Experimental evidence of a liquid-liquid critical point in supercooled water.

The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water. Editor's summary: Polyamorphism and the resulting liquid-liquid critical point (LLCP) in supercooled water are intriguing phenomena in condensed-matter science. Rapid spontaneous ice formation that the water can undergo when probed makes finding the LLCP extremely challenging experimentally. Previously, evidence for polyamorphism relied on indirect signs such as extrapolating unusual physical properties, the presence of two amorphous ice forms, and liquid-liquid transitions in simulations. Experiments presented by You et al. with isochoric heating of high- and low-density amorphous ices using infrared ultrafast laser pulses followed by x-ray scattering at time scales before ice formation have directly and convincingly demonstrated two liquid states near a critical point in supercooled water (see the Perspective by Paesani). —Yury Suleymanov [ABSTRACT FROM AUTHOR]