Water keeps puzzling scientists because of its numerous properties which behave oppositely to usual liquids: for instance, water expands upon cooling, and liquid water is denser than ice. To explain this anomalous behaviour, several theories have been proposed, with different predictions for the properties of supercooled water (liquid at conditions where ice is more stable). However, discriminating between those theories with experiments has remained elusive because of spontaneous ice nucleation. Here we measure the sound velocity in liquid water stretched to negative pressure, and derive an experimental equation of state, which reveals compressibility anomalies. We show by rigorous thermodynamic relations how these anomalies are intricately linked with the density anomaly. Some features we observe are necessary conditions for the validity of two theories of water.Liquid water exhibits numerous anomalies and different scenarios have been proposed to explain them (1). In particular, the existence of a line of density maxima along isobars has been related to putative maxima in compressibility (2) and heat capacity (3). These maxima may arise from an intriguing phase separation of water in two distinct liquids (1,4), although they can also be explained without resorting to such a phase separation (2). However, the compressibility and heat capacity maxima, whose existence is predicted by molecular dynamics simulations (3,5), have hitherto not been observed in experiments. Alternative theoretical scenarios, namely 1 arXiv:1708.00063v1 [physics.chem-ph] 31 Jul 2017 the stability limit conjecture (6) and the critical-point-free scenario (7, 8), do not require the existence of compressibility and heat capacity maxima, but rather predict a divergence of these quantities at low temperature.Another type of anomaly, namely a minimum in sound velocity along one isochore, was recently discovered at negative pressure (9,10). At negative pressure, the liquid is mechanically stretched, in a state metastable with respect to vapor. To date, the only method able to reach significantly negative pressures (beyond −100 MPa) uses 3 − 10 µm fluid inclusions (FIs) of water in a quartz crystal, and stretching is obtained by cooling liquid water at nearly constant volume (11,12). In our previous work (9, 10), we could only study two FIs along different isochores, and only one clearly showed a minimum in sound velocity. In the present work, we have measured more FIs showing a sound velocity minimum, and reached more negative pressures. We have thus established a more accurate experimental equation of state (EoS) down to −137 MPa. The new EoS strongly supports the existence of the compressibility maxima predicted by some theories of water. Furthermore, we establish new thermodynamic relations between the line of density maxima and the sound velocity anomalies. In contrast to previous works, this provides a relation between quantities that are directly observable in experiments. The corresponding lines of extrema obtained from our experimenta...

The fast evaporative cooling of micrometer-sized water droplets in a vacuum offers the appealing possibility to investigate supercooled water-below the melting point but still a liquid-at temperatures far beyond the state of the art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a train of perfectly uniform water droplets allows us to measure the variation in droplet size resulting from evaporative mass losses with an absolute precision of better than 0.2%. This finding proves crucial to an unambiguous determination of the droplet temperature. In particular, we find that a fraction of water droplets with an initial diameter of 6379±12  nm remain liquid down to 230.6±0.6  K. Our results question temperature estimates reported recently for larger supercooled water droplets and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.