Shear Viscosity Of Water Research Articles

Viscosity in water from first-principles and deep-neural-network simulations

We report on an extensive study of the viscosity of liquid water at near-ambient conditions, performed within the Green-Kubo theory of linear response and equilibrium ab initio molecular dynamics (AIMD), based on density-functional theory (DFT). In order to cope with the long simulation times necessary to achieve an acceptable statistical accuracy, our ab initio approach is enhanced with deep-neural-network potentials (NNP). This approach is first validated against AIMD results, obtained by using the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional and paying careful attention to crucial, yet often overlooked, aspects of the statistical data analysis. Then, we train a second NNP to a dataset generated from the Strongly Constrained and Appropriately Normed (SCAN) functional. Once the error resulting from the imperfect prediction of the melting line is offset by referring the simulated temperature to the theoretical melting one, our SCAN predictions of the shear viscosity of water are in very good agreement with experiments.

On the validity of the Stokes–Einstein relation for various water force fields

The translational self-diffusion coefficient and the shear viscosity of water are related by the fractional Stokes–Einstein relation. We report extensive novel molecular dynamics simulations for the self-diffusion coefficient and the shear viscosity of water. The SPC/E and TIP4P/2005 water models are used in the temperature range 220–560 K and at 1 or 1,000 bar. We compute the fractional exponents t, and s that correspond to the two forms of the fractional Stokes–Einstein relation and respectively. We analyse other available experimental and numerical simulation data. In the current analysis two temperature ranges are considered (above or below 274 K) and in both cases deviations from the Stokes–Einstein relation are observed with different values for the fractional exponents obtained for each temperature range. For temperatures above 274 K, both water models perform comparably, while for temperatures below 274 K TIP4P/2005 outperforms SPC/E. This is a direct result of the ability of TIP4P/2005 to predict water densities more accurately and thus predict more accurately the water self-diffusion coefficient and the shear viscosity.

Nanoporous organosilica membrane for water desalination: Theoretical study on the water transport

An unconventional nanoporous organosilica membrane has been tested in a vacuum membrane distillation (MD) process for water desalination. We propose a modified approach to understand the transport mechanism of water molecules through the nanopores of this membrane. The modified approach stems from the fact that the membrane has a hydrophilic surface (contact angle <90°) and so capillary pressure, which draws liquid water into the nanopore, must be considered when establishing the mathematical model. However, increased friction arising from the dramatic increase in shear viscosity of water in nano-confined spaces balances the capillary flow against the evaporative mass transport to avoid pore wetting. Notably, the liquid/vapor interface is no longer formed at the pore entrance as with a conventional hydrophobic membrane, but rather exists deeper in the pore channel as a consequence of capillary pressure. This was backed by experimental observations (no pore wetting) and SEM evidence which showed salt nucleation and growth existed only on the membrane surface, and did not infiltrate the membrane support layers. The impacts of pore size, membrane thickness, substrate thickness, concentration polarization, porosity, and contact angle on water flux and pore intrusion depth were tested using the model. Pore size was the most influential parameter with an >80% increase in permeation flux if the pore size increased from 2 to 3nm at 60°C. However, pore wetting is expected if dp>3.4nm, particularly at low temperatures where the slower evaporation rate promoted greater pore intrusion. Concentration polarization was shown to be negligible which agreed well with experimentally observed water fluxes which remained relatively constant despite feed salinity increasing from 0 to 150gL−1. Lastly, the membrane hydrophilicity was found to impact on water flux and pore intrusion in a complex relationship with pore size. Ultimately, hydrophilic pores less than 3nm in diameter offer a good combination of good water flux and minimal water intrusion suggesting that ordered mesoporous organosilica membranes have potential in MD applications.

Effects of Temperature Control Algorithms on Transport Properties and Kinetics in Molecular Dynamics Simulations.

Temperature control algorithms in molecular dynamics (MD) simulations are necessary to study isothermal systems. However, these thermostatting algorithms alter the velocities of the particles and thus modify the dynamics of the system with respect to the microcanonical ensemble, which could potentially lead to thermostat-dependent dynamical artifacts. In this study, we investigate how six well-established thermostat algorithms applied with different coupling strengths and to different degrees of freedom affect the dynamics of various molecular systems. We consider dynamic processes occurring on different times scales by measuring translational and rotational self-diffusion as well as the shear viscosity of water, diffusion of a small molecule solvated in water, and diffusion and the dynamic structure factor of a polymer chain in water. All of these properties are significantly dampened by thermostat algorithms which randomize particle velocities, such as the Andersen thermostat and Langevin dynamics, when strong coupling is used. For the solvated small molecule and polymer, these dampening effects are reduced somewhat if the thermostats are applied to the solvent alone, such that the solute's temperature is maintained only through thermal contact with solvent particles. Algorithms which operate by scaling the velocities, such as the Berendsen thermostat, the stochastic velocity rescaling approach of Bussi and co-workers, and the Nosé-Hoover thermostat, yield transport properties that are statistically indistinguishable from those of the microcanonical ensemble, provided they are applied globally, i.e. coupled to the system's kinetic energy. When coupled to local kinetic energies, a velocity scaling thermostat can have dampening effects comparable to a velocity randomizing method, as we observe when a massive Nose-Hoover coupling scheme is used to simulate water. Correct dynamical properties, at least those studied in this paper, are obtained with the Berendsen thermostat applied globally, despite the fact that it yields the wrong kinetic energy distribution.

Thermal conductivity, shear viscosity and specific heat of rigid water models

The thermal conductivity and shear viscosity of water are evaluated for eight rigid water models with reverse non-equilibrium molecular dynamics (RNEMD). Specific heats under constant volume and constant pressure are calculated within NVT and NPT ensembles. The five-site models (TIP5P and TIP5P-Ew) can predict thermal conductivity more accurately than other rigid water models. It appears that TIP5P-Ew is the best potential to predict the trend of the thermal conductivity. Both five-site models can accurately predict shear viscosity values as well as its trends along with temperature. Three-site model (TIP3P) has better performance in calculating specific heats.

Shear viscosity enhancement in water–nanoparticle suspensions

Equilibrium molecular dynamics simulations characterize the increase in the shear viscosity of water around a suspended silicon dioxide nanoparticle. Water layering on the solid surface decreases the fraction of adjacent fluid molecules that are more mobile and hence less viscous, thereby increasing the shear viscosity. The contribution of the nanoparticle surface area to this rheological behavior is identified and an empirical model that accounts for it is provided. The model successfully reproduces the shear viscosity predictions from previous experimental measurements as well as our simulations.

Prediction of self-diffusion coefficient and shear viscosity of water and its binary mixtures with methanol and ethanol by molecular simulation

Density, self-diffusion coefficient, and shear viscosity of pure liquid water are predicted for temperatures between 280 and 373 K by molecular dynamics simulation and the Green-Kubo method. Four different rigid nonpolarizable water models are assessed: SPC, SPC/E, TIP4P, and TIP4P/2005. The pressure dependence of the self-diffusion coefficient and the shear viscosity for pure liquid water is also calculated and the anomalous behavior of these properties is qualitatively well predicted. Furthermore, transport properties as well as excess volume and excess enthalpy of aqueous binary mixtures containing methanol or ethanol, based on the SPC/E and TIP4P/2005 water models, are calculated. Under the tested conditions, the TIP4P/2005 model gives the best quantitative and qualitative agreement with experiments for the regarded transport properties. The deviations from experimental data are of 5% to 15% for pure liquid water and 5% to 20% for the water + alcohol mixtures. Moreover, the center of mass power spectrum of water as well as the investigated mixtures are analyzed and the hydrogen-bonding structure is discussed for different states.

Laser-field-induced coherent molecular librations in liquid

The temperature evolution of rotational spectra is demonstrated for four-photon scattering in water in the interval 0.1–8 cm−1 (3–240 THz). A detailed numerical simulation of the spectra is performed. The best agreement is reached using the frequencies of the rotational resonances of free molecules. It is demonstrated that the contribution of coherent librations into the measured signal increases proportionally to the decrease in the shear viscosity of water. The four-photon-scattering spectra of water corresponding to an increase in the temperature and dilution with H2O2 are compared.

Temperature dependence of the Brillouin linewidth in water

In the frequency spectrum of light that is scattered in liquid water there is a central elastically scattered peak that is due mainly to scattering by suspended particles; and, there is a peak on each side of the central peak that is displaced by the Brillouin frequency shift. The Brillouin shift is a direct measure of sound speed. The linewidth of the Brillouin shifted lines is dependent on the bulk and shear viscosity of water as well as its density, thermal conductivity, and specific heat. The linewidth of the Brillouin peaks has been investigated in laboratory experiments over a range 1°C to 35°C. The frequency spectrum of back-scattered laser light was analysed using a scanning Fabry-Perot etalon. A strong dependence of the linewidth of the Brillouin shifted lines on temperature was found. In particular, for low temperatures in the range 10°C down to 1°C the linewidth shows an increase from 750 MHz to 1.4 GHz.

Viscosity of liquid water from computer simulations with a polarizable potential model

The longitudinal and shear viscosity of water are calculated by molecular dynamics simulation with a polarizable potential model at room temperature. To overcome the difficulty of evaluating directly the stress autocorrelation function of a system with intrinsically many-body forces, we have resorted to the analysis of the wave-vector-dependent longitudinal and transverse-current correlation functions. In a memory function formalism, the generalized viscosity can be evaluated as a function of the wave vector k. By extrapolating to k=0, we find longitudinal and shear viscosity values in better agreement with the experimental value than the corresponding quantities evaluated by making use of a nonpolarizable potential model. This result points out that for a realistic reproduction of transport quantities, it is crucial to take into account many-body contributions to the interaction potential.

Shear viscosity of polar fluids: Molecular dynamics calculations of water

The shear viscosity of water at 303.15 K is calculated for the extended simple point charge (SPC/E) model of Berendsen et al. [J. Phys. Chem. 91, 6269 (1987)] using both equilibrium and nonequilibrium molecular dynamics (NEMD) methods. Reciprocal space sums to handle long-range electrostatic forces in a noncubic simulation box under shear are used in conjunction with box dynamics to evaluate the Coulombic interactions. At the state point studied, the shear viscosity of SPC/E water is found to be 6.6±0.8×10−4 Pa s, which is about 18% less than the experimental value.

A new mechanism of ultrasonic absorption in aqueous solutions of rigid macromolecules

The temperature- and frequency-dependences of the ultrasonic absorption in lyotropic liquid crystals of rod-like FD virus particles have been measured with the ultrasonic beam being parallel or perpendicular to the rod axis. The experimental results show a strongly anisotropic ultrasonic absorption in the lyotropic liquid crystals, which varies with temperature exactly as the shear viscosity of water. Experimental results are in good agreement with the following model for the absorption process. The authors believe that the absorption arises from differential flow of water between the relatively long and rigid rod-like macromolecules. This mechanism of ultrasonic absorption becomes dominant for frequencies of 1-100 MHz. The validity of this mechanism is, however, not restricted to the special lyotropic crystals discussed here. Instead, this absorption process is expected to be important also for other macromolecular solutions whenever the dissolved macromolecular objects have a sufficiently large linear dimension in the direction parallel to the sound beam.

Activation energy for water transport in toad bladder.

Activation energies (Ea) for water movement across vasopressin-(ADH) sensitive epithelia have been reported to be about 10 kcal/mol (1, 12). The present study shows that measurements of Ea for osmotic water flow across toad bladders are unreliable, because a temperature change induces marked alterations in membrane permeability to water within a 2.5-min interval. Thus bladders equilibrated with ADH either at room temperature or at 33 degrees C and then suddenly subjected to a lower temperature were found to exhibit a marked increase in membrane permeability to water. This observation suggests that there is a rapid turn-over of water permeability sites and that sudden exposure to cold inhibits the removal more than the induction of sites by ADH. To stabilize ADH-induced water channels for Ea measurements, bladders were exposed to ADH at room temperature, fixed with glutaraldehyde, and subjected to osmotic gradients at different temperatures. The Ea values for osmotic water flow across these ADH-permeabilized, glutaraldehyde-fixed bladders were 5.1 (4-12 degrees C), 4.3 (12-21 degrees C), 3.6 (21-36 degrees C), and 3.6 kcal/mol (30-38 degrees C). Ea values for shear viscosity of water in these temperature ranges were calculated to be 4.7, 4.2, 4.1, and 3.6 kcal/mol, respectively. The close correlation between Ea values for bulk water viscosity and osmotic water flow across the bladder wall suggests that an equivalent number of hydrogen bonds must be broken to achieve an increase in water flow through ADH-induced channels and an increase in fluidity of water in bulk solution.

A dislocation model for the viscosity of water

A dislocation model for the shear viscosity of water is proposed, based upon the dislocation theory of melting and liquid state. This model, studied extensively within the last years in relation to phase transitions in two-dimensional systems, makes rigorous use of the similarity between the structure of a liquid and that of a crystalline solid. Viscous flow in water due to slip of dislocation loops is proposed as an alternative point of view to the hole theories and the cluster models of hydrogen bonded molecules. The model correctly yields the temperature dependence of the viscosity at normal pressure between melting and boiling point.