Percolating networks and liquid–liquid transitions in supercooled water

The anomalous behaviour of various properties of liquid water upon cooling may be explained by approaching the percolation transition of the four-coordinated water molecules or by approaching the liquid–liquid transition. We have found these two explanations are intrinsically closely related by locating the line of the percolation transitions of the four-coordinated water molecules with respect to the liquid–vapour and liquid–liquid transitions of ST2 water. The saturated liquid water upon cooling crosses the percolation threshold of the four-coordinated water molecules, which is close to the binodal (spinodal) of the first (lowest-density) liquid–liquid transition of water. This finding agrees with the known close relation between the phase transition and the percolation transition of physical clusters. The lowest-density amorphous water phase is characterized by presence of a percolating network of the four-coordinated water molecules. The line of the percolation transitions of tetrahedrally ordered water molecules (with arbitrary coordination number) envelops the whole region where liquid–liquid transitions occur. So, the percolating network of the tetrahedrally ordered water molecules is absent in the highest-density amorphous water phase.