Surface Tension of Supercooled Water Determined by Using a Counterpressure Capillary Rise Method

Abstract

Measurements of the surface tension of supercooled water down to −25°C have been reported recently (Hrubýet al. J. Phys. Chem. Lett. 2014, 5, 425−428). These experiments did not show any anomalous temperature dependence of the surface tension of supercooled water reported by some earlier measurements and molecular simulations. In the present work, this finding is confirmed using a counterpressure capillary rise method (the counterpressure method) as well as through the use of the classical capillary rise method (the height method). In the counterpressure method, the liquid meniscus inside the vertical capillary tube was kept at a fixed position with an in-house developed helium distribution setup. A preset counterpressure was applied to the liquid meniscus when its temperature changed from a reference temperature (30°C) to the temperature of interest. The magnitude of the counterpressure was adjusted such that the meniscus remained at the same height, thus compensating the change of the surface tension. One advantage of the counterpressure method over the height method consists of avoiding the uncertainty due to a possible variation of the capillary diameter along its length. A second advantage is that the equilibration time due to the capillary flow of the highly viscous supercooled water can be shortened. For both the counterpressure method and the height method, the actual results are relative values of surface tension with respect to the surface tension of water at the reference temperature. The combined relative standard uncertainty of the relative surface tensions is less than or equal to 0.18%. The new data between −26 and +30°C lie close to the IAPWS correlation for the surface tension of ordinary water extrapolated below 0.01°C and do not exhibit any anomalous features. ■ INTRODUCTIONThe surface tension of supercooled liquids, in particular, water and aqueous mixtures, is an important property both in academia and in industry. It plays an essential role in atmospheric research of the nucleation and growth of water droplets 1 and ice crystals. 2 It is known that water in clouds can persist in a supercooled liquid form at temperatures down to −38°C. 3 Manka et al. 4 recently showed that liquid water nanodroplets rather than ice crystals form by homogeneous nucleation at temperatures down to −73°C. Reliable data for the surface tension of supercooled aqueous systems are also important in technical applications such as operation of wind turbines, 5 aircraft icing, 6 or design of secondary refrigeration systems operating with brine. 7 Compared to other fluids, water shows several anomalies at low temperatures, e.g., the well-known maximum in the liquid density at +4°C at atmospheric pressure. The unusual behavior of liquid water becomes more distinct in the metastable supercooled region below 0°C. For instance, the isobaric heat capacity and the isothermal compressibility seemingly diverge (or approach a sharp maximum) when extrapolated to temperatures around −45°C, i.e., below the homogeneous nuc...

Improvements of the experimental apparatus for measurement of the surface tension of supercooled liquids using horizontal capillary tube

“…Figure 7 shows the experimental data for the surface tension of supercooled water. The new measurements at temperatures between -11 °C and + 15 °C (red diamonds) are in good agreement with both the capillary rise data [4][5][6] and the IAPWS correlation (2) extrapolated below 0.01 °C. The reference temperature Tref for the evaluation of the inner diameter from equation (4) was + 15 °C.…”

“…The relatively low degrees of supercooling were measured due to a potential risk of crystallization of the liquid sample. Unlike in the modified capillary rise method [4][5][6], the horizontal capillary tube is open to ambient, which increases the risk of abrupt solidification of the water sample. Figure 7 shows the experimental data for the surface tension of supercooled water.…”

“…The reason is also the fact that the data by Hacker have relatively low scattering and good internal consistency. However, recent experiments performed with another technique based on the modified capillary rise method show that the surface tension of water does not show any anomaly at temperatures down to -26 °C [4][5][6]. The surface tension of supercooled water can be predicted by the IAPWS (International Association for Properties of Water and Steam) standard [6] extrapolated below 0.01 °C.…”

“…The IAPWS standard has following form 1.256 235. 8 1 0.625 [6]. The IAPWS standard is currently being revised with the use of recent experimental data in order to improve the estimates of its uncertainties and to extend the range of validity also to the supercooled region [8].…”

“…The improved setup will allow achieving higher degrees of supercooling of up to 20 °C. Main motivation for this work is to obtain new valuable data for supercooled water with an experimental method different from the recent measurements with the modified capillary rise technique [4][5][6]. Moreover, the new measurements with the horizontal capillary tube may clarify the SIP anomaly in the Hacker's data [3], as Hacker used also the horizontal technique.…”