Experimental evidence of a liquid-liquid critical point in bulk supercooled water observed through the ultrafast heating of amorphous ice.
ABSTRACT
Water is the most important liquid for our existence, and many of its macroscopic properties show unique anomalous behavior. One of the most important topics in physics and chemistry is what causes this anomalous behavior. One major hypothesis is that there could exist two separate macroscopic liquid water phases, high-density liquid (HDL) and low-density liquid (LDL), with a coexistence line in the P-T diagram deep in the supercooled regime at elevated pressure. In this hypothesis, a liquid-liquid transition (LLT) line is proposed to end in a liquid-liquid critical point (LLCP) and its extension into the one-phase region corresponds to the Widom line.
However, since water at the expected temperature-pressure condition of the LLCP crystallizes into ice on the microsecond timescale, it has been impossible to directly prove the existence of the LLCP through experiments over the past few decades. To overcome this challenge and prove the existence of the LLCP experimentally, we generated bulk deeply supercooled water by ultrafast heating of amorphous ice and measured the structural changes that occur before crystallization using time-resolved X-ray scattering. By adjusting the laser power used for heating amorphous ice, we could generate supercooled water at temperatures both higher and lower than the LLCP.
We observed that a discontinuous transition between two liquid water phases occurs at low temperatures, whereas a continuous transition occurs at high temperatures. We also observed a rapid increase in heat capacity, which diverges at Tc = 210±8 K. These results indicate that our experiment was able to access the LLCP of supercooled water directly.
