The structure of ambient water

We review the spectroscopic techniques and scattering experiments used to probe the structure of water, and their interpretation using empirical and ab initio models, over the last 5 years. We show that all available scientific evidence overwhelmingly favors the view of classifying water near ambient conditions as a uniform, continuous tetrahedral liquid. While there are controversial issues in our understanding of water in the supercooled state, in confinement, at interfaces, or in solution, there is no real controversy in what is understood as regards bulk liquid water under ambient conditions.     

Imaging density disturbances in water with a 41.3-attosecond time resolution

We show that the momentum flexibility of inelastic x-ray scattering may be exploited to invert its loss function, allowing real time imaging of density disturbances in a medium. We show the disturbance arising from a point source in liquid water, with a resolution of 41.3 attoseconds (4.13 x 10(-17) s) and 1.27 A (1.27 x 10(-8) cm). This result is used to determine the structure of the electron cloud around a photoexcited chromophore in solution, as well as the wake generated in water by a 9 MeV gold ion. We draw an analogy with pump-probe techniques and suggest that energy-loss scattering may be applied more generally to the study of attosecond phenomena.     

Superstability of surface nanobubbles

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to -6 MPa. We denote this feature as superstability.     

The puzzling unsolved mysteries of liquid water: Some recent progress

Water is perhaps the most ubiquitous, and the most essential, of any molecule on earth. Indeed, it defies the imagination of even the most creative science fiction writer to picture what life would be like without water. Despite decades of research, however, water's puzzling properties are not understood and 63 anomalies that distinguish water from other liquids remain unsolved. We introduce some of these unsolved mysteries, and demonstrate recent progress in solving them. We present evidence from experiments and computer simulations supporting the hypothesis that water displays a special transition point (which is not unlike the “tipping point” immortalized by Malcolm Gladwell). The general idea is that when the liquid is near this “tipping point,” it suddenly separates into two distinct liquid phases. This concept of a new critical point is finding application to other liquids as well as water, such as silicon and silica. We also discuss related puzzles, such as the mysterious behavior of water near a protein.     

Resolving the optical spectrum of water: coordination and electrostatic effects

The optical absorption of small water clusters, water chains, liquid water, and crystalline ice is analyzed computationally. We identify two competing mechanisms determining the onset of the optical absorption: Electronic transitions involving surface molecules of finite clusters or chains cause a redshift upon molecular aggregation compared to monomers. On the other hand, a strong blueshift is caused by the electrostatic environment experienced by water monomers embedded in a hydrate shell. Concerning the recent dispute over the structure of the liquid, the present results support the conventional fourfold coordinated water, as obtained from ab initio molecular-dynamics simulations.     

Measurement of the viscosity-density product using multiple reflections of ultrasonic shear horizontal waves

We have developed an on-line computer-controlled sensor, based on ultrasound reflection measurements, to determine the product of the viscosity and density of a liquid or slurry for Newtonian fluids and the shear impedance of the liquid for non-Newtonian fluids. A 14 MHz shear wave transducer is bonded to one side of a 45-90 degrees fused silica wedge and the base is in contract with the liquid. Twenty-eight echoes were observed due to the multiple reflections of an ultrasonic shear horizontal (SH) wave within the wedge. The fast Fourier transform of each echo was obtained for a liquid and for water, which serves as the calibration fluid, and the reflection coefficient at the solid-liquid interface was obtained. Data were obtained for 11 sugar water solutions ranging in concentration from 10% to 66% by weight. The viscosity values are shown to be in good agreement with those obtained independently using a laboratory viscometer. The data acquisition time is 14s and this can be reduced by judicious selection of the echoes for determining the reflection coefficient. The measurement of the density results in a determination of the viscosity for Newtonian fluids or the shear wave velocity for non-Newtonian fluids. The sensor can be deployed for process control in a pipeline, with the base of the wedge as part of the pipeline wall, or immersed in a tank.     

Polyhedral shaped gold nanoparticles with outstanding near-infrared light absorption

A time-resolved X-ray absorption study of the structural dynamics of liquid water on a picosecond timescale is presented. We apply femtosecond midinfrared pulses to resonantly excite the intramolecular O–H stretching band of liquid water and monitor the transient response in the oxygen K-edge absorption spectrum with picosecond X-ray pulses. In this way, structural changes in the hydrogen bond network of liquid water upon an ultrafast temperature jump of approximately 20 K are investigated. The changes of the X-ray absorption as induced by such a temperature jump are about 3.2%. This demonstrates that our method serves as a sensitive probe of transient structural changes in liquid water and that combined infrared-laser–synchrotron experiments with substantially shorter X-ray pulses, such as generated with a femtosecond slicing scheme, are possible.     

Application of the two-liquid model for the interpretation of the observed electrophysical properties of supercooled water in nanopores

We propose a model combining the first-order liquid-liquid and the second-order ferroelectric phase transitions phenomenology to explain various features of the λ-point of liquid water. We suggest that the long-range dipole-dipole interaction of water molecules leads to a ferroelectric phase transition, which occurs only in the low-density component of the liquid and is the origin of the singularity of the dielectric constant recently observed in experiments with supercooled liquid water in nanoporous materials. Finally, we establish the model parameters and prove the consistency of the combined model by comparing its predictions with experimental data and the results of recent molecular-dynamics simulations.     

Structure of supercooled and glassy water under pressure

We use molecular-dynamics simulations to study the effect of temperature and pressure on the local structure of liquid water in parallel with neutron-scattering experiments. We find, in agreement with experimental results, that the simulated liquid structure at high pressure is nearly independent of temperature, and remarkably similar to the known structure of the high-density amorphous ice. Further, at low pressure, the liquid structure appears to approach the experimentally measured structure of low-density amorphous ice as temperature decreases. These results are consistent with the postulated continuity between the liquid and glassy phases of H2O.     

Molecular-scale density oscillations in water adjacent to a mica surface

High-resolution specular x-ray reflectivity of the mica(001)-water interface under ambient conditions reveals oscillations in water oxygen density in the surface-normal direction, giving evidence of interfacial water ordering. The spacings between neighboring water layers in the near-surface, strongly oscillatory region are 2.5(2)-2.7(2) A, approximately the size of the water molecule. The density oscillations extend to about 10 A above the surface and do not strictly maintain a solvent-size periodicity as that in interfacial liquid metal and hard-sphere molecular liquids. We interpret this oscillatory density profile of the interfacial water as due to the "hard-wall" effect of the molecularly smooth mica surface.     

Molecular structure of alcohol-water mixtures

We use x-ray emission spectroscopy to elucidate the molecular structure of liquid methanol, water, and methanol-water solutions. We find that molecules in the pure liquid methanol predominantly persist as hydrogen-bonded chains and rings with six and/or eight molecules of equal abundance. For water-methanol solutions we find evidence of incomplete mixing at the microscopic level. Our results provide a new explanation for a smaller entropy increase in the solution due to water molecules bridging methanol chains to form rings.     

Dynamics of capillary drying in water

We use atomistic simulations to address the question when capillary evaporation of water confined in a hydrocarbonlike slit is kinetically viable. Activation barriers and absolute rates of evaporation are estimated using open ensemble Monte Carlo-umbrella sampling and molecular dynamics simulations. At ambient conditions, the evaporation rate in a water film four molecular diameters thick is found to be of the order 10(5)(nm(2) s)(-1), meaning that water readily evaporates. Films more than a few nanometers thick will persist in a metastable liquid state. Dissolved atmospheric gas molecules do not significantly decrease the activation barrier.     

Quantum cluster equilibrium theory treatment of hydrogen-bonded liquids: water,methanol and ethanol

The quantum cluster equilibrium (QCE) theory was used in order to predict the composition of the hydrogen bonded liquids: water, methanol and ethanol. The calculations were based on high accuracy theoretical data obtained at the DFT/B3LYP/6-311 G(d,p) level of theory. All investigated liquids are predicted to be composed of big clusters: hexamers in the case of water, tetramers, pentamers, hexamers and heptamers in the case of methanol and pentamers in the case of ethanol. The content of big clusters in a liquid phase as predicted by QCE is overestimated. We have found two confirmations of this. First of all, the behaviour of the liquid water isobar clearly demonstrates that there should be a substantial amount of small clusters in order to obtain the correct temperature dependence of the molar volume. Indeed, the theoretical molar volume close to the boiling point is by about 0.6cm³ lower than the experimental one. The molar volume is too low due to the overestimated population of big clusters resulting in too high a liquid density. Second, the temperature dependence of the chemical shift of the hydroxyl protons in liquid methanol and ethanol, obtained as the population weighted average of the chemical shift of individual clusters, is shifted down field as compared to experiment by as much as 2ppm. This is because big clusters with strongly deshielded hydroxyl protons contribute too much to the weighted average. Possible shortcomings of the QCE approach are discussed.     

Optical depth and multiple scattering depolarization in liquid clouds

In this paper, we calculate multiply scattered lidar signals with Monte Carlo method for measuring optical depth (extinction coefficient), effective size of water droplets, and liquid water content of clouds, and present algorithms that implement our method. We calculated multiply scattered lidar signals for various water droplet sizes and liquid water contents using a Monte Carlo method. A simple correspondence between water droplet optical depth and the degree of polarization in a modified gamma size distribution (C₁ cloud) is found. We also calculated the degree of polarization of a lidar signal for a given liquid water content, finding that the degree of polarization is only dependent on optical depth. Since the Raman lidar signal of liquid water depends on the total volume of the water droplet, the effective radius of the water droplet can thus be recovered from the degree of polarization of the lidar signal and the Raman signal of the liquid water.     

三维倾斜平板液氧降膜润湿数值模拟研究

建立三维倾斜平板降膜模型,利用VOF两相流模型计算了液氧降膜的润湿情况,研究了工质物性、倾斜角、液膜入口高度对润湿面积的影响。结果表明:Weber数(We)相同时,液氧和水的润湿比均随Kapitza数(Ka)增大而减小;相同Ka下,液氧和水的润湿比均随We增大而增大,而液氧润湿比一直小于水润湿比,两者的差值也随We增大而增大。拟合得到液氧在液膜入口高度0.4 mm、接触角70°时的界面润湿比经验关联式,拟合值和模拟值相对误差≤±20%;在We=0.76时,液氧的润湿比随倾斜角增大而减小,但降低速率随Ka增大而减小;在倾斜角为90°时,易出现液膜脱离壁面的现象;当We固定时,液氧的润湿比随液膜入口高度增大而增大。     

Water flows induced by microwave electric fields in microsystems

AC electric fields are of increasing importance for the generation of fluid flows in microsystems. We analyse numerically the use of AC electric fields at microwave frequencies for electro-thermal actuation of water in microdevices. Water is heated because of its significant dielectric loss at microwave frequencies. Buoyancy and dielectric forces actuate in the liquid bulk, and the relative importance between them is studied. The microwave liquid actuation can be used for pure water as well as for water saline solutions, such as bio-fluids. Therefore, it is of interest for the Lab-on-a-Chip technology.     

Recent experimental aspects of the structure and dynamics of liquid and supercooled water

Liquid water, the most familiar liquid, is still not completely understood, even less so all the processes in which it participates. The directionality of the bonds and quantum aspects make the establishment of a complete theory difficult, particularly in the case of effective potentials built with spherical electrostatic forces. Recent work has focused on the hydrogen bonds formed between water molecules or with hydrophilic substrates. We describe the present situation of research concerning the so-called anomalies of liquid water at low temperature. Although without direct applications, this problem is consistently an object of discussion, enhanced by results from molecular dynamics simulations. Conversely, because in many situations where water plays a major role, such as, for example, in biology, only a few molecules are involved, the study of confined water is extremely important, sometimes decoupled from the more fundamental studies of bulk water. A short, but far from exclusive, summary of some of the more active domains of research concerning liquid water is given, mainly concerning interactions with other media.     

Is there any fast sound in water?

We measured the dynamic structure factor S(Q,omega) of liquid and undercooled water down to 253 K in the Q approximately 0.02-0.1 nm;{-1} momentum transfer region. We observe the neat departure of the apparent speed of sound from the adiabatic regime as a function of decreasing temperature. Our evaluation of the infinite-frequency limit of sound velocity, c_{infinity}, matches with the results obtained in the high momentum transfer limit by inelastic neutron and x-ray scattering. These results strongly support the viscoelastic interpretation of the dynamics of water. Hence, we propose to call c_{infinity} the high-frequency speed of sound and to abandon the term fast sound, which recalls a propagation mechanism through lighter atoms, like in gas mixtures.