Configurational entropy and diffusivity of supercooled water

As a liquid approaches the glass transition, its properties are dominated by local potential minima in its energy landscape. The liquid experiences localized vibrations in the basins of attraction surrounding the minima, and rearranges via relatively infrequent inter-basin jumps. As a result, the liquid dynamics at low temperature are related to the system's exploration of its own configuration space. The 'thermodynamic approach' to the glass transition considers the reduction in configuration space explored as the system cools, and predicts that the configurational entropy (a measure of the number of local potential energy minima sampled by the liquid) is related to the diffusion constant. Here we report a stringent test of the thermodynamic approach for liquid water (a convenient system to study because of an anomalous pressure dependence in the diffusion constant). We calculate the configurational entropy at points spanning a large region of the temperature-density plane, using a model that reproduces the dynamical anomalies of liquid water. We find that the thermodynamic approach can be used to understand the characteristic dynamic anomalies, and that the diffusive dynamics are governed by the configurational entropy. Our results indicate that the thermodynamic approach might be extended to predict the dynamical behaviour of supercooled liquids in general.     

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。     

Structural relaxation in supercooled water by time-resolved spectroscopy

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water. But until now, such a characterization has only been realized in simulations that agree with the predictions of simple mode-coupling theory; unambiguous experimental support for this surprising conclusion is, however, not yet available. Here we report time-resolved optical Kerr effect measurements that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.     

'Inverse' melting of a vortex lattice

Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.     

Direct observation of molecular cooperativity near the glass transition

The increasingly sluggish response of a supercooled liquid as it nears its glass transition (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking. Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.     

多相混输体系析蜡热力学研究进展

针对海底多相混输体系析蜡问题,开展了蜡热力学预测模型及析蜡实验测量方法的研究。通过对Won 模型、
Ji 模型等析蜡热力学预测模型的分析,认为热力学预测模型的建立过程中需考虑到液相描述一致性、固–固转变、纯组
分热力学性质、Poynting 修正项处理以及活度系数计算等问题。并对可用于多相体系析蜡实验测量的激光法、超声波
法、声共振法、石英晶体微天平法以及高压微差示扫描量热法等进行了分析研究。认为石英晶体微天平法因其较高的
灵敏性,高压微差示扫描量热法因其简便、耗样量少、再现性好且测试过程中基本避免了人为因素的影响,具有较好的
应用前景。     

Fragile-to-strong transition and polyamorphism in the energy landscape of liquid silica

Liquid silica is the archetypal glass former, and compounds based on silica are ubiquitous as natural and man-made amorphous materials. Liquid silica is also the extreme case of a 'strong' liquid, in that the variation of viscosity with temperature closely follows the Arrhenius law as the liquid is cooled toward its glass transition temperature. In contrast, most liquids are to some degree 'fragile', showing significantly faster increases in their viscosity as the glass transition temperature is approached. Recent studies have demonstrated the controlling influence of the potential energy hypersurface (or 'energy landscape') of the liquid on the transport properties near the glass transition. But the origin of strong liquid behaviour in terms of the energy landscape has not yet been resolved. Here we study the static and dynamic properties of liquid silica over a wide range of temperature and density using computer simulations. The results reveal a change in the energy landscape with decreasing temperature, which underlies a transition from a fragile liquid at high temperature to a strong liquid at low temperature. We also show that a specific heat anomaly is associated with this fragile-to-strong transition, and suggest that this anomaly is related to the polyamorphic behaviour of amorphous solid silica.     

Observation of scale invariance and universality in two-dimensional Bose gases

The collective behaviour of a many-body system near a continuous phase transition is insensitive to the details of its microscopic physics; for example, thermodynamic observables follow generalized scaling laws near the phase transition. The Berezinskii-Kosterlitz-Thouless (BKT) phase transition in two-dimensional Bose gases presents a particularly interesting case because the marginal dimensionality and intrinsic scaling symmetry result in a broad fluctuation regime and an extended range of universal scaling behaviour. Studies of the BKT transition in cold atoms have stimulated great interest in recent years, but a clear demonstration of critical behaviour near the phase transition has remained elusive. Here we report in situ density and density-fluctuation measurements of two-dimensional Bose gases of caesium at different temperatures and interaction strengths, observing scale-invariant, universal behaviours. The extracted thermodynamic functions confirm the existence of a wide universal region near the BKT phase transition, and provide a sensitive test of the universality predicted by classical-field theory and quantum Monte Carlo calculations. Our experimental results provide evidence for growing density-density correlations in the fluctuation region, and call for further explorations of universal phenomena in classical and quantum critical physics.     

A thermodynamic connection to the fragility of glass-forming liquids

Although liquids normally crystallize on cooling, there are members of all liquid types (including molecular, ionic and metallic) that supercool and then solidify at their glass transition temperature, Tg. This continuous solidification process exhibits great diversity within each class of liquid-both in the steepness of the viscosity-temperature profile, and in the rate at which the excess entropy of the liquid over the crystalline phase changes as Tg is approached. However, the source of the diversity is unknown. The viscosity and associated relaxation time behaviour have been classified between 'strong' and 'fragile' extremes, using Tg as a scaling parameter, but attempts to correlate such kinetic properties with the thermodynamic behaviour have been controversial. Here we show that the kinetic fragility can be correlated with a scaled quantity representing excess entropy, using data over the entire fragility range and embracing liquids of all classes. The excess entropy used in our correlation contains both configurational and vibration-related contributions. In order to reconcile our correlation with existing theory and simulations, we propose that variations in the fragility of liquids originate in differences between their vibrational heat capacities, harmonic and anharmonic, which we interpret in terms of an energy landscape. The differences evidently relate to behaviour of low-energy modes near and below the boson peak.     

Poisson's ratio and the fragility of glass-forming liquids

The nature of the transformation by which a supercooled liquid 'freezes' to a glass--the glass transition--is a central issue in condensed matter physics but also affects many other fields, including biology. Substantial progress has been made in understanding this phenomenon over the past two decades, yet many key questions remain. In particular, the factors that control the temperature-dependent relaxation and viscous properties of the liquid phase as the glass transition is approached (that is, whether the glass-forming liquid is 'fragile' or 'strong') remain unclear. Here we show that the fragility of a glass-forming liquid is intimately linked to a very basic property of the corresponding glass phase: the relative strength of shear and bulk moduli, or Poisson's ratio.     

Geometric phase shifts in chemical oscillators

One of the most remarkable developments in quantum mechanics in recent years has been the discovery that when a system is moved adiabatically around a closed loop in parameter space there occurs, besides the familiar dynamical phase shift, an additional phase shift (sometimes referred to as 'Berry's phase') that is purely geometric in nature. The dynamical phase shift, which results from the variation of the period of the oscillatory system with the change in parameters, is relatively easily understood and is proportional to the time over which the parameter change occurs. The geometric phase shift, on the other hand, is less intuitive and depends on the curvature of the surface in parameter space bounded by the closed path, but is independent of the time taken to traverse the circuit. Here we present evidence for time-independent geometric phase shifts in numerical solutions for a model of an oscillating chemical reaction. The conditions for the occurrence of such shifts seem to be sufficiently general that geometric phase effects should be experimentally observable in essentially all chemical oscillators as well as in biological networks such as the brain and the central nervous system, where phase control is of vital importance.     

Gelation of particles with short-range attraction

Nanoscale or colloidal particles are important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behaviour to a wide variety of complex fluids. This behaviour arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA). This is understood as a purely kinetic phenomenon that can form solid-like gels at arbitrarily low particle volume fraction. Far more important technologically are systems with weaker attractions, where gel formation requires higher volume fractions. Numerous scenarios for gelation have been proposed, including DLCA, kinetic or dynamic arrest, phase separation, percolation and jamming. No consensus has emerged and, despite its ubiquity and significance, gelation is far from understood-even the location of the gelation phase boundary is not agreed on. Here we report experiments showing that gelation of spherical particles with isotropic, short-range attractions is initiated by spinodal decomposition; this thermodynamic instability triggers the formation of density fluctuations, leading to spanning clusters that dynamically arrest to create a gel. This simple picture of gelation does not depend on microscopic system-specific details, and should thus apply broadly to any particle system with short-range attractions. Our results suggest that gelation-often considered a purely kinetic phenomenon-is in fact a direct consequence of equilibrium liquid-gas phase separation. Without exception, we observe gelation in all of our samples predicted by theory and simulation to phase-separate; this suggests that it is phase separation, not percolation, that corresponds to gelation in models for attractive spheres.     

汽－液相变中成核的热力学理论

对现有汽－液相变过程有关文献中关于汽泡形成过程的分析提出了不同看法，并利用热力学理论从亚稳平衡态的特性出发对汽泡形成的热力学机制作出了分析，对系统从液态转变到汽－液共存状态的各阶段作出了说明。     

Phonon interpretation of the 'boson peak' in supercooled liquids

Glasses are amorphous solids, in the sense that they display elastic behaviour. In crystalline solids, elasticity is associated with phonons, which are quantized vibrational excitations. Phonon-like excitations also exist in glasses at very high (terahertz; 10(12) Hz) frequencies; surprisingly, these persist in the supercooled liquids. A universal feature of such amorphous systems is the boson peak: the vibrational density of states has an excess compared to the Debye squared-frequency law. Here we investigate the origin of this feature by studying the spectra of inherent structures (local minima of the potential energy) in a realistic glass model. We claim that the peak is the signature of a phase transition in the space of the stationary points of the energy, from a minima-dominated phase (with phonons) at low energy to a saddle-point-dominated phase (without phonons). The boson peak moves to lower frequencies on approaching the phonon-saddle transition, and its height diverges at the critical point. Our numerical results agree with the predictions of euclidean random matrix theory on the existence of a sharp phase transition between an amorphous elastic phase and a phonon-free one.     

乙醇汽油体系热力学性质的研究

用低温绝热量热计测量了国产93#无铅汽油和乙醇混合体系(10wt％乙醇 90wt汽油)在78～320K温区的热容．用最小二乘法拟合了热容与温度的函数关系．由热容随温度的变化曲线可知,混合体系在93.17K发生玻璃化转变;在150．86K发生固-液相变,分别计算了相变过程中的焓变和熵变．计算了以298．15K为基准的该混合体系的热力学函数。     

Liquid fragility——A key togoing deep into materials of glassy states

“Liquid fragility“ is a concept that has been widely used in the investigation on the glass community,though it was presented less than two decades ago. The concept enables the comparison between the glass-forming liquids with different dynamic characters by using a general criterion, in which the temperature scale is reduced by the glass transition temperature. In order to illuminate the significance of the concept in the fields of the glass transition,structural relaxation process and the structure of supercooled liquids, the accomplished progress and the faced challenges are summarized from different aspects such as on the correlation between dynamics and thermodynamic characters of condensed matters, on the energy landscape, on the nonexponential relaxation and on the theoretical model of microstructure and medium-range order. The tendency of investigation in “liquid fragility“ is also evaluated.     

Clarifying the glass-transition behaviour of water by comparison with hyperquenched inorganic glasses

The formation of glasses is normal for substances that remain liquid over a wide temperature range (the 'good glassformers') and can be induced for most liquids if cooling is fast enough to bypass crystallization. During reheating but still below the melting point, good glassformers exhibit glass transitions as they abruptly transform into supercooled liquids, whereas other substances transform directly from the glassy to the crystalline state. Whether water exhibits a glass transition before crystallization has been much debated over five decades. For the last 20 years, the existence of a glass transition at 136 K (ref. 3) has been widely accepted, but the transition exhibits qualities difficult to reconcile with our current knowledge of glass transitions. Here we report detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures. We compare our results to the behaviour of glassy water and find that small endothermic effects, such as the one attributed to the glass transition of water, are only a 'shadow' of the real glass transition occurring at higher temperatures, thus substantiating the conclusion that the glass transition of water cannot be probed directly.     

Decompression-induced disorder to order phase transition in low-melting ionic liquid [OMIM][PF6]

In situ pressure-induced Raman spectral changes of 1-octyl-3-methyl imidazolium hexafluorophosphate （[OMIM][PF6]） have been investigated under the pressure up to 5.86 GPa at room temperature. The results indicated that [OMIM][PF6] experienced a phase transition at about 4.12 GPa during compression, and it was thought as a phase transition of liquid to a superpressurized glass. Upon decompression, from the obvious change of Raman spectra of [OMIM][PF6] at about 0.48 GPa, it could be inferred that a decompression-induced disorder to order phase transition in [OMIM][PF6] occurred. The phase behavior of [OMIM] [PF6] at low temperature under atmospheric pressure was also investigated in detail. The result showed that Raman spectra of [OMIM][PF6] varied slightly and no crystallization occurred upon cooling. These facts suggested that a disorder to order phase transition was induced by decompression in [OMIM][PF6], and [OMIM][PF6] served as a superpressurized glass under the pressure above 4.12 GPa, which was similar to the glassy state at low temperature.     

蒙特卡罗方法模拟无序涡旋系统中的相图

从GL(Ginzburg-Landau)自由能出发,在最低朗道能级近似下,用蒙特卡罗的方法模拟了二维无序第二类超导体涡旋态系统.对于纯净的涡旋系统,得到了从涡旋固态到涡旋液态的一级相变,验证了前人的理论.对于无序的涡旋系统,发现低温区域存在Bragg玻璃态.