Speaker
Prof.
Hajime Tanaka
(University of Tokyo)
Description
The anomalous thermodynamic and kinetic behaviour of water
is known to play a fundamental role in many chemical,
biological, geological and terrestrial processes. However,
its origin is still much debated despite decades of intense
research. An early insight, which dates back to Rontgen, is
that the complexity of water may be modelled by a mixture of
two structural motifs, but the idea has found limited
support due to the lack of microscopic evidence. Unlike
assuming the presence of two structural motifs, we proposed
a two-order-parameter model of liquid [1,2], which focuses
on the (cooperative) formation of locally favoured
structures in a sea of random liquid structures. Here we
provide a simple physical description of water anomaly from
microscopic data obtained through computer simulations. We
introduce a novel structural order parameter, which
quantifies the degree of translational order of the second
shell, and show that this parameter alone, which measures
the amount of locally favored structures, accurately
characterizes the state of water [3]. A two-state modeling
of these microscopic structures [1,2] is used to describe
the behavior of liquid water over a wide region of the phase
diagram, correctly identifying the density and
compressibility anomalies, and being compatible with the
existence of a second critical point in the deeply
supercooled region. Furthermore, we reveal that locally
favored structures in water not only have translational
order in the second shell, but also contain five-membered
rings of hydrogen-bonded molecules. This suggests their
mixed character: the former helps crystallization, whereas
the latter causes frustration against crystallization [3].
We also show that this local structural ordering plays a key
role in ice crystallization of a deeply supercooled water,
through a novel metastable ice crystal phase, which we named
Ice 0 [4]. We also discuss the glass-forming ability of
water-type tetrahedral liquids, focusing on the friendliness
of local structural orderings with a crystal structure to be
nucleated. This work is performed in collaboration with John
Russo and Flavio Romano.
[1] H. Tanaka, Thermodynamic anomaly and polyamorphism of
water, Europhy. Lett. 50, 340 (2000).
[2] H. Tanaka, Bond orientational order in liquids: Towards
a unified description of water-like anomalies, liquid-liquid
transition, glass transition, and crystallization, Eur.
Phys. J. E 35, 113 (2012).
[3] J. Russo and H. Tanaka, Understanding water's anomalies
with locally favored structures, Nat. Commun. 5, 3556 (2014).
[4] J. Russo, F. Romano, and H. Tanaka, New metastable form
of ice and its role in the homogeneous crystallization of
water, Nature Mater. 13, 733 (2014).
