Contrary to the historical data, several recent experiments indicate that the surface tension of supercooled water follows a smooth extrapolation of the IAPWS equation in the supercooled regime. It can be seen, however, that a small deviation from the IAPWS equation is present in the recent experimental measurements. It is shown with simulations using the WAIL water potential that the small deviation in the experimental data is consistent with the tail of an exponential growth in surface tension as temperature decreases. The emergence temperature, Te, of a substantial deviation from the IAPWS equation is shown to be 227 K for the WAIL water and 235 K for real water. Since the 227 K Te is close to the Widom line in WAIL water, we argue that real water at 235 K approaches a similar crossover line at one atmospheric pressure.
The surface tension of nanoscale droplets of water was studied with molecular dynamics simulations using the BLYPSP-4F water potential. The internal pressure of the droplet was measured using an empirical correlation between the pressure and density, established through a series of bulk simulations performed at pressures from 1 to 1000 bars. Such a procedure allows for reliable determination of internal pressure without the need to calculate the local virial. The surface tension, estimated with the Young-Laplace relation, shows good agreement with the Tolman equation with a Tolman length of -0.48 Å. The interface of a liquid water droplet is shown to be around 1.1-1.3 nm thick depending on radii. The fairly thick interface region puts a lower limit on the size of droplets that still have a bulk-like interior.
A closed-loop flow circuit, incorporating a canned pump, has been developed for calorimetric studies on compressed water at pressures up to 100 MPa (MN/m2). The pump, which has a peripheral impeller, develops a maximum differential pressure of over 0.30 MPa and produces mass flow rates of up to 0.25 kg/s. The pump and associated flow circuits exhibit especially good flow stability. A unique pressurizing system is employed to raise and adjust the pressure in the pump and circuit. The oil and water pressures are equalized in a vessel where the oil/water interface is monitored using a capacitance method. Pump characteristics and associated calibrations are reported.
The effect for removing weak longtime correlation is studied using a model system that contains a driven atom at liquid density under strong thermal fluctuations. The force that drives the tagged particle is about 1% of the average random force experienced by the particle. The tagged particle is allowed to assume a range of masses from 1/8 to 80 times that of a surrounding particle to study the effects of inertia. The driving force is indefinitely correlated but much weaker than "random" fluctuations from the environment. From this study, it is shown that the environmental influence is not fully random leading to the force autocorrelation function being a poor metric for detecting the correlated driving force. Although the velocity autocorrelation function shows stronger correlation for systems with higher inertia, the velocity autocorrelation function decays to a very small value of 2.5×10 −3 even for the most massive driven particle. For systems with small inertia, our study reveals that discarding longtime correlation has negligible influence on the first passage time (FPT) estimate, whereas for particles with large inertia, the deviation can indeed be appreciable. It is interesting that the Markov State Model (MSM) still produces reasonable estimates on the FPT even when a very short lag time that clearly violates the Markovianity assumption is used. This is likely a result of favorable error cancellations when the MSM transition probability matrices were constructed using trajectories that are much longer than the lag time.
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