Phase transition and bistable phenomenon of granular flows down a chute with successive turnings

This paper studies the granular flow down a chute with two successive turnings, which play the role of bottlenecks for the granular flow system and determine the granular flow state in main section between them. With the increase of main section width D, phase transition from dilute to dense granular flow is observed: When the main section width D is small (large), the granular flow at upper (lower) bottleneck is dense and the granular flow is dilute (dense) in the main section. More interestingly, a bistable region is exhibited, in which either dilute flow or dense flow may occur and continue for the entire run. In this region, the packing in the reservoir will affect initial flow rate and then affect the flow pattern. This study can be viewed as a paradigm for the jamming and unjamming transitions under shear due to gravity.     

Entropic phase separation in polymer-microemulsion networks

We study theoretically a model system of a transient network of microemulsion droplets connected by telechelic polymers and explain recent experimental findings. Despite the absence of any specific interactions between either the droplets or polymer chains, we predict that as the number of polymers per drop is increased, the system undergoes a first-order phase separation into a dense, highly connected phase, in equilibrium with dilute droplets, decorated by polymer loops. The phase transition is purely entropic and is driven by the interplay between the translational entropy of the drops and the configurational entropy of the polymer connections between them. Because it is dominated by entropic effects, the phase behavior of the system is extremely robust and is independent of the detailed properties of either polymers or drops.     

Proton momentum distribution of liquid water from room temperature to the supercritical phase

Measurements of the proton momentum distribution n(p) in water from ambient conditions to above the supercritical point are compared with theoretical calculations based on a recently developed polarizable water model. The n(p) along the H-bond direction is narrower in the dense phases, and approaches that of the isolated molecule in the more dilute phases. The theoretical model, which includes only electrostatic interactions, is unable to explain the softening of the local potential experienced by the proton in the dense phases, but it accurately predicts the n(p) for the dilute phases.     

Study of compact QED with light fermions

We present a numerical study of compact QED including dynamical fermions of mass = 0.10 (lattice units) on a 6⁴ lattice. Staggered lattice fermions and the Wilson action and mixed gauge actions were studied. The simulations used two modified hybrid methods: one with stochastic fields and the other with deterministic ones. Working with a variable number of flavors we found that the chiral transition which separates the strong and weak coupling phases is of first order in all cases. The strength of these discontinuous transitions increase with the number of flavors.     

Phase behavior of flowerlike micelles in a SCF cell model

We study the interactions between flowerlike micelles, self-assembled from telechelic associative polymers, using a molecular self-consistent field (SCF) theory and discuss the corresponding phase behavior. In these calculations we do not impose properties such as aggregation number, micellar structure and number of bridging chains. Adopting a SCF cell model, we calculate the free energy of interaction between a central micelle surrounded by others. Based on these results, we predict the binodal for coexistence of dilute and dense liquid phases, as a function of the length of the hydrophobic and hydrophilic blocks. In the same cell model we compute the number of bridges between micelles, allowing us to predict the network transition. Several quantitative trends obtained from the numerical results can be rationalized in terms of transparent scaling arguments.     

Lamellar phase coexistence induced by electrostatic interactions

Membranes containing highly charged biomolecules can have a minimal free-energy state at small separations that originates in the strongly correlated electrostatic interactions mediated by counterions. This phenomenon can lead to a condensed, lamellar phase of charged membranes that coexists in thermodynamic equilibrium with a very dilute membrane phase. Although the dilute phase is mostly water, entropy dictates that this phase must contain some membranes and counterions. Thus, electrostatics alone can give rise to the coexistence of a condensed and an unbound lamellar phase. We use numerical simulations to predict the nature of this coexistence when the charge density of the membrane is large, for the case of multivalent counterions and for a membrane charge that is characteristic of biomolecules. We also investigate the effects of counterion size and salt on the two coexisting phases. With increasing salt concentration, we predict that electrostatic screening by salt can destroy the phase separation.     

Dyson-Schwinger approach to color superconductivity at finite temperature and density

We investigate the phases of dense QCD matter at finite temperature with Dyson-Schwinger equations for the quark propagator for N _f = 2 + 1 flavors. For the gluon propagator we take a fit to quenched lattice data and add quark-loop effects perturbatively in a hard-thermal-loop-hard-dense-loop approximation. We consider 2SC and CFL-like pairing with chiral up and down quarks and massive strange quarks and present results for the condensates and the phase diagram. We find a dominant CFL phase at chemical potentials larger than 500-600MeV. At lower values of the chemical potential we find a 2SC phase, which also exists in a small band at higher temperatures for larger chemical potentials. With values of 20–30 MeV, the critical temperatures to the normal phase turn out to be quite small.     

Influence of granule velocity on gravity-driven granular flow

In this Letter, we present the results of our study on the influence of granule velocity on the granular flow state. The maximum outflow rate of the dilute flow state increases with an increase in granule velocity, whereas the outflow rate of the dense flow state remains unchanged. In addition, increasing granule velocity can extend the region of the dilute flow state and shrink that of the dense flow state. These observations can be explained by the relationship between the flow rate and the packing fraction at the bottleneck under a fixed particle number condition.     

通道宽度和初始流量对颗粒稀疏流-密集流转变临界开口的影响

通过实验研究斜面上二维颗粒流，当出口尺寸减小到临界值Dc时，发生稀疏流到密集流的突变.发现临界尺寸Dc与初始流量和通道宽度有关，通道宽度一定的情况下，临界开口尺寸Dc近似随初始流量Q0的平方根增大.在初始流量Q0一定时，临界开口尺寸Dc随通道宽度W近似线性增大.给出了这些关系的表达式，理论计算与实验观测结果一致.同时，也讨论了通道宽度影响临界开口尺寸的原因. 关键词： 颗粒物质 颗粒流     

Internal organisation in polyelectrolytes/oppositely charged surfactants colloidal complexes anticipating precipitated nanostructures

In this paper, we relate the periodic nanostructures found in the colloidal complexes and the concentrated phases obtained with polyelectrolyte/surfactant aqueous solutions. We present small-angle X-ray scattering studies of the self-organisation of the anionic polymer carboxymethylcellulose with three cationic quaternary ammonium surfactants with different head and tail groups: hexadecyl trimethyl, hexadecyl ethyl dimethyl and didodecyl dimethyl ammonium bromides. We investigated the mesophases obtained above a precipitation threshold. The mixed solutions with the double-chained surfactant led to lamellar phases, in which the repeat distance only depends on the surfactant/carboxyl charge molar ratio. We show that an internal lamellar organisation already takes place in the dilute phase containing colloidal complexes found below the precipitation threshold.     

Elastically driven linker aggregation between two semiflexible polyelectrolytes

The behavior of mobile linkers connecting two semiflexible charged polymers, such as polyvalent counterions connecting DNA or F-actin chains, is studied theoretically. The chain bending rigidity induces an effective repulsion between linkers at large distances while the interchain electrostatic repulsion leads to an effective short-range interlinker attraction. We find a rounded phase transition from a dilute linker gas where the chains form large loops between linkers to a dense disordered linker fluid connecting parallel chains. The onset of chain pairing occurs within the rounded transition.     

二维颗粒流通道宽度效应的分子动力学模拟

通过用分子动力学方法对颗粒物质流的计算机模拟，研究发现增大通道宽度可以使二维颗粒流从稀疏流转变为密集流状态.通过对不同通道宽度下，固定开口为9.5d的颗粒流和 漏斗口以上9.5d×8d区域记录的模拟结果分析，发现随通道宽度增大，密度变大、温 度降低.当“颗粒温度”T较低时(T/m<0.05 J/kg),颗粒流内部接触数开始超过1.2 ，同时出现较为牢固的横向链状颗粒团簇，是造成流量突变以及密集流的原因. 关键词： 颗粒物质 颗粒流 计算机模拟     

基于拟颗粒的气固两相曳力模型研究

为了研究气固两相流动大涡模拟中合适的曳力计算模型,本文引入拟颗粒和拟颗粒表面能的概念,通过拟颗粒表面能与外界输入能量之间的平衡关系来确定拟颗粒的粒径。根据拟颗粒粒径,得到运算量较小且考虑颗粒团聚效应的曳力计算模型。应用本文的曳力计算模型对二维竖直槽道内稠密气固两相流动进行了大涡模拟,结果表明颗粒的浓度分布具有上稀下浓,壁面附近浓中心稀及颗粒聚集等特点。这与实验结果在定性上是一致的。对气相和颗粒相的瞬时速度场进行了分析,发现气相和颗粒相速度场分布的非对称性是形成颗粒浓度分布壁面附近浓中心稀的重要原因之一。     

Counterion phase transitions in dilute polyelectrolyte solutions

In dilute solutions of rodlike polyelectrolytes some counterions are distributed far from polyions while others are located in their vicinity in the regions of cylindrical symmetry of the electrostatic potential. For these cylindrical regions around rodlike polyelectrolytes we find an exact solution of the nonlinear Poisson-Boltzmann equation for the case of nonzero net charge in these regions. This exact solution implies three qualitatively different phases of counterion distribution around the polyions with second order phase transitions between these phases.     

Phase Transition and Critical Phenomenon Occurring in Granular Matter

We investigate the granular flow states in a channel with bottleneck by molecular dynamics simulations.Our study is restricted only on a selected key area rather than on the whole system to focus on the flow properties of a single granular state.A random force field is introduced to control the granular temperature.It is also pointed out that the flow rate in the granular flow can be correlated with the pressure,which leads us to carry out a comprehensive study similar to the classical study for general liquid-gas phase transition.Our results show that the dilute flow state and the dense flow state of the granules are similar to the gas state and the liquid state of general substances,respectively,and the properties of phase transition and critical phenomenon are also similar to those occurring in general substances.     

Three-phase osmotic equilibria using the Gibbs ensemble simulation method

The Gibbs Ensemble Monte Carlo method has been used to simulate osmotic equilibria for Lennard-Jones mixtures. When the simulations are performed with two independent boxes, one containing solvent and the other a mixture of solute and solvent significantly negative osmotic pressures (Π) develop. Following a sugestion of Powles et al. (1997, Molec. Phys., 90, 665), we have extended these simulations to include a third box and the possibility of modelling three coexisting phases. The simulations show that the two phase equilibria with negative values of 77 are metastable and that the system spontaneously separates into three phases: pure solvent, dilute solute-solvent and dense solute-solvent with a resulting osmotic pressure that is normally small and positive.     

Electric-field-induced polarization of the layer of condensed ions on cylindrical colloids

In concentrated suspensions of charged colloids, interactions between colloids can be induced by an external electric field through the polarization of charge distributions (within the diffusive double layer and the layer of condensed ions) and/or electro-osmotic flow. In case of rod-like colloids, these field-induced inter-colloidal interactions have recently been shown to lead to anomalous orientation perpendicular to the external field, and to phase/state transitions and dynamical states, depending on the field amplitude and frequency of the external field. As a first step towards a (semi-) quantitative understanding of these phenomena, we present a linear-response analysis of the frequency-dependent polarization of the layer of condensed ions on a single, long and thin cylindrical colloid. The in-phase and out-phase response functions for the charge distribution and the electric potential are calculated for arbitrary orientation of the cylindrical colloid. The frequency-dependent degree of alignment, which is proportional to the electric-field-induced birefringence, is calculated as well, and compared to experiments on dilute fd virus suspensions.     

Tight and loose shapes in flat entangled dense polymers

We investigate the effects of topological constraints (entanglements) on two-dimensional polymer loops in the dense phase, and at the collapse transition ( -point). Previous studies have shown that in the dilute phase the entangled region becomes tight, and is thus localised on a small portion of the polymer. We find that the entropic force favouring tightness is considerably weaker in dense polymers. While the simple figure-eight structure, created by a single crossing in the polymer loop, localises weakly, the trefoil knot and all other prime knots are loosely spread out over the entire chain. In both the dense and conditions, the uncontracted-knot configuration is the most likely shape within a scaling analysis. By contrast, a strongly localised figure-eight is the most likely shape for dilute prime knots. Our findings are compared to recent simulations.Received: 7 October 2003, Published online: 21 November 2003PACS: 87.15.-v Biomolecules: structure and physical properties - 82.35.-x Polymers: properties; reactions; polymerization - 02.10.Kn Knot theory     

Phase diagrams and kinetics of phase transitions in protein solutions

The phase behavior of proteins is of interest for fundamental and practical reasons. The nucleation of new phases is one of the last major unresolved problems of nature. The formation of protein condensed phases (crystals, polymers, and other solid aggregates, as well as dense liquids and gels) underlies pathological conditions, plays a crucial role in the biological function of the respective protein, or is an essential part of laboratory and industrial processes. In this review, we focus on phase transitions of proteins in their properly folded state. We first summarize the recently acquired understanding of physical processes underlying the phase diagrams of the protein solutions and the thermodynamics of protein phase transitions. Then we review recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We explore the transition from nucleation to spinodal decomposition for liquid-liquid separation and introduce the new concept of solution-to-crystal spinodal. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. The concepts and mechanisms reviewed here provide powerful tools for control of the nucleation process by varying the solution thermodynamic parameters.     

可压稠密气固两相流动过程的模拟计算

基于稠密气体分子运动论和颗粒动理学,建立可压稠密气固两相流动模型。采用梯度模拟来考虑气相可压缩性对气相湍流的影响。模拟计算表明气固两相射流速度沿轴向和径向减小,颗粒浓度下降。气固两相射流具有高的颗粒温度,呈现强烈的气固两相湍流流动特性。