Experimental Observation of Mesoscopic Fluctuations to Identify Origin of Thermodynamic Anomalies of Ambient Liquid Water

Abstract

We report a new experimental approach for observing mesoscopic fluctuations underlying the thermodynamic anomalies of ambient liquid water. In this approach, two sound velocity measurements with different frequencies, namely inelastic X-ray scattering (IXS) in the terahertz band and ultrasonic (US) in the megahertz band, are required to investigate the relaxation phenomenon of sound waves with the characteristic frequency between the two aforementioned frequencies. We performed IXS measurements to obtain the IXS sound velocity of liquid water from the ambient conditions to the supercritical region of liquid-gas phase transition (LGT) and compared the results with the US sound velocity reported in the literature. We found that the ratio of the sound velocities, S f , which corresponds to the relaxation intensity, obtained using these two methods exhibits a simple but significant change. Two distinct rises were observed in the high-temperature and low-temperature regions, implying that two relaxation phenomena exist. In the high-temperature region, a peak was observed near the LGT critical ridge line, which was linked with changes in the magnitude of density fluctuation and isochoric and ishobaric specific heat capacities. This result indicates that the high-temperature relaxation originates from the LGT critical fluctuation, proving that this method is effective for observing such mesoscopic fluctuations. Meanwhile, in the low-temperature region, S f increased from 550 K toward the low-temperature region and reached a high value, attaining the "fast sound" state near ambient conditions. This result indicates that another mechanism of relaxation exists, which causes the sound velocity anomaly, including the "fast sound" phenomenon of liquid water near ambient conditions. The change in S f in the low-temperature region is linked with the change in the isochoric heat capacity, which implies that this relaxation causes the well-known heat capacity anomaly of liquid water. This low-temperature relaxation corresponds to the critical fluctuation of the liquid-liquid phase transition (LLT) that is speculated to exist in the supercooled region. In this study, both LGT and LLT critical fluctuations were observed, and the relationship between thermodynamics and the critical fluctuations was comprehensively discussed by analyzing the similarities and differences between the two phase transitions.

Experimental Observation of Maximum Density Fluctuation in Liquid Te

“…With cooling from 1123 K to approximately 750 K (1b), S 0 gradually decreased, which can be interpreted as the liquid becoming more homogeneous because it moved away from the liquid-gas (LGT) critical point, which should be located at a higher temperature. This behavior is more clearly seen for liquid water [24]. In contrast, in the LLT region, S 0 clearly increases to a maximum at approximately 620-630 K. Around this temperature, (1a) α P , (1d) isobaric heat ca-pacity C P , and ultrasonic sound velocity v US [4] all exhibit peak values.…”

“…Meanwhile, liquid water exhibits thermodynamic anomalies (review [21]) similar to those of liquid Te [8,[22][23][24] and is often referred to as an "anomalous liquid". The density is greatest at 277 K, which is slightly above T m (273 K), the pressure variation of T m is negative, the heat capacity shows a divergent-like increase in the supercooled region, and the ultrasonic sound velocity shows a maximum at approximately 350 K. Furthermore, the LLT concept (review [25]) and the critical point of the LLT hypothesis [26,27] have been proposed independently of the aforementioned model of liquid Te.…”

“…Further, we believe that important physics is hidden in the "dynamic" feature. In fact, the SAXS measurements implicitly revealed that the "static" density inhomogeneity in both liquid Te and liquid water [30] were too small in quantity to be detectable [24]. However, such a small inhomogeneity in density is accompanied by quantitatively large underlying thermodynamic anomalies.…”

“…The inhomogeneity in density detected by SAXS measurements is only the tip of the iceberg of the inhomogeneity or fluctuation that truly affects thermodynamics. We recently proposed a method for detecting such true inhomogeneity or "dynamical fluctuation" using sound waves [24]. In fact, we could measure large changes in real liquid regions above T m for both Te [10] and water [24] using this method.…”

“…We recently proposed a method for detecting such true inhomogeneity or "dynamical fluctuation" using sound waves [24]. In fact, we could measure large changes in real liquid regions above T m for both Te [10] and water [24] using this method. By making full use of these static and dynamical fluctuation measurement methods and clarifying the mesoscopic fluctuations in various liquids, we hope that the understanding of liquids, including glass transition liquids, will be significantly advanced.…”