Water is a substance that challenges traditional notions of liquids, exhibiting numerous unusual characteristics. In 1992, a significant theory was put forth proposing the presence of two separate liquid phases of water, which can be distinguished under extreme temperature and pressure conditions, particularly below freezing. This daring hypothesis has initiated extensive investigation, and currently, fresh experimental outcomes suggest considerable evidence supporting this theory.
An international research team, headed by Anders Nilsson from Stockholm University, effectively utilized ultrafast laser pulses to rapidly melt ice at exceedingly low temperatures and high pressures, environments favorable for the proposed liquid states. Through x-ray scattering, they recorded a liquid-liquid phase transition, similar to the shift between liquid and gas states. This research emphasizes the critical point where these supercooled liquid phases meet, prompting physicist Greg Kimmel to assert that these findings provide convincing proof of a liquid-liquid critical point (LLCP) in water.
This LLCP sheds light on water’s strange anomalies, suggesting that normal water may exist as a supercritical liquid above this critical threshold. The two proposed phases vary where one is a denser form, lacking the hydrogen bonds usually present in typical water, which contributes to water’s exceptional density-maximum anomaly just above freezing. The competition between these molecular configurations aids in clarifying why ice is less dense than liquid water.
The liquid-liquid transition (LLT) is theorized to occur below -50°C and at 1000 atmospheres of pressure. However, exploring this area is difficult due to the swift crystallization of ice. Nilsson’s group, working alongside Kyung Hwan Kim and others, extended these limits by employing rapid laser heating and x-ray scattering to observe the LLT in microseconds before ice can form.
Recent experimental modifications aimed to further clarify the LLCP. They noted a considerable increase in the heat capacity of the sample approaching -63°C, signifying a critical point phenomenon. Francesco Sciortino from Sapienza University, who contributed to the 1992 LLCP prediction, commends these endeavors, recognizing some lingering experimental uncertainties but considering them a testament to the long-desired LLCP scenario.
This new evidence strengthens the credibility of the LLCP in water, altering the comprehension of this common yet mysterious liquid.