For a limited time, water can exist in the liquid phase at temperatures below the equilibrium freezing point as a metastable supercooled liquid. This phenomenon is common in the clouds and mist. Water exhibits numerous anomalies, the best known is the density maximum at 4°C and a negative coefficient of thermal expansion below this temperature. In the supercooled region, the anomalies get stronger. Anomalous thermodynamic properties of supercooled water can be explained by a hypothesis of two liquid phases and a liquid-liquid critical point, which is supported by indirect evidence.

In the Institute of Thermomechanics of the Czech Academy of Sciences in Prague, the density and surface tension of supercooled water and aqueous solutions are investigated. Three modifications of the capillary rise method were used to study the surface tension. In all of them, we measure a difference between the surface tension at a reference temperature (typically 15°C) and the temperature of interest. The part of the capillary containing the liquid meniscus is precisely temperature-controlled. The temperature alternates between the reference temperature and the temperature of interest. Surface tension of ordinary water was measured down to −31.4 °C. The motivation was to verify existence of an anomalous phenomenon - the second inflection point in the temperature course of the surface tension. Earlier measurements indicated a strong effect a few degrees below 0°C. We found that these measurements were not correct. However, at the lower end of the experimental range, we observe a weak inflection, which supports theoretical expectations. Further experiments included seawater and solutions of salts and alcohols. The density of supercooled water was measured with an in-house developed apparatus allowing pressures up to 200 MPa. Liquid sample is contained in two capillaries of different length. We measure the change of the height of the liquid column in the capillary for a temperature change between a reference temperature and the temperature of interest. The capillaries are firmly sealed from one side and enclosed by a movable mercury plug at the other side. The capillaries are pressurized from all sides. The pressure difference acting across the capillary wall is merely a tiny hydrostatic head. Density of supercooled ordinary water, heavy water, and seawater was measured down to about 15°C below the equilibrium freezing temperature at given pressure. The measurements are very accurate - estimated uncertainty of determined densities is 50 ppm. We developed thermodynamic models describing analytically the experimental data and enabling computation of other thermodynamic quantities.