arXiv subjects

Ning Xu

Publications and source records attributed to Ning Xu.

At least 181 records · Page 10Linked to original sources

arXiv2015

Intra-and-Inter-Constraint-based Video Enhancement based on Piecewise Tone Mapping

Video enhancement plays an important role in various video applications. In this paper, we propose a new intra-and-inter-constraint-based video enhancement approach aiming to 1) achieve high intra-frame quality of the entire picture where multiple region-of-interests (ROIs) can be adaptively and simultaneously enhanced, and 2) guarantee the inter-frame quality consistencies among video frames. We first analyze features from different ROIs and create a piecewise tone mapping curve for the entire frame such that the intra-frame quality of a frame can be enhanced. We further introduce new inter-frame constraints to improve the temporal quality consistency. Experimental results show that the proposed algorithm obviously outperforms the state-of-the-art algorithms.

cs.CV↗

arXiv2015

Mechanical properties of jammed packings of frictionless spheres under applied shear stress

By minimizing a thermodynamic-like potential, we unbiasedly sample the potential energy landscape of soft and frictionless spheres under constant shear stress. We obtain zero-temperature jammed states under desired shear stresses and investigate their mechanical properties as a function of the shear stress. As a comparison, we also obtain jammed states from the quasistatic-shear sampling in which the shear stress is not well-controlled. Although the yield stresses determined by both samplings show the same power-law scaling with the compression from point $J$, i.e.~the jamming transition point at zero temperature and shear stress, for finite size systems, the quasistatic-shear sampling leads to a lower yield stress and a higher critical volume fraction of point $J$. The shear modulus of jammed solids decreases when increasing the shear stress. However, the shear modulus does not decay to zero at yielding. This discontinuous change of the shear modulus implies the discontinuous nature of the unjamming transition under nonzero shear stress, which is further verified by the observation of a discontinuous jump of the pressure from jammed solids to shear flows. The pressure jump decreases upon decompression and approaches zero at the critical-like point $J$, in analogy with well-known phase transitions under external field. The analysis of force networks in jammed solids reveals that the force distribution is more sensitive to the increase of the shear stress near point $J$. The force network anisotropy increases with the shear stress. Weak particle contacts near the average force and under large shear stresses exhibit asymmetric angle distribution.

cond-mat.soft↗

arXiv2014

Visualizing kinetic pathways of homogeneous nucleation in colloidal crystallization

When a system undergoes a transition from a liquid to a solid phase, it passes through multiple intermediate structures before reaching the final state. However, our knowledge on the exact pathways of this process is limited, mainly due to the difficulty of realizing direct observations. Here, we experimentally study the evolution of symmetry and density for various colloidal systems during liquid-to-solid phase transitions, and visualize kinetic pathways with single-particle resolution. We observe the formation of relatively-ordered precursor structures with different symmetries, which then convert into metastable solids. During this conversion, two major cross-symmetry pathways always occur, regardless of the final state and the interaction potential. In addition, we find a broad decoupling of density variation and symmetry development, and discover that nucleation rarely starts from the densest regions. These findings hold for all our samples, suggesting the possibility of finding a unified picture for the complex crystallization kinetics in colloidal systems.

cond-mat.soft↗

arXiv2014

Disordered Solids Without Well-Defined Transverse Phonons: The Nature of Hard Sphere Glasses

We probe the Ioffe-Regel limits of glasses with repulsions near the zero-temperature jamming transition by measuring the dynamical structure factors. At zero temperature, the transverse Ioffe-Regel frequency vanishes at the jamming transition with a diverging length, but the longitudinal one does not, which excludes the existence of a diverging length associated with the longitudinal excitations. At low temperatures, the transverse and longitudinal Ioffe-Regel frequencies approach zero at the jamming-like transition and glass transition, respectively. As a consequence, glasses between the glass transition and jamming-like transition, which are hard sphere glasses in the low temperature limit, can only carry well-defined longitudinal phonons and have an opposite pressure dependence of the ratio of the shear modulus to the bulk modulus from glasses beyond the jamming-like transition.

cond-mat.soft↗

arXiv2014

Structural heterogeneity and its role in determining properties of disordered solids

We construct a new order parameter from the normal modes of vibration, based on the consideration of energy equipartition, to quantify the structural heterogeneity in disordered solids. The order parameter exhibits strong spatial correlations with low-temperature single particle dynamics and local structural entropy. To characterize the role of particles with the most defective local structures identified by the order parameter, we pin them and study how properties of disordered solids respond to the pinning. It turns out that these particles are responsible to the quasilocalized low-frequency vibration, instability, softening, and nonaffinity of disordered solids.

cond-mat.soft↗

arXiv2014

Probing the glass transition from structural and vibrational properties of zero-temperature glasses

We find that the density dependence of the glass transition temperature of Lennard-Jones (LJ) and Weeks-Chandler-Andersen (WCA) systems can be predicted from properties of the zero-temperature ($T=0$) glasses. Below a crossover density $ρ_s$, LJ and WCA glasses show different structures, leading to different vibrational properties and consequently making LJ glasses more stable with higher glass transition temperatures than WCA ones. Above $ρ_s$, structural and vibrational quantities of the $T=0$ glasses show scaling collapse. From scaling relations and dimensional analysis, we predict a density scaling of the glass transition temperature, in excellent agreement with simulation results. We also propose an empirical expression of the glass transition temperature using structural and vibrational properties of the $T=0$ glasses, which works well over a wide range of densities.

cond-mat.soft↗

arXiv2014

Tortuosity Entropy: a measure of spatial complexity of behavioral changes in animal movement data

The goal of animal movement analysis is to understand how organisms explore and exploit the complex and varying environment. Animals usually exhibit varied and complicated movements, from apparently deterministic behaviors to highly random ones. This is critical for assessing movement efficiency and strategies that are used to quantify and analyze movement trajectories. Here we introduce a tortuosity entropy (TorEn) based on comparison of parameters, e.g. heading, bearing, speed, of consecutive points in movement trajectory, which is a simple measure for quantifying the behavioral change in animal movement data in a fine scale. In our approach, the differences between pairwise successive track points are transformed inot symbolic sequences, then we map these symbols into a group of pattern vectors and calculate the information entropy of pattern vector. Tortuosity entropy can be easily applied to arbitrary real-world data-deterministic or stochastic, stationary or non-stationary. We test the algorithm on both simulated trajectories and real trajectories and show that both mixed segments in synthetic data and different phases in real movement data are identified accurately. The results show that the algorithm is applicable to various situations, indicating that our approach is a promising tool to reveal the behavioral pattern in movement data.

q-bio.QM↗

arXiv2013

Finite size analysis of zero-temperature jamming transition under applied shear stress

By finding local minima of an enthalpy-like energy, we can generate jammed packings of frictionless spheres under constant shear stress $σ$ and obtain the yield stress $σ_y$ by sampling the potential energy landscape. For three-dimensional systems with harmonic repulsion, $σ_y$ satisfies the finite size scaling with the limiting scaling relation $σ_y\simϕ- ϕ_{_{c,\infty}}$, where $ϕ_{_{c,\infty}}$ is the critical volume fraction of the jamming transition at $σ=0$ in the thermodynamic limit. The width or uncertainty of the yield stress decreases with decreasing $ϕ$ and decays to zero in the thermodynamic limit. The finite size scaling implies a length $ξ\sim (ϕ-ϕ_{_{c,\infty}})^{-ν}$ with $ν=0.81\pm 0.05$, which turns out to be a robust and universal length scale exhibited as well in the finite size scaling of multiple quantities measured without shear and independent of particle interaction. Moreover, comparison between our new approach and quasi-static shear reveals that quasi-static shear tends to explore low-energy states.

cond-mat.soft↗

arXiv2013

Signatures of shear thinning-thickening transition in dense athermal shear flows

In non-equilibrium molecular dynamics simulations of dense athermal shear flows, we observe the transition from shear thinning to shear thickening at a crossover shear rate $\dotγ_c$. Shear thickening occurs when $\frac{{\rm d (ln} T_g)}{{\rm d (ln}\dotγ)}>2$ with $T_g$ the granular temperature. At the transition, the pair distribution function shows the strongest anisotropy. Meanwhile, the dynamics undergo apparent changes, signified by distinct scaling behaviors of the mean squared displacement and relaxation time on both sides of $\dotγ_c$. These features serve as robust signatures of the shear thinning-thickening transition.

cond-mat.soft↗

arXiv2012

Non-monotonic pressure dependence of the dynamics of soft glass-formers at high compressions

In molecular dynamics simulations of soft glass-formers interacting via repulsions, we find that the glass transition temperature, dynamical heterogeneity, and glass fragility reach their maxima at the same crossover pressure $P_d$. Our analysis of the zero-temperature jammed states indicates that states at $P_d$ have the highest bond orientational order with the largest spatial fluctuation. Correspondingly, the low-frequency normal modes of vibration are the least localized and the average potential energy barrier along these modes are the highest for jammed states in the vicinity of $P_d$. The reentrant glass transition and dynamics of supercooled liquids are thus predictable by these structural and vibrational precursors in the zero-temperature jammed states.

cond-mat.soft↗

arXiv2012

Propagation of Delays in the National Airspace System

The National Airspace System (NAS) is a large and complex system with thousands of interrelated components: administration, control centers, airports, airlines, aircraft, passengers, etc. The complexity of the NAS creates many difficulties in management and control. One of the most pressing problems is flight delay. Delay creates high cost to airlines, complaints from passengers, and difficulties for airport operations. As demand on the system increases, the delay problem becomes more and more prominent. For this reason, it is essential for the Federal Aviation Administration to understand the causes of delay and to find ways to reduce delay. Major contributing factors to delay are congestion at the origin airport, weather, increasing demand, and air traffic management (ATM) decisions such as the Ground Delay Programs (GDP). Delay is an inherently stochastic phenomenon. Even if all known causal factors could be accounted for, macro-level national airspace system (NAS) delays could not be predicted with certainty from micro-level aircraft information. This paper presents a stochastic model that uses Bayesian Networks (BNs) to model the relationships among different components of aircraft delay and the causal factors that affect delays. A case study on delays of departure flights from Chicago O'Hare international airport (ORD) to Hartsfield-Jackson Atlanta International Airport (ATL) reveals how local and system level environmental and human-caused factors combine to affect components of delay, and how these components contribute to the final arrival delay at the destination airport.

cs.AI↗

arXiv2012

Structural signature of jamming transition in thermal amorphous systems

In thermal amorphous systems, the first peak of the pair correlation function $g(r)$ shows a maximum height $g_1^{\rm max}$ at a volume fraction $ϕ=ϕ_v$ that increases with the temperature. $g_1^{\rm max}$ diverges at the T=0 jamming transition at $ϕ=ϕ_c$. Molecular dynamics simulations show that some typical quantities, such as the pressure, bulk modulus, shear modulus, and boson peak frequency that behave power law scalings with $ϕ-ϕ_c$ in marginally jammed solids at T=0, all show scalings with $ϕ-ϕ_c$ when $ϕ> ϕ_v$, while the scalings break down when $ϕ< ϕ_v$. The presence of $g_1^{\rm max}$ is thus not only a thermal vestige of the T=0 jamming transition, but more importantly the structural signature of the jamming transition.

cond-mat.soft↗

arXiv2012

Critical scalings and jamming in thermal colloidal systems

During the jamming of thermal colloids, the first peak of the pair distribution function shows a maximum height $g_1^{\rm max}$. We find that $g_1^{\rm max}$ is accompanied by significant change of material properties and thus signifies the transition from unjammed to jammed glasses. The scaling laws at $g_1^{\rm max}$ lead to scaling collapse of structural and thermodynamic quantities, indicating the criticality of the T=0 jamming transition. The physical significance of $g_1^{\rm max}$ is highlighted by its coincidence with the equality of the kinetic and potential energy and the maximum fluctuation of the coordination number. In jammed glasses, we find the strong coupling between the isostaticity and flattening of the density of vibrational states at the isostatic temperature scaled well with the compression.

cond-mat.soft↗

arXiv2011

Products of redial derivative and integral-type operators from Zygmund spaces to Bloch spaces

Let $H(\mathbb{B})$ denote the space of all holomorphic functions on the unit ball $\mathbb{B}\in \mathbb{C}^n$. In this paper we investigate the boundedness and compactness of the products of radial derivative operator and the following integral-type operator $$ I_ϕ^g f(z)=\int_0^1 \Re f(ϕ(tz))g(tz)\frac{dt}{t},\ z\in\mathbb{B} $$ where $g\in H(\mathbb{B}), g(0)=0$, $ϕ$ is a holomorphic self-map of $\mathbb{B}$,\ between Zygmund spaces and Bloch spaces.

math.CV↗

arXiv2011

Direct determination of the size of basins of attraction of jammed solids

We propose a free-energy based Monte-Carlo method to measure the volume of potential-energy basins in configuration space. Using this approach we can estimate the number of distinct potential-energy minima, even when this number is much too large to be sampled directly. We validate our approach by comparing our results with the direct enumeration of distinct jammed states in small packings of frictionless spheres. We find that the entropy of distinct packings is extensive and that the entropy of distinct hard-sphere packings must have a maximum as a function of packing fraction.

cond-mat.soft↗

arXiv2011

Understanding the Low-Frequency Modes in Disordered Systems at Single-Particle Level

Normal modes provide a fundamental basis for understanding crucial properties of solids, such as the thermal conductivity, the heat capacity and the sound propagation. While the normal modes are excellently described by plane waves in crystals, they are far less understood in disordered systems, due to the great difficulties in characterizing the heterogeneous vibrational behaviors. Using charged colloids with long-range repulsion, we successfully make different disordered systems without any contact friction, whose normal modes can be visualized at single-particle level. In these systems, we directly tackle the long-time outstanding puzzle in condensed matter physics: the microscopic origin of the low-frequency modes in disordered systems. For the first time, we experimentally clarify that the low-frequency modes are caused by the collective resonance of relatively disordered particles (or soft structures) coupled with long-wavelength transverse excitations, settling this puzzle at single-particle level. Next to these low-frequency modes in the density of states, we also observe a plateau due to isostaticity, verifying the fundamental prediction of jamming model. Moreover, we reveal the intrinsic correlation between the low-frequency modes and the real dynamics, which may lead to a universal mechanism for aging, melting and yielding.

cond-mat.soft↗

arXiv2011

New jamming scenario: From marginal jamming to deep jamming

We study properties of jammed packings of frictionless spheres over a wide range of volume fractions. There exists a crossover volume fraction which separates deeply jammed solids from marginally jammed solids. In deeply jammed solids, all the scalings presented in marginally jammed solids are replaced with remarkably different ones with potential independent exponents. Correspondingly, there are structural changes in the pair distribution function associated with the crossover. The normal modes of vibration of deeply jammed solids also exhibit some anomalies, e.g. strengthened quasi-localization and absence of Debye-like density of states at low frequencies. Deeply jammed systems may thus be cataloged to a new class of amorphous solids.

cond-mat.soft↗

arXiv2010

Soft modes and elasticity of nearly isostatic lattices: randomness and dissipation

The square lattice with central-force springs on nearest-neighbor bonds is isostatic. It has a zero mode for each row and column, and it does not support shear. Using the Coherent Potential Approximation (CPA), we study how the random addition, with probability $\mathcal{P}=(z-4)/4$ ($z$ = average number of nearest neighbors), of springs on next-nearest-neighbor ($NNN$) bonds restores rigidity and affects phonon structure. We find that the CPA effective $NNN$ spring constant $\tildeκ_m(ω)$, equivalent to the complex shear modulus $G(ω)$, obeys the scaling relation, $\tildeκ_m(ω) = κ_m h(ω/ω^*)$, at small $\mathcal{P}$, where $κ_m = \tildeκ'_m(0)\sim \mathcal{P}^2$ and $ω^* \sim \mathcal{P}$, implying that elastic response is nonaffine at small $\mathcal{P}$ and that plane-wave states are ill-defined beyond the Ioffe-Regel limit at $ω\approx ω^*$. We identify a divergent length $l^* \sim \mathcal{P}^{-1}$, and we relate these results to jamming.

cond-mat.soft↗