Factors determining crystal--liquid coexistence under shear

The interaction between an imposed shear flow and an order--disorder transition underlies a broad range of phenomena. Under the influence of shear flow, a variety of soft matter is observed to spontaneously form bands characterized by different local order---for example, thermotropic liquid crystals subjected to shear flow exhibit rich phase behaviour. The stability of order under the influence of shear flow is also fundamental to understanding frictional wear and lubrication. Although there exists a well developed theoretical approach to the influence of shear flow on continuous transitions in fluid mixtures, little is known about the underlying principles governing non-equilibrium coexistence between phases of different symmetry. Here we show, using non-equilibrium molecular dynamics simulations of a system of spherical particles, that a stationary coexistence exists between a strained crystal and the shearing liquid, and that this coexistence cannot be accounted for by invoking a non-equilibrium analogue of the chemical potential. Instead of such thermodynamic arguments, we argue that a balancing of the crystal growth rate with the rate of surface erosion by the shearing melt can account for the observed coexistence.     

小世界网络上的Ising模型

在一维规则环链的基础上，构造了小世界网络，研究了其上的Ising模型的相变行为．Monte Carlo方法被用来作为研究的方法．模拟结果发现，所构造的小世界网络上的Ising模型存在相变，模拟结果给出了临界温度．     

Computer simulations on the gas-liquid phase diagram of Stockmayer fluids

Particle exchange molecular dynamics （PEMD） simulation technique is proposed to study the gas-liquid phase diagram of fluids. In the simulations, the fluid particles can be transferred between the two coupled boxes, which possess constant total number of particles and volume. The particle transfer is controlled by the difference of chemical potential in the respective simulation box. After equilibrium the two boxes have the same pressure, temperature and chemical potential. The method is further used to study the gas-liquid phase diagram of Stockmayer fluid. Increasing the dipole strength will enhance the critical temperature. The predicted critical points are in agreement with those from Gibbs ensemble Monte Carlo simulations, while the small systematic difference is attributed to the system size effects and the thermostat methods.     

Atomic simulations of packing patterns and thermal behavior in Ti clusters

Atomic simulations using an embedded atom method (EAM) potential were performed to study changes of packing patterns in titanium clusters containing tens to thousands of atoms. The findings revealed that the changes came from the movements and rearrangements of surface atoms in a large temperature range, and they were strongly dependent on cluster size and elevated temperatures. Small size clusters with icosahedron (Ih) configurations of geometric shell closures were preferred. As the cluster size increased, most of the atoms in these clusters could hold their HCP packing patterns. With the increasing temperature, the coexistence of different packing patterns including HCP, BCC, and Ih occurred. At a high temperature, the disorderly packing quickly extended into the entire region of a large size cluster. The calculations of specific heat were compared with Dulong-Petit law. The simulations provide the possibility to straddle structural transformations and determine critical size suitable for classical theory under certain thermal conditions.     

Simulation of mesoscale interfacial properties using the lattice Boltzmann method

We derive the mesoscopic interparticle potentials from macroscopic thermodynamics for van der Waals, Redlich-Kwong, and Redlich-Kwong-Soave equations of state and find that all these potentials are very similar to the Lennard-Jones potential. To in-vestigate the interfacial property at the mesoscale level, we incorporate free energy functions into the single-component multiphase lattice Boltzmann model and obtain the saturated density coexistence curves and interface mass density profiles across the interface using this method with different equations of state. The simulation results accurately reproduce the properties of equilibrium thermodynamics. Numerical results for single-component phase transitions indicate that a bubble-growth process is obtained and the equilibrium phase diagram is achieved at a given temperature. Bulk free energy, the interfacial energy coefficient, and other properties of nonequilibrium thermodynamic parameters, which are used to examine interfacial properties, are obtained in these simulations, and all these parameters are found to obey irreversible thermodynamics.     

Two coexisting vortex phases in the peak effect regime in a superconductor

The critical current in the vortex phase of a type-II superconductor such as NbSe2 displays a striking anomaly in the vicinity of the superconductor-to-normal-metal transition. Instead of going to zero smoothly, it rebounds to a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect, has remained an unsolved problem for 40 years. Here we use a scanning a.c. Hall microscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter phases with intrinsically different pinning strengths coexist on a macroscopic scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects and anomalous voltage response commonly seen when external parameters such as temperature, magnetic field or transport current are varied. We argue that the observed phase coexistence is, in fact, the hallmark of a disorder-driven non-thermal phase transition.     

Metropolis蒙特卡罗计算方法的新发展及应用

本文从统计物理角度阐述了Ｍｅｔｒｏｐｏｌｉｓ蒙特卜罗计算方法原理，了近年来成功应用于统计物理模型相变研究上这一方法的新发展，然后采用该方法考察淬火座稀释的二维ＸＹ模型以分析其相变状况。计算结果显示该模型相变存在，并同时给出相变温度近似值。     

具有无穷多个吸收态的二维模型的密度演化

从一个自然初始状态的二维模型(准三维模型)出发,模拟了模型态密度的演化.结果显示:模型从完全的激活态过渡到激活态与吸收态共存的状态,是一个连续的相变过程;而从两相共存的状态到完全吸收态可能存在一个非连续的相变.动力学的驱动机制方式控制和影响整个演化行为.     

粘弹性效应对高分子混合物相分离动力学的影响

采用双流模型考察了粘弹性效应对二元高分子复杂流体相分离动力学的影响。发现高分子混合物的两组分粘弹性相同时，相分离形态与简单二元流体类型，即非临界组成时少数相呈分散相，临界组成时呈双连续相，散射函数呈单峰；但相区域增长指数比简单二元流体混合物的增长指数小得多。另一方面，对于高分子混合物的两组分存在粘弹性反差的情况，非临界组成时出现相反转，并导致相区域的增长指数变大，临界组 成时出现一种持久的海绵状结构，相区域增长指数反而下降。同时由于网络结构的出现导致散射函数出现双峰和肩形峰，表明有两种特征尺寸，此时用全程圆平均的散射函数来求增长指数将失去意义，应双峰分开处理或采用界面轮廓线统计的方法来计算增长指数。     

苯甲腈与正十四烷溶液的临界性质

寻找并配制苯甲腈和正十四烷溶液的临界温度和临界组成,用折射率方法测定共存曲线.在离临界温度11.4 K范围内测定了温度-折射率数据,通过计算临界指数β和指前因子B,结果与3D-Ising模型的理论值一致.     

非对称排斥过程中的稳态性质

通过数值模拟研究了ASEP模型的稳态性质,主要考察了当注入概率α和输出概率β变化时系统稳态密度和流量表现出的相变性质.研究发现,同以往TASEP模型的结果相比较,ASEP模型中系统的稳态密度具有相似的性质,然而其稳态流量表现出非常显著的新特性.首先,在TASEP模型中一个很大参数范围内存在的极大流量区在ASEP模型中退化为一个临界值,系统的极大流量仅仅当注入概率α等于某一临界值cα时才出现.此外,研究发现ASEP模型中存在着一个TASEP模型不具备的流量随注入概率α的增加而递减的流量减小区.     

一类具有密度制约的捕食-食饵扩散模型的定性分析

研究了一类具有密度制约的捕食-食饵模型共存解的唯一性和渐近行为。利用不动点指数理论给出了共存解存在的充分条件。然后采用线性算子的扰动理论讨论了共存解的唯一性和稳定性。最后运用抛物方程的比较原理得到了系统的持久性和灭绝性条件,并通过数值模拟对其理论结果进行了验证。结果表明在一定条件下两物种共存且系统存在稳定的唯一共存解。     

High entropy state of orientationally-disordered clusters in Zr-based bulk metallic glass

A phase transformation of a metastable Zr2Ni intermetallic crystalline phase to vitrify was studied with molecular dynamics (MD) simulations based on a plastic crystal model (PCM), which utilizes the orientational disorder of molecules of plastic crystals in a class of soft matter. The simulation results show ed that computational operations in MD-PCM for the random rotati ons of the clusters in the crystalline phase around their center of gravity and subsequent annealing lead to form a dense random pa cking structure from the metastable Zr2Ni phase. These randomly-rotated clusters provide high degrees of freedom in terms of atomic positions, which results in enhancing the glass-forming ability of the alloy. The critical fraction of the number of rotated cl usters for forming the glassy phase ( fc^R) is evaluated to be 0.75 ? 0.80. Thus, the critical fraction of the number of un-rotated clusters (fc^U=1?fc^R) is close to the critical concentration of site percolation for metallic materials. The mechanism of the metastable Zr2Ni phase to vitrify with increasing f^R was analyzed with a concept of communal entropy in the free volume theory with solid- and liquid-like cells proposed by Cohen and Grest. The MD-PCM for the metastable Zr2Ni phase suggests that Zr-based bulk metallic glass (BMG) can be regarded as an alloy in a high rotation entropy state of cl usters and that glass transition takes place by percolation of th e nuclei of a liquid-like glassy phase.     

绕水翼超空化流场的数值模拟

基于Reynolds平均法,采用RNG k-ε模型及Mixture两相流模型,对绕水翼ys930的超空化流动进行了数值模拟,得到了绕水翼各个超空化阶段的超空化形态及各阶段的临界空化数与水翼攻角、来流雷诺数之间的关系,确定了临界空化数与来流雷诺数在不同攻角条件下的0.4次方经验关系式,给出了系数的取值区间.结果表明:超空化的发展经历了超空穴形成、汽液两相共存和完全发展3个阶段;各阶段的临界空化数随水翼攻角的增大而增大,基本呈线性关系;由于回射流的作用,在水翼前缘处形成一个逆时针旋涡,使逐渐长大的空泡脱离水翼吸力面并向下游脱落;在不同空化数下,空泡脱落的位置不同,导致空泡长度的变化.     

鄂尔多斯盆地构造与耦合成矿关系思考

目的探索多种矿产耦合成矿的盆地构造动力学问题。方法综合剖析鄂尔多斯盆地构造与油、气、煤和砂岩型铀矿等多种矿产共存富集的时空配置关系。结果鄂尔多斯盆地多种矿产相互关联、共存富集于古生界和中生界的不同地质层位，促使它们耦合成矿的时代主要发生在晚侏罗-早白垩世的燕山中晚期，主体受控于中新生代多构造演化及其关键变革时期的构造转换过程。结论探索、研究鄂尔多斯盆地中新生代构造动力学演化及其叠加转换关系，有助于客观认识多种矿产共存富集可能受控的统一盆地动力学环境及其耦合成矿效应。     

一类捕食食饵系统中交叉扩散诱导的图灵不稳和斑图

研究了一类具有交叉扩散的捕食食饵模型的斑图形成.证明了交叉扩散破坏了正平衡点的稳定性,得到了交叉扩散导致不稳的条件.另外,数值模拟结果不仅支持了该理论,还显示模型具有复杂的空间斑图:条状斑图、点状斑图以及点条混合斑图.研究结果表明交叉扩散在斑图的形成中起着重要的作用.     

二维Lennard-Jones系统气/液相转变的Monte Carlo模拟研究

为深入研究微小尺度系统的热力学性质和相转变机制,以粒子间通过Lennard-Jones势能作用的2维系统为模型,通过正则系综Monte Carlo模拟细致地考察了气/液相转变区的平衡态参数。沿等温线的模拟结果显示了在气/液相转变区内系统状态连续变化的特征,与固/液相转变区的状态变化趋势相同。热力学性质和系统微观行为的模拟结果都表明,模拟系统的气/液相转变具有一级相变的特征,与已知的固/液一级相变相对应;从而揭示了固/液相转变和气/液相转变机制的一致性,建立了2维Lennard-Jones系统中完整并自洽的一级相变图景。     

Sexual selection enables long-term coexistence despite ecological equivalence

Empirical data indicate that sexual preferences are critical for maintaining species boundaries, yet theoretical work has suggested that, on their own, they can have only a minimal role in maintaining biodiversity. This is because long-term coexistence within overlapping ranges is thought to be unlikely in the absence of ecological differentiation. Here we challenge this widely held view by generalizing a standard model of sexual selection to include two ubiquitous features of populations with sexual selection: spatial variation in local carrying capacity, and mate-search costs in females. We show that, when these two features are combined, sexual preferences can single-handedly maintain coexistence, even when spatial variation in local carrying capacity is so slight that it might go unnoticed empirically. This theoretical study demonstrates that sexual selection alone can promote the long-term coexistence of ecologically equivalent species with overlapping ranges, and it thus provides a novel explanation for the maintenance of species diversity.     

非线性媒质中时空光孤子之间的能量交换

采用分裂步长傅立叶法研究了在色散立方-五次非线性媒质中碰撞的(3+1)维时空光孤子之间的能量交换.通过大量的数值模拟发现(3+1)维时空光孤子之间的能量交换对它们之间的相位差很敏感,两迎面相撞的时空孤子如果有相同的速率且它们之间的相位差在（0~π）之间,那么能量从初始相位滞后的光子弹转移到初始相位领先的光子弹;如果它们之间的相位差在（0~π）之间,那么能量从初始相位领先的光子弹转移到初始相位滞后的光子弹.     

Formation of ordered ice nanotubes inside carbon nanotubes

Following their discovery, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary, mould or template in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions. Particularly intriguing is the conjecture that matter within the narrow confines of a carbon nanotube might exhibit a solid-liquid critical point beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid-liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.