Václav Vinš

Surface Tension of Supercooled Water: No Inflection Point down to −25 °C

A dramatic increase in the surface tension of water with decreasing temperature in the supercooled liquid region has appeared as one of the many anomalies of water. This claimed anomaly characterized by the second inflection point at about +1.5 °C was observed in older surface tension data and was partially supported by some molecular simulations and theoretical considerations. In this study, two independent sets of experimental data for the surface tension of water in the temperature range between +33 and -25 °C are reported. The two data sets are mutually consistent, and they lie on a line smoothly extrapolating from the stable region. No second inflection point and no other anomalies in the course of the surface tension were observed. The new data lies very close to the extrapolated IAPWS correlation for the surface tension of ordinary water, which hence can be recommended for use, e.g., in atmospheric modeling.

Influence of the light source on the liquid optical element planarity measurement

The liquid variable focus lenses and mirrors are extensively studied for its variety of applications nowadays because it presents one of the simplest ways for applications as varying optical elements. Instead of other principles the liquid lenses and mirrors can be easily operated by changing the pressure pushing the liquid in and out of the base to change its surface curvature. In case of small diameter optical elements the surface curvature is given by its aperture diameter, applied pressure, and surface tension of the liquid. This feature enables to use such a variable optical element for fine surface tension measurement of small volume of the liquid sample, when the optical element, diameter, and pressure are measured. We are focused on surface tension measurement of small sample of liquid water under supercooled conditions by use of pressure measurement and optical identification of free liquid meniscus planarity. We developed a measurement stand with planar meniscus shape determination by its confocal visualization. We tried different light sources for 0.3 mm diameter meniscus illumination. We discuss an influence of the light source on the liquid meniscus optical element planarity measurement.

Predictions of homogeneous nucleation rates for n-alkanes accounting for the diffuse phase interface and capillary waves

Homogeneous droplet nucleation has been studied for almost a century but has not yet been fully understood. In this work, we used the density gradient theory (DGT) and considered the influence of capillary waves (CWs) on the predicted size-dependent surface tensions and nucleation rates for selected n-alkanes. The DGT model was completed by an equation of state (EoS) based on the perturbed-chain statistical associating fluid theory and compared to the classical nucleation theory and the Peng-Robinson EoS. It was found that the critical clusters are practically free of CWs because they are so small that even the smallest wavelengths of CWs do not fit into their finite dimensions. The CWs contribute to the entropy of the system and thus decrease the surface tension. A correction for the effect of CWs on the surface tension is presented. The effect of the different EoSs is relatively small because by a fortuitous coincidence their predictions are similar in the relevant range of critical cluster sizes. The difference of the DGT predictions to the classical nucleation theory computations is important but not decisive. Of the effects investigated, the most pronounced is the suppression of CWs which causes a sizable decrease of the predicted nucleation rates. The major difference between experimental nucleation rate data and theoretical predictions remains in the temperature dependence. For normal alkanes, this discrepancy is much stronger than observed, e.g., for water. Theoretical corrections developed here have a minor influence on the temperature dependency. We provide empirical equations correcting the predicted nucleation rates to values comparable with experiments.

Corrections to the classical work of formation of critical clusters

We show that some corrections to the work of formation predicted by the Classical Nucleation Theory (CNT) can be derived from a simple cluster model based on the density profile for the planar phase interface. These corrections are related to the Tolman length and to the thickness of the phase interface. We analyze the temperature dependencies of these corrections based on critical scaling and compare them with results of the density gradient theory (DGT).