Mean-field cage theory for the random close packed state of a metastable hard-sphere glass

In 2005, we developed a mean-field cage theory for the freezing of a stable hard-sphere fluid using the character of a stable hard-sphere fluid, some observations and the mean-configuration approximation [X.Z. Wang, J. Chem. Phys. 122 (2005) 044515]. It was found that near the freezing point, a thermal fluctuation of a cage causes the hard sphere in this cage to exchange positions with one of its nearest neighbors. In this paper, we extend the theory to the random close packed state of a metastable hard-sphere glass. It is found that near the random close packing point, a thermal fluctuation of a cage sets three of the hard spheres in this cage and its nearest cages into the local circulatory motion, resulting in indirect position exchanges among these three hard spheres. We obtain an analytical formula for the random close packing density. The predicted values are in good agreement with the experimental and simulation results for spatial dimensions d=2–7$d=2\text{–}7$.     

Digi-FH-OFDM: An all-digital wideband frequency-hopped OFDM system

Frequency-hopped spread-spectrum systems (FHSS) traditionally employ a super-heterodyne receiver architecture to perform frequency hopping in the passband. Such an architecture consists of analog blocks such as the mixer and the local oscillator that contribute greatly to the overall cost and hardware complexity of the system. The recent development of direct radio-frequency (RF) data converters has led to the possibility of having an all-digital receiver architecture where an RF signal is digitized directly to baseband, without the need to translate it to an intermediate frequency. Motivated by this, we propose an all-digital wideband frequency-hopped orthogonal frequency division multiplexing system which is abbreviated as Digi-FH-OFDM in this paper. The system performs two-stage frequency hopping — one in the wideband and the other in the baseband. The system architecture and the implementation details are presented. Real-time power spectra of the hopped signals in the wideband are obtained after transmitting them over the air via the RF data converters of the reconfigurable Xilinx Ultrascale ZCU111 RFSoC board and the Qorvo RF front end card. The bit error rate performance of the system is studied against eavesdropping and jamming attacks under a slow-fading channel and pilot-based channel estimation. The proposed Digi-FH-OFDM system outperforms the existing analog and partially digital FH-OFDM systems in terms of hardware complexity, robustness to eavesdropping and jamming, and the overall latency.     

Effect of magnetic field orientation on fluidized beds of magnetic particles: Theory and experiment

Geldart-A fluidized beds of fine particles experience a jamming transition between a fluid-like state and a solid-like state at a certain superficial gas velocity, that depends on the relative strength of interparticle attractive forces with respect to particle weight. Interparticle forces provide the bed with a certain tensile strength in the jammed state. In the work presented here we analyze the behavior of a fluidized bed of magnetic particles subjected to an externally applied magnetic field, which contributes to enhance interparticle forces. The importance of the magnetic contribution to interparticle forces is measured by the changes in the tensile strength and the superficial gas velocity at the jamming transition. The link of the field orientation with the microstructure of the bed is discussed.     

Scheme for nonlinear frequency modulated signal cancelling system

A cancelling system for the four nonlinear frequency modulated (NLFM) signal (i.e. Taylor window, Tangent-based, Combination linear frequency modulation (LFM) and tan-FM, Stepped NLFM) is presented. It is mainly composed of a digital radio frequency memory (DRFM) and a field programmable gate array (FPGA) chip etc. The received signals are stored and reproduced by DRFM, and the system delay time is controlled by the FPGA chip. According to the target's radar cross section (RCS), the radar echo cancelling wave will be generated by the FPGA and DRFM on signal processing. The effect of error on the cancelling wave is analysed and the method for reducing nonlinear phase errors is presented. Theoretical analysis and simulation show that the system effectively reduces the signal power received by the radar receiver. Numerical simulation results show that about 11.3 dB target gain reduction can be achieved under the condition of large deviation.     

Near-field scattering techniques: Novel instrumentation and results from time and spatially resolved investigations of soft matter systems

Recently the investigation of non-stationary systems exhibiting spatial and dynamic heterogeneities has propelled the development of innovative optical techniques providing the missing link between the scattering and imaging approaches. The novel techniques are characterized by the fact that the scattered radiation is measured close to the sample. They allow to recover information on the structure and dynamics of the system under investigation equivalent to that available with scattering techniques, with the great advantage that they simultaneously allow to achieve a local characterization of the sample, which is lost in traditional scattering measurements.     

Local Transient Jamming in Stress Relaxation of Bulk Amorphous Polymers

Bulk amorphous polymers become stretched and parallel-aligned under loading stress,and their intermolecular cooperation slows down the subsequent stress relaxation process.By means of dynamic Monte Carlo simulations,we employed the linear viscoelastic Maxwell model for stress relaxation of single polymers and investigated their intermolecular cooperation in the stress relaxation process of stretched and parallel-aligned bulk amorphous polymers.We carried out thermal fluctuation analysis on the reproduced Debye relaxation and Arrhenius fluid behaviors of bulk polymers.We found a transient state with stretch-coil coexistence among polymers in the stress relaxation process.Further structure analysis revealed a scenario of local jamming at the transient state,resulting in an entropy barrier for stretch-coil transition of partial polymers.The microscopic mechanism of intermolecular cooperation appears as unique to polymer stress relaxation,which interprets the hydrodynamic interactions as one of essential factors raising a high viscosity in bulk amorphous polymers.Our simulations set up a platform of molecular modeling in the study of polymer stress relaxation,which brought new insights into polymer dynamics and the related mechanical/rheological properties.     

Random unfriendly seating arrangement in a dining table

A detailed study is made of the number of occupied seats in an unfriendly seating scheme with two rows of seats. An unusual identity is derived for the probability generating function, which is itself an asymptotic expansion. The identity implies particularly a local limit theorem with optimal convergence rate. Our approach relies on the resolution of Riccati equations. We also clarify some simple yet delicate stochastic dominance relations.     

Jamming: A simple introduction

Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can jam into a rigid, disordered state where they withstand finite shear stresses before yielding. Here we give a simple introduction to the surprising physics displayed by a very simple model system for the jamming transition: frictionless, soft spheres at zero temperature and zero shear that act through purely repulsive contact forces. This system starts to become rigid, i.e. goes through the jamming transition, whenever the confining pressure becomes positive. We highlight some of the remarkable geometrical features of the zero pressure jamming point and discuss the peculiar mechanical properties of these systems for small pressures.     

Jamming and diode effects for vortices in nanostructured superconductors

We examine jamming and ratchet effects for vortex matter in superconductors with asymmetric funnel geometries. We show that the vortex–vortex interactions can induce a clogging or jamming effect where it becomes increasingly difficult for the vortices to move through the system. We also find that commensurability effects can arise when certain vortex configurations form highly symmetrical structures in the funnel plaquettes. Due to the asymmetry, the critical currents are different for driving in different directions, leading to a diode effect. We also discuss other possible geometries and approaches that could be used to explore jamming in vortex matter, such as an analog to a granular hopper and a single driven vortex probe moving through an array of other vortices.