The relationship between fragility, configurational entropy and the potential energy landscape of glass-forming liquids

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a manner that it does not crystallize. Almost all types of materials are capable of glass formation, including polymers, metal alloys and molten salts. Given such diversity, general principles by which different glass-forming materials can be systematically classified are invaluable. One such principle is the classification of glass-formers according to their fragility. Fragility measures the rapidity with which a liquid's properties (such as viscosity) change as the glassy state is approached. Although the relationship between the fragility, configurational entropy and features of the energy landscape (the complicated dependence of energy on configuration) of a glass-former have been analysed previously, a detailed understanding of the origins of fragility is lacking. Here I use simulations to analyse the relationship between fragility and quantitative measures of the energy landscape for a model liquid whose fragility depends on its bulk density. The results reveal that fragility depends on changes in the vibrational properties of individual energy minima in addition to their total number and spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with kinetic fragilities obtained from the liquid's diffusivity.     

Thermodynamic determination of fragility in La-based glass-forming liquid

Differences in the thermodynamic functions between the liquid and crystalline states of La-based bulk metallic glasses alloys were calculated with the specific heat capacity Cp and the fusion heat ΔHf,which we measured. Fragility indexes having different thermodynamic definitions were calculated from the temperature dependence of excess entropy ΔSliq-cry. It is ambiguous for La-based glass-forming liquid to evaluate fragility from the intercepts of ΔSliq-cry-temperature curves. We found that the thermodynam...     

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.     

体相CO2约化自扩散系数的标度律关联模型

用巨正则转换矩阵蒙特卡罗（GC-TMMC）和分子动力学（MD）模拟方法研究了体相CO2单位分子的过剩熵与其约化自扩散系数（D*s）的标度关系。过剩熵主要考虑3种定义：总体过剩熵sex，以分子重心径向分布函数定义的二体熵s(2)c和所有原子径向分布函数定义的二体熵s(2)t。结果表明，总体过剩熵sex与D*s之间存在明显的标度关系，并且独立于状态点所处的相态，可以用热力学量-过剩熵来关联动力学量-自扩散系数（约化方式）；二体熵在小于超临界温度范围内的标度结果偏离了-sex-D*s的“共同曲线”。本文拓展了过剩熵标度律在分子流体中的应用范围，得到了其在体相CO2自扩散系数关联中的应用规律。     

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.     

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.     

低密度玻璃化液体中力学模量与脆性的关联研究

利用低密度玻璃化液体的径向分布函数,导出过冷液体脆性与玻璃化温度、体模量和剪切模量的比值及与柏松比之间简单明确的关系.此结论定性地与实验结果一致.并为各种玻璃化液体的研究,提供有效的途径.     

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.     

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.     

林分结构多样性参数与纹理信息相关分析中最优窗口的确定

【目的】遥感影像中的纹理指数在林业领域的应用日益广泛,窗口大小对纹理指数与林分结构多样性指数的相关性影响较大,因此研究试图利用纹理指数与林分结构多样性指数的相关性大小确定最优窗口,以期实现林分结构多样性指数的准确预估。【方法】基于广西壮族自治区一类调查的 68 块杉木纯林或混交林样地,计算得出林分结构多样性指数; 根据 SPOT5 全色波段,分别计算7种窗口(3×3、5×5、7×7、9×9、11×11、13×13 和 15×15)下的纹理指数,对窗口大小与纹理指数两者进行Pearson相关性分析,并确定最优窗口。【结果】熵、同质性、角二阶矩阵、对比度、非相似度、变化量以及平均值与林分结构多样性的相关性随窗口的变化不显著,但纹理相关性指数与 DBH 标准差的相关系数的变化率达到了86%。【结论】基于DBH标准差与纹理指数相关关系的变化规律,确定研究区最优窗口大小为11×11。     

Propagation of the polyamorphic transition of ice and the liquid-liquid critical point

Water has a rich metastable phase behaviour that includes transitions between high- and low-density amorphous ices, and between high- and low-density supercooled liquids. Because the transitions occur under conditions where crystalline ice is the stable phase, they are challenging to probe directly. In the case of the liquids, it remains unclear whether their mutual transformation at low temperatures is continuous, or discontinuous and terminating at a postulated second critical point of water that is metastable with respect to crystallization. The amorphous ices are more amenable to experiments, which have shown that their mutual transformation is sharp and reversible. But the non-equilibrium conditions of these studies make a firm thermodynamic interpretation of the results difficult. Here we use Raman spectroscopy and visual inspection to show that the transformation of high-density to low-density amorphous ices involves the propagation of a phase boundary-a region containing a mixture of both ices. We find that the boundary region becomes narrower as the transformation progresses, and at higher transformation temperatures. These findings strongly suggest that the polyamorphic ice transition is discontinuous; a continuous transformation should occur uniformly over the entire sample. Because the amorphous ices are structurally similar to their supercooled liquid counterparts, our results also imply that the liquids transform discontinuously at low temperatures and thus support the liquid-liquid critical-point theory.     

Quantum phase transition in a resonant level coupled to interacting leads

A Luttinger liquid is an interacting one-dimensional electronic system, quite distinct from the 'conventional' Fermi liquids formed by interacting electrons in two and three dimensions. Some of the most striking properties of Luttinger liquids are revealed in the process of electron tunnelling. For example, as a function of the applied bias voltage or temperature, the tunnelling current exhibits a non-trivial power-law suppression. (There is no such suppression in a conventional Fermi liquid.) Here, using a carbon nanotube connected to resistive leads, we create a system that emulates tunnelling in a Luttinger liquid, by controlling the interaction of the tunnelling electron with its environment. We further replace a single tunnelling barrier with a double-barrier, resonant-level structure and investigate resonant tunnelling between Luttinger liquids. At low temperatures, we observe perfect transparency of the resonant level embedded in the interacting environment, and the width of the resonance tends to zero. We argue that this behaviour results from many-body physics of interacting electrons, and signals the presence of a quantum phase transition. Given that many parameters, including the interaction strength, can be precisely controlled in our samples, this is an attractive model system for studying quantum critical phenomena in general, with wide-reaching implications for understanding quantum phase transitions in more complex systems, such as cold atoms and strongly correlated bulk materials.     

Dynamics of supercooled water in confined geometry

As with most liquids, it is possible to supercool water; this generally involves cooling the liquid below its melting temperature (avoiding crystallization) until it eventually forms a glass. The viscosity and related relaxation times (tau) of glass-forming liquids typically show non-Arrhenius temperature (T) dependencies. Liquids with highly non-Arrhenius behaviour in the supercooled region are termed 'fragile'. In contrast, liquids whose behaviour is close to the Arrhenius law (In tau infinity 1/T) are termed 'strong'. A unique 'fragile-strong' transition around 228 K has been proposed for supercooled water; however, experimental studies of bulk supercooled water in this temperature range are generally hampered because crystallization occurs. Here we use broad-band dielectric spectroscopy to study the relaxation dynamics of supercooled water in a wide temperature range, including the usually inaccessible temperature region. This is possible because the supercooled water is held within a layered vermiculite clay-the geometrical confinement and presence of intercalated sodium ions prevent most of the water from crystallizing. We find a relaxational process with an Arrhenius temperature dependence, consistent with the proposed strong nature of deeply supercooled bulk water. Because water that is less supercooled has been established as highly fragile, our results support the existence of a fragile-strong transition.     

Functional waters in intraprotein proton transfer monitored by FTIR difference spectroscopy

Much progress has been made in our understanding of water molecule reactions on surfaces, proton solvation in gas-phase water clusters and proton transfer through liquids. Compared with our advanced understanding of these physico-chemical systems, much less is known about individual water molecules and their cooperative behaviour in heterogeneous proteins during enzymatic reactions. Here we use time-resolved Fourier transform infrared spectroscopy (trFTIR) and in situ H2(18)O/H2(16)O exchange FTIR to determine how the membrane protein bacteriorhodopsin uses the interplay among strongly hydrogen-bonded water molecules, a water molecule with a dangling hydroxyl group and a protonated water cluster to transfer protons. The precise arrangement of water molecules in the protein matrix results in a controlled Grotthuss proton transfer, in contrast to the random proton migration that occurs in liquid water. Our findings support the emerging paradigm that intraprotein water molecules are as essential for biological functions as amino acids.     

Supercooled liquids and the glass transition

Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state-supercooling-has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.     

The jamming route to the glass state in weakly perturbed granular media

It has been suggested that a common conceptual framework known as 'jamming' (refs 1 and 2) may be used to classify a wide variety of physical systems; these include granular media, colloidal suspensions and glass-forming liquids, all of which display a critical slowdown in their dynamics before a sudden transition to an amorphous rigid state. Decreasing the relevant control parameter (such as temperature, drive or inverse density) may cause geometrical constraints to build up progressively and thus restrict the accessible part of the system's phase space. In glass-forming liquids (thermal molecular systems), jamming is provided by the classical vitrification process of supercooling, characterized by a rapidly increasing and apparently diverging viscosity at sufficiently low temperatures. In driven (athermal) macroscopic systems, a similar slowdown has been predicted to occur, notably in sheared foam or vibrated granular media. Here we report experimental evidence for dynamic behaviour, qualitatively analogous to supercooling, in a driven granular system of macroscopic millimetre-size particles. The granular medium is perturbed by isolated tapping or continuous vibration, with the perturbation intensity serving as a control parameter. We observe the random deflection of an immersed torsion oscillator that moves each time the grains rearrange, like a 'thermometer' sensing the granular noise. We caution that our granular analogy to supercooling is based on similarities in the dynamical behaviour, rather than quantitative theory.     

Si-Ca-Ba三元合金熔体热力学性质的计算

根据Miedema二元合金生成热模型和Toop非对称几何模型,对1 873 K时Si-Ca-Ba三元合金熔体过剩自由能、生成热、过剩熵以及各组元间的活度交互作用系数等热力学性质进行了计算.结果表明,在1 873 K时Si-Ca-Ba合金熔体的过剩自由能、生成热和过剩熵均为负值.在富硅区域和硅含量较少的区域,各热力学性质的变化趋势较大.过剩自由能、生成热和过剩熵的最小值均出现在xSi=55%,xCa=45%,xBa=0成分处.Si-Ca-Ba合金中组元钡的加入,对合金的过剩自由能、生成热和过剩熵影响不大,随钡含量的增加,各热力学性质无显著变化.     

Observation of five-fold local symmetry in liquid lead

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.     

Fluidity of water confined to subnanometre films.

The fluidity of water in confined geometries is relevant to processes ranging from tribology to protein folding, and its molecular mobility in pores and slits has been extensively studied using a variety of approaches. Studies in which liquid flow is measured directly suggest that the viscosity of aqueous electrolytes confined to films of thickness greater than about 2-3 nm remains close to that in the bulk; this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers. Here we observe that the effective viscosity of water remains within a factor of three of its bulk value, even when it is confined to films in the thickness range 3.5 +/- 1 to 0.0 +/- 0.4 nm. This contrasts markedly with the behaviour of organic solvents, whose viscosity diverges when confined to films thinner than about 5-8 molecular layers. We attribute this to the fundamentally different mechanisms of solidification in the two cases. For non-associative liquids, confinement promotes solidification by suppressing translational freedom of the molecules; however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing.     

离子液体CnpyNTf2(n=2,4,5)甲醇溶液的密度和超额摩尔体积

离子液体作为一种新型的环境友好的溶剂,已被应用到越来越多的领域.合成了同系列的3种离子液体CnpyNTf2(n=2,4,5),并对其物化性质进行了研究.使用韦氏天平测量了CnpyNTf2(n=2,4,5)及其甲醇溶液的密度.通过一系列的测量和计算,得到了不同温度、不同摩尔分数下离子液体的密度和超额摩尔体积,为离子液体的...