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

The experimental search for the liquid-liquid critical point in water is
challenging due to rapid ice crystallization. We studied supercooled water
at time scales prior to ice formation by isochoric heating of 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 spanning the region of the predicted critical point. Our results
revealed 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 consistent with a critical divergence at
210±10 K coincident with enhanced density fluctuations.  These
results suggest that our experiments have directly probed states in the
vicinity of a critical point in supercooled water.