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Abstract:
We present evidence from experiments and computer simulations supporting
the hypothesis that water displays polyamorphism, i.e., water separates into
two distinct liquid phases. This concept of a new liquid–liquid phase transition is
finding potential application to other liquids as well as water, such as silicon and
silica. Here we review the relation between changes in dynamic and thermodynamic
anomalies arising from the presence of the liquid–liquid critical point in (i) Two
models of water, TIP5P and ST2, which display a first order liquid–liquid phase
transition at low temperatures; (ii) Two spherically symmetric two-scale potentials
known to possess a liquid–liquid critical point, in which the competition between
two liquid structures is generated by repulsive and attractive ramp interactions; and
(iii) A Hamiltonian model of water where the idea of two length/energy scales is
built in. This model also displays a first order liquid–liquid phase transition at low
temperatures besides the first order liquid-gas phase transition at high temperatures.
We find a correlation between the dynamic fragility crossover and the locus of
specific heat maxima Cmax
P (“Widom line”) emanating from the critical point. Our
findings are consistent with a possible relation between the previously hypothesized
liquid-liquid phase transition and the transition in the dynamics recently observed
in neutron scattering experiments on confined water. More generally, we argue that
this connection between Cmax
P and the dynamic crossover is not limited to the case
of water, a hydrogen bonded network liquid, but is a more general feature of crossing
the Widom line, an extension of the first-order coexistence line in the supercritical
region.
