Water under extreme conditions

Water is a simple molecule that forms a liquid with complex properties. Its numerous anomalies, such as a negative expansion coefficient, are even more pronounced in supercooled water, where the liquid is metastable with respect to the formation of ice. Such special properties of water are under intense scrutiny for decades.

We seek to extend the study of water to another region of metastability, that of negative of pressure, where liquid can remains before breaking into a bubble of vapor. In this region where the liquid has a low density, attractive interactions and hydrogen bonding are put under heavy strain. We have set up a microscope coupled to Brillouin and Raman spectrometers to characterize water under those extreme conditions.

We have also built an experimental set-up designed to measure the viscosity of supercooled water at positive pressure, that will allow to test recent predictions obtained with molecular dynamics simulations. This aspect of our work falls into the Liquid at interfaces group. Our research are funded through the  WASSR project (Starting Grant ERC), and involve an international joint project (AnomWater ANR-NSF project) with Abraham Stroock's laboratory at Cornell.

  

 

Members:

Berthelard, Romain Caupin, Frédéric  Guillerm, Emmanuel 
Issenmann, Bruno

 

Highlights

 

 

The more, the swifter

Commuting during rush hours teaches us that the denser the crowd, the slower the motion. In contrast, increasing pressure in water makes it denser, but less viscous! At 20°C, the effect is modest. By supercooling water to -29°C, we find that viscosity drops by nearly one half for a 2000 atm pressure increase. We propose an explanation based on a two-state model.

Pressure dependence of viscosity in supercooled water and a unified approach for thermodynamic and dynamic anomalies of water, L.P. Singh et al., PNAS (2017).

 

       

Close-packed fishes flow fast, as supercooled water under pressure.

 

Viscosity of supercooled water

So far accurate viscosity data were lacking in supercooled water. Using Brownian spheres suspended in water, we have measured it down to −34°C. Whereas contrary to most fluids, viscosity decouples from molecular translation upon cooling, it remains coupled to rotation. This anormal behaviour could be related to a liquid-liquid phase transition, predicted by simulations.

Viscosity of deeply supercooled water and its coupling to molecular diffusion, Dehaoui et al., PNAS (2015)

 

       

Left : The setup. Brownian spheres suspended in supercooled water are placed in a thermal stage. Thanks to differential dynamic microscopy, the viscosity of water is deduced. Right : Example of obtained image.

 

Anomalies of water at negative pressure

Supercooled water exhibits pronounced anomalies. Their origin remains elusive because measurements are limited by crystallization of the system. By applying negative pressure and using Brillouin spectroscopy, we have observed a minimum in the sound velocity before crystallization occurs. Together with molecular dynamics simulations, these results put more constraints on theoretical scenarios proposed to explain water anomalies.

Anomalies of bulk supercooled water at negative pressure, Pallares et al. PNAS (2014).

 

Molecular dynamics simulations done in C. Valeriani’s group (Madrid) confirm the sound velocity measurements and allow to propose an explanation involving a putative phase transition in water between two different liquids.

 

Water at negative pressure

Liquide water under tension breaks into gaz: this is cavitation. To understand this phenomena, we have relied on a method where inclusions of liquid within a quartz crystal are cooled down at constant volume.The observed cavitation statistics are in good agreement with the classical mechanism of homogeneous cavitation and rule out other mechanisms. The results also enable us to locate the temperature of maximum density at negative pressure. This work is an advance on the phase diagram of water, which is still under debate.  

A coherent picture for water at extreme negative pressure, Mouna El Mekki Azouzi et al.   Nature Physics (2012).

 

Experiments performed up to now on these systems would measure the threshold for bubble appearance on many inclusions at the same time, whence a large dispersion in the result. To get very accurate measurements, we measured the threshold on a single, well-chosen inclusion (right), but repeated the process many times.

 

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