Strong pressure-energy correlations in van der Waals liquids

Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in computer simulations of the Lennard-Jones liquid and other simple liquids, but not for hydrogen-bonding liquids such as methanol and water. The correlations that are present also in the crystal and glass phases reflect an effective inverse power-law repulsive potential dominating fluctuations, even at zero and slightly negative pressure. In experimental data for supercritical argon, the correlations are found to be approximately 96%. Consequences for viscous liquid dynamics are discussed.     

Glass-forming liquids,anomalous liquids,and polyamorphism in liquids and biopolymers

Summary Glass formation in nature and materials science is reviewed and the recent recognition of polymorphism within the glassy state, polyamorphism, is discussed. The process by which the glassy state originates during the continuous cooling or viscous slowdown process, is examined and the three canonical characteristics of relaxing liquids are correlated through the fragility. The conversion of strong liquids to fragile liquids by pressure-induced coordination number increases is discussed, and then it is shown that for the same type of system it is possible to have the same conversion accomplished via a first-order transition within the liquid state. The systems in which this can happen are of the same type which exhibit polyamorphism, and the whole phenomenology can be accounted for by a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The concept of complex amorphous systems which can lose a significant number of degrees of freedom through weak first-order transitions is then used to discuss the relation between native and denatured hydrated proteins, since the latter have much in common with plasticized chain polymer systems. Finally, we close the circle by taking a short-time-scale phenomenon given much attention by protein physicists,viz., the onset of an anomaly in the Debye-Waller factor with increasing temperature, and showing that for a wide variety of liquids, including computer-simulated strong and fragile ionic liquids, this phenomenon is closely correlated with the experimental glass transition temperature. This implies that the latter owes its origin to the onset of strong anharmonicity in certain components of the vibrational density of states (evidently related to the boson peak) which then permits the system to gain access to its configurational degrees of freedom. The more anharmonic these vibrational components, the closer to the Kauzmann temperature will commence the exploration of configuration space and, for a given configurational microstate degeneracy, the more fragile the liquid will be. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Inherent Structure Approach to the Study of Glass-Forming Liquids

The inherent structure approach, wherein thermodynamic and structural changes in glass-forming liquids are analyzed in terms of local potential energy minima that the liquid samples, has recently been applied extensively to the study of thermodynamic aspects of glass-forming liquids. The evaluation of the configurational entropy, which arises from the multiplicity of local energy minima, plays a central role in such analyses. Results are presented here concerning the calculation of configurational entropy based on computer simulations of a model liquid; and the application of the inherent structure formalism to the study of the glass transition locus, and the fragility of glass-forming liquids.     

Measuring the configurational heat capacity of liquids

A high electric field impedance experiment on supercooled molecular liquids is employed to transfer energy to the slow modes by absorption from the field and detect the increase of their "configurational temperature", T(cfg), via the change of the relaxation times. This allows us to determine the configurational heat capacity, which accounts for most of the excess heat capacity for stronger liquids, but for only half of the heat capacity step in the case of more fragile systems. It is also observed that T(cfg) gradually approaches the phonon temperature on the structural relaxation time scale.     

Description peculiarities of viscous and elastic properties of dense molecular liquids on the basis of the model of Lennard-Jones (n−6) fluid

Results previously obtained in molecular dynamics experiment with Lennard-Jones (n−6) (L-J (n−6)) fluid were applied for the determination of viscous and elastic properties of real molecular fluids (shear viscosity coefficient and pressure). Parameters σ and of real liquids (liquid hydrocarbons) were determined by fitting p–ρ–T data of model fluids to experimental p–ρ–T data of real liquids. Using the data obtained in that way, parameters σ and viscous and elastic characteristics of real liquids were determined. The comparison of experimental and calculated viscous and elastic characteristics revealed lesser dependence of viscous properties from n in comparison with elastic properties.     

The radio frequency conductivity of some aliphatic alcohols at different temperatures

Activation energies for viscous flow and conductivity are computed from experimental values of RF conductivity (frequency 400 kHz) at different temperatures for eight polar liquids. It is observed that for aliphatic alcohols the activation energy for conductivity is greater than that for viscous flow. However, their ratio is slightly greater than unity and is different for different alcohols whereas for nonassociating polar liquids the ratio is almost equal to unity. The dependence of conductivity on viscosity for the liquid alcohols does not follow the Walden’s law but can be well represented by Adamczewskis relation.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers: a leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers A leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.     

弛豫铁电体弥散相变的玻璃化特性研究

根据弛豫铁电材料在相变区域的介电弥散行为和玻璃化 液体材料在过冷状态下黏度与温度的行为所共同满足的Vogel-Fulcher 函数关系, 分析了施主替代钛酸钡系列陶瓷的缺陷补偿原理, 通过引入玻璃化液体的构型熵概念, 研究了弛豫铁电材料中钛阳离子缺陷作用势的温度关系, 得到了如下结论: 施主掺杂含量的增加导致了无序度的增加, 钛离子缺陷浓度的增大和平均极性区域尺寸的减小; 在构型熵满足Vogel-Fulcher 函数关系的条件下, 温度越低, 钛离子缺陷作用的范围越大, 极化区域也越大. 缺陷作用的范围随温度的变化导致了弛豫铁电材料的弥散性. 温度下降到一定程度, 冻结效应发生, 介电弥散现象消失.     

Production of Fine Particles from Melts of Metals or Highly Viscous Fluids by ultrasonic standing wave atomization

Ultrasonic standing wave atomization (USWA) is a new process capable of atomizing both high surface energy liquids and highly viscous liquids. Atomization is achieved through acoustic forces acting upon a liquid jet which is guided into the central pressure node of a standing wave field. Spherical metal powders with minimum mass median diameters of less than 15 μm have been produced from metal melts with surface tensions of about 0.5 N/m. Organic liquids with viscosities between 1 and 10 Pas have been atomized, yielding mass median diameters from 20 to 330 μm. The influence of different operating parameters on the mass median diameter of metal melts and highly viscous liquids was evaluated. Parameters which were varied were ambient gas pressure, vibration amplitude of the transducers, mass flow rate, density of liquid, viscosity of the liquid, surface tension and the outlet diameter. The powders and sprays were analyzed with laser diffraction particle sizers. The physical background of the atomization process is discussed and an equation for the prediction of the mass median diameter is derived.     

Statistical field theory for simple fluids: mean field and Gaussian approximations

We present an exact field theoretical representation of the statistical mechanics of simple classical liquids with short-range pairwise additive interactions. The action of the field theory is obtained by performing a Hubbard-Stratonovich transformation of the configurational Boltzmann factor. The mean field and Gaussian approximations of the theory are derived and applications to the liquid-vapour transition considered.     

On derivation of the analytical expression for damping ratios in a low-viscosity approximation

The damping ratios of waves and oscillations in nonlinear dispersion equations are found for planar, cylindrical, and spherical geometries as applied to finite-volume liquids. For a cylindrical jet and a plane interface between viscous liquids, the damping ratios are determined for the first time. When the radius of curvature of the liquid jet surface decreases, so does the damping ratio of capillary waves. In a system of immiscible liquids, the damping ratio may be both larger and smaller than that for the pure liquid depending on the viscosity of the liquids and the ratio of their densities. This is because the damping ratio depends on the kinematic viscosities of pure liquids. The damping ratio is also estimated for waves arising at the liquidgas interface due to a tangential discontinuity of the velocity field.     

粘性流体对涡轮流量计仪表系数影响的实验研究

流体的粘性对涡轮流量计的仪表系数有很大影响．本文使用活塞式液体标准体积管在不同粘度下对涡轮流量计进行了实验研究，得出了仪表系数与流量、流体的粘度之间关系曲线．     

Shear viscosity of glass-forming melts in the liquid-glass transition region

A new approach to interpreting the hole-activation model of a viscous flow of glass-forming liquids is proposed. This model underlies the development of the concept on the exponential temperature dependence of the free energy of activation of a flow within the range of the liquid-glass transition in complete agreement with available experimental data. The “formation of a fluctuation hole” in high-heat glass-forming melts is considered as a small-scale low-activation local deformation of a structural network, i.e., the quasi-lattice necessary for the switching of the valence bond, which is the main elementary event of viscous flow of glasses and their melts. In this sense, the hole formation is a conditioned process. A drastic increase in the activation free energy of viscous flow in the liquid-glass transition region is explained by a structural transformation that is reduced to a limiting local elastic deformation of the structural network, which, in turn, originates from the excitation (critical displacement) of a bridging atom like the oxygen atom in the Si-O-Si bridge. At elevated temperatures, as a rule, a necessary amount of excited bridging atoms (locally deformed regions of the structural network) always exists, and the activation free energy of viscous flow is almost independent of temperature. The hole-activation model is closely connected with a number of well-known models describing the viscous flow of glass-forming liquids (the Avramov-Milchev, Nemilov, Ojovan, and other models).     

Cavitation breakup of low-and high-viscosity liquids

A cavitation criterion based on the notion of the incubation time is applied in analysis of experimental data for the tensile strength of water and glycerol (by analogy with successful analysis of spalling in solids carried out earlier). Good agreement between analytical results and experimental data is observed for both liquids. Behavioral models (equations) for a bubble under external pressure are considered for both inviscid and viscous liquids. These equations allow for transformations of similitude in both cases, but the exponents in these transformations are different.     

Heating liquid dielectrics by time dependent fields

Steady state and time-resolved dielectric relaxation experiments are performed at high fields on viscous glycerol and the effects of energy absorption from the electric field are studied. Time resolution is obtained by a sinusoidal field whose amplitude is switched from a low to a high level and by recording voltage and current traces with an oscilloscope during this transition. Based on their distinct time and frequency dependences, three sources of modifying the dynamics and dielectric loss via an increase in the effective temperature can be distinguished: electrode temperature, real sample temperature, and configurational temperatures of the modes that absorbed the energy. Isothermal conditions that are desired for focusing on the configurational temperature changes (as in dielectric hole burning and related techniques) are maintained only for very thin samples and for moderate power levels. For high frequencies, say ν > 1 MHz, changes of the real temperature will exceed the effects of configurational temperatures in the case of macroscopic samples. Regarding microwave chemistry, heating via cell phone use, and related situations in which materials are subject to fields involving frequencies beyond the MHz regime, we conclude that changes in the configurational (or fictive) temperatures remain negligible compared with the increase of the real temperature. This simplifies the assessment of how time dependent electric fields modify the properties of materials.     

Folding of viscous threads in diverging microchannels

We study the folding instability of a viscous thread surrounded by a less viscous miscible liquid flowing from a square to a diverging microchannel. Because of the change in the flow introduced by the diverging channel, the viscous thread minimizes viscous dissipation by oscillating to form bends rather than by simply dilating. The folding frequency and the thread diameter can be related to the volume flow rates and thus to the characteristic shear rate. Diffusive mixing at the boundary of the thread can significantly modify the folding flow morphologies. This microfluidic system enables us to control the bending of the thread and to enhance mixing between liquids having significantly different viscosities.     

Effect of a capillary meniscus on the Faraday instability threshold

Threshold for Faraday instability has been experimentally measured for slightly viscous liquids. Changing the size of the container containing the fluid allows us to emphasize the role played by the capillary meniscus on the onset for instability. As the container is getting smaller, an upset of the critical acceleration is observed. Below a given container diameter, eigenmodes are observed along the stability curve. A dissipation term is proposed for considering the viscous dissipation against the walls of the container.     

Self-propelled Leidenfrost droplets

We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchetlike) topology. The pumping effect is observed when the liquid is in the Leidenfrost regime (the film-boiling regime), for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid.     

Dynamic rheological and dynamic thermomechanical properties of poly(trimethylene terephthalate)/short carbon fibre composites

The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.