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Surface tension of supercooled water

Surface tension apparatus is designed for measurement of surface tension of aqueous liquids both in the thermodynamic equilibrium and in the metastable state. The operating temperatures lie between – 30 °C and + 50 °C. Main aim of the measurements is to collect new experimental data under the metastable conditions in order to clarify some of the anomalies of the supercooled water, e.g., the second inflection point in the surface tension of water detected by previous experiments and some molecular simulations. Surface tension of supercooled water plays an important role by investigation of nucleation and growth of droplets and ice crystals in upper atmosphere and also by many technical applications, e.g., freezing of aircrafts in clouds formed by the supercooled droplets or freezing of wind turbines blades.


Fig. 1 Fused silica capillary tube with inner diameter around 0.3 mm placed inside the temperature-controlled glass chamber
 
The surface tension of supercooled water is measured inside fused silica capillary tubes with small inner diameter (around 0.3 mm). The apparatus allows measurement with two different methods. In the main method based on the capillary rise technique, kthe liquid column in a vertical capillary tube elevates to a certain height (several centimeters) given by the surface tension and the density of liquid, or eventually the hydrostatic pressure of the liquid column. The surface tension can be determined from the height of the liquid column, the inner diameter of the capillary tube, and the liquid density. On the apparatus for supercooled liquids, the upper part of the elevated liquid column, located inside the temperature-controlled chamber, can be rapidly cooled down for a short time such that the solidification of liquid sample is prevented. The surface tension increases with decreasing temperature, thus the liquid column elevates to a higher position. The temperature dependence of the surface tension can be obtained from the measured difference of height corresponding to the given temperature change.
 
Fig. 2 Principle of the capillary rise method for the measurement of surface tension
 

Fig. 3 Main components of the capillary rise apparatus

 
The second method for the measurement of surface tension of supercooled liquids uses the capillary tube in a horizontal position. Liquid sample of a length of 2 to 3 cm is sucked inside a horizontal capillary tube with one precisely grinded planar end. The grinded end of the capillary tube is left open to ambient. It is observed with sensitive optics. The other end of the capillary tube is connected to a system with inert gas allowing precise adjustment of a low counterpressure in order of several hundreds of Pascal. With increasing counterpressure of inert gas, the liquid meniscus at the open end of the capillary tube gradually changes from concave (inside the capillary tube) to planar and subsequently convex (meniscus outside of the capillary tube). In case of the exactly planar outer meniscus, the counterpressure is in equilibrium with surface forces at the second meniscus inside the capillary tube. Consequently, the surface tension at the inner meniscus can be determined from the measured counterpressure of inert gas and the inner diameter of the capillary tube.

Fig. 4 The horizontal capillary tube installed inside the temperature-controlled chamber with the heat transfer liquid (mostly ethanol)

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