Evidence of two distinct local structures of water from ambient to supercooled conditions

Abstract

The liquid and supercooled states of water show a series of anomalies whose nature is debated. A key role is attributed to the formation of structural aggregates induced by critical phenomena occurring deep in the supercooled region; the nature of the water anomalies and of the hidden critical processes remains elusive. Here we report a time-resolved optical Kerr effect investigation of the vibrational dynamics and relaxation processes in supercooled bulk water. The experiment measures the water intermolecular vibrations and the structural relaxation process in an extended temperature range, and with unprecedented data quality. A mode-coupling analysis of the experimental data enables to characterize the intermolecular vibrational modes and their interplay with the structural relaxation process. The results bring evidence of the coexistence of two local configurations, which are interpreted as high-density and low-density water forms, with an increasing weight of the latter at low temperatures.

Possible Further Evidence for the Thixotropic Phenomenon of Water

“…Namely, LDL tetrahedral motifs decrease in number with increasing pressure in favor of the HDL disordered motifs undergoing normal stochastic fluctuations. And recently, measurements of optical Kerr effects by Taschin et al gave strong evidence for two distinct, high-density and low-density water forms [31].…”

Understanding water’s anomalies with locally favoured structures

“…Some evidence for the inhomogenous structure of water was also reported in experimental studies of X-ray absorption spectroscopy, X-ray emission spectroscopy and X-ray small angle scattering 31,32 , but these results are highly debated 33,34 , especially since the majority component at room temperature was proposed to be associated with the break-up of the tetrahedral structure. Recently, evidence for two distinct local structures was found in the vibrational dynamics and relaxation processes in supercooled bulk water 35 . Despite these pieces of evidence supporting a two-state picture, the lack of a clear connection between these experimental observables and the amount of locally favoured structures has made it difficult to estimate the fractions of the two states in a convincing manner.…”

Viscosity of deeply supercooled water and its coupling to molecular diffusion

“…As an example, crystallization of water is accompanied by one of the largest known relative changes in sound velocity, which has been attributed to the relaxation effects of the hydrogen bond network (8,9). Indeed, whereas the sound velocity is around 1,400 m · s −1 in liquid water at 273 K, it reaches around 3,300 m · s −1 in ice at 273 K and a similar value in the known amorphous phases of ice at 80 K (10).…”

“…However, pioneering measurements on bulk supercooled water by NMR (11) and quasi-elastic neutron scattering (12), as well as recent ones by optical Kerr effect (8,9), reveal a large super-Arrhenius behavior between 340 and 240 K, similar to what is observed in fragile glassformers (13,14). The temperature dependence of the relaxation time is well described by a power law (8,9), as expected from mode-coupling theory (15,16), which usually applies well to liquids with a small change of sound velocity upon vitrification. Based on these and other observations, it has been hypothesized that supercooled water experiences a fragile-tostrong transition (17).…”