polymer systems close to solid interfaces. The simulations utilize simplified coarse-grained models: The polymers are represented by bead-spring chains, and the walls by a crystalline layer of Lennard-Jones particles or by a smooth impenetrable barrier. This approach has two advantages. First, it reduces the complexity of the simulation. Often, it is only then possible that the interesting length and time scales can be studied at all. Second, the approach concentrates on generic features that are believed to determine the physics of the problem under consideration. The results of the simulation can thus help to single out those features which should be incorporated in an analytical treatment. In this paper, we want to illustrate the versatility of these models by applying them to a broad spectrum of different problems. The situations considered range from the adsorption of a polymer from dilute solution onto a wall, over the importance of sub-monolayer monomeric or polymeric lubricants for kinetic friction, to the crystallization or glass transition of dense polymer films.     

Isoenergetic jamming transition in particle-filled systems

A full understanding of the jamming transition remains elusive, but recent advances which draw upon the common features of frustrated systems are encouraging. Herein, we show that, for mixtures of oil and silica particles, the dependence of the dejamming stress on filler volume fraction, phi is consistent with the shape of a reported jamming phase diagram [Trappe, Nature (London) 411, 772 (2001)]. We discover for the first time, however, that the role of phi disappears when mechanical energy input, defined as stress multiplied by strain, is used instead of stress as the critical parameter. We also examine literature results for aqueous suspensions of boehmite alumina powders, latex dispersions of polystyrene particles, and carbon black-filled elastomers in order to illustrate the universality of our finding. This study provides evidence for a thermodynamic interpretation of the jamming transition.     

Elevation localization and head-related transfer function analysis at low frequencies

Monaural spectral features due to pinna diffraction are the primary cues for elevation. Because these features appear above 3 kHz where the wavelength becomes comparable to pinna size, it is generally believed that accurate elevation estimation requires wideband sources. However, psychoacoustic tests show that subjects can estimate elevation for low-frequency sources. In the experiments reported, random noise bursts low-pass filtered to 3 kHz were processed with individualized head-related transfer functions (HRTFs), and six subjects were asked to report the elevation angle around four cones of confusion. The accuracy in estimating elevation was degraded when compared to a baseline test with wideband stimuli. The reduction in performance was a function of azimuth and was highest in the median plane. However, when the source was located away from the median plane, subjects were able to estimate elevation, often with surprisingly good accuracy. Analysis of the HRTFs reveals the existence of elevation-dependent features at low frequencies. The physical origin of the low-frequency features is attributed primarily to head diffraction and torso reflections. It is shown that simple geometrical approximations and models of the head and torso explain these low-frequency features and the corresponding elevations cues.     

Emergent <Emphasis Type="Italic">SO</Emphasis>(3) Symmetry of the Frictionless Shear Jamming Transition

We study the shear jamming of athermal frictionless soft spheres, and find that in the thermodynamic limit, a shear-jammed state exists with different elastic properties from the isotropically-jammed state. For example, shear-jammed states can have a non-zero residual shear stress in the thermodynamic limit that arises from long-range stress-stress correlations. As a result, the ratio of the shear and bulk moduli, which in isotropically-jammed systems vanishes as the jamming transition is approached from above, instead approaches a constant. Despite these striking differences, we argue that in a deeper sense, the shear jamming and isotropic jamming transitions actually have the same symmetry, and that the differences can be fully understood by rotating the six-dimensional basis of the elastic modulus tensor.     

Jamming percolation and glass transitions in lattice models

A new class of lattice gas models with trivial interactions but constrained dynamics is introduced. These models are proven to exhibit a dynamical glass transition: above a critical density rhoc ergodicity is broken due to the appearance of an infinite spanning cluster of jammed particles. The fraction of jammed particles is discontinuous at the transition, while in the unjammed phase dynamical correlation lengths and time scales diverge as exp[C(rhoc-rho)-mu]. Dynamic correlations display two-step relaxation similar to glass formers and jamming systems.     

Physical layer security of vehicular networks with cooperative jamming helpers

As an important part of the intelligent transportation system (ITS), vehicular networks can provide drivers and passengers with more comfortable and convenient services such as efficient traffic management and infotainment. However, the security threats on data exchanges over vehicular networks have become increasingly severe. Different from conventional cryptographic technologies, the application of physical layer security (PLS) to vehicular networks has been investigated to prevent the security of exchanging data from the eavesdropper and measure precisely the leaked information to the eavesdropper, due to its low complexity and communication overhead. In this work, we are concerned with the PLS of cooperative vehicular networks consisting of a source vehicle, a destination vehicle, an eavesdropping vehicle and a cooperative jamming vehicle. First, to improve the secrecy performance, the cooperative jamming helper emits jamming signals to degrade the eavesdropping channel without harming the legitimate channel. Then, based on the Rayleigh fading channel models and the traffic models, the closed-form expressions of the secrecy outage probability (SOP) and the average secrecy capacity (ASC) of the considered vehicular networks are derived, which deliver more implications of various system parameters on SOP and ASC performances and can be computed without simulations at a lower complexity. Second, a definition of the optimal jamming vehicle is introduced and then the cooperative jamming vehicle selection strategy is presented. The existence of the optimal jamming vehicle is measured in probability, which is explored analytically. Third, the optimal power allocation that maximizes the secrecy capacity is found analytically for the source vehicle and the cooperative jamming helper. Finally, simulations are also presented to demonstrate the validation of these analytical results and confirm the advantages of the cooperative jamming strategy and the optimal power allocation. From the numerical results, more observations on the effects of the main system parameters on secrecy performances are obtained, which provides some useful guides for practice.     

Shear- and vibration-induced order-disorder transitions in granular media

By molecular dynamics simulations we investigate the order-disorder transitions induced in granular media by an applied drive combining vibrations and shear. As the steady state is attained, the pack is found in disordered configurations for comparatively high intensities of the drive; conversely, ordering and packing fractions exceeding the random close packing are found when vibrations and shear are weak. As forcing amplitudes get smaller, we find diverging time scales in the dynamics, as the system enters a jamming region. Under this perspective, our picture supports the intuition that externally applied forcing has, in driven granular media, a role similar to temperature in thermal systems.     

A low-dimensional dynamical system to describe low-frequency fluctuations in a semiconductor laser with optical feedback

We derive a low-dimensional dynamical system to describe a semiconductor laser with optical feedback. This model captures many features from the original time delayed dynamical system and explains the origin of the low-frequency instability observed in the time-delayed equation.     

Microscopic mean-field theory of the jamming transition

Dense particle packings acquire rigidity through a nonequilibrium jamming transition commonly observed in materials from emulsions to sandpiles. We describe athermal packings and their observed geometric phase transitions by using equilibrium statistical mechanics and develop a fully microscopic, mean-field theory of the jamming transition for soft repulsive spherical particles. We derive analytically some of the scaling laws and exponents characterizing the transition and obtain new predictions for microscopic correlation functions of jammed states that are amenable to experimental verifications and whose accuracy we confirm by using computer simulations.     

Study of random sequential adsorption by means of the gradient method

By using the gradient method (GM) we study random sequential adsorption (RSA) processes in two dimensions under a gradient constraint that is imposed on the adsorption probability along one axis of the sample. The GM has previously been applied successfully to absorbing phase transitions (both first and second order), and also to the percolation transition. Now, we show that by using the GM the two transitions involved in RSA processes, namely percolation and jamming, can be studied simultaneously by means of the same set of simulations and by using the same theoretical background. For this purpose we theoretically derive the relevant scaling relationships for the RSA of monomers and we tested our analytical results by means of numerical simulations performed upon RSA of both monomers and dimers. We also show that two differently defined interfaces, which run in the direction perpendicular to the axis where the adsorption probability gradient is applied and separate the high-density (large-adsorption probability) and the low-density (low-adsorption probability) regimes, capture the main features of the jamming and percolation transitions, respectively. According to the GM, the scaling behaviour of those interfaces is governed by the roughness exponent α = 1/(1 + ν), where ν is the suitable correlation length exponent. Besides, we present and discuss in a brief overview some achievements of the GM as applied to different physical situations, including a comparison of the critical exponents determined in the present paper with those already published in the literature.     

Critical scaling in linear response of frictionless granular packings near jamming

We study the origin of the scaling behavior in frictionless granular media above the jamming transition by analyzing their linear response. The response to local forcing is non-self-averaging and fluctuates over a length scale that diverges at the jamming transition. The response to global forcing becomes increasingly nonaffine near the jamming transition. This is due to the proximity of floppy modes, the influence of which we characterize by the local linear response. We show that the local response also governs the anomalous scaling of elastic constants and contact number.