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吉野 元@阪大サイバー
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2017年10月16日(月) 16:30 - 大阪大学豊中キャンパス サイバーメディアセンター7F会議室
http://www.cp.cmc.osaka-u.ac.jp
Identification of time-scales that support violation or preservation of
Stokes-Einstein relation in supercooled water
川﨑 猛史 氏 (名古屋大学 理学研究科 物理学教室 R研)
The Stokes–Einstein (SE) relationship between the shear viscosity and
the translational diffusion constant of liquids is an important
characteristic of their transport properties. The SE relationship breaks
down in the case of glassy states. In fact, such violation of the SE
relationship is "generally" observed in supercooled liquids. However,
the physical origin of this SE violation remains elusive. In particular,
for supercooled water, the SE violation coincides with other significant
unsolved problems such as fragile-to-strong crossover in transport
properties and liquid-liquid transition between a high- and a low
density liquid. Therefore, clarification of the SE violation is very
important for achieving a deeper understanding of the problems.
Unfortunately, the high computational costs of the calculation of the
shear viscosity have prevented a thorough investigation of the SE
violation in supercooled liquid water to date.
In our study, we describe a systematic investigation of the SE
relationship in supercooled liquid water [1]. We present the results of
molecular dynamics simulations over a wide temperature range, including
comprehensive numerical calculations of the shear viscosity using the
shear stress correlation function. In particular, the role of the
time-scales associated with various dynamical properties including
hydrogen-bond (HB) breakage on the SE relationship is discussed. Most
importantly, we found that the SE relationship is violated when the
structural relaxation time is utilized; however the SE relation holds,
even for supercooled states, when the HB lifetime is used. That is, the
appropriate time scale supporting the SE relationship is surprisingly
determined by the HB network rearrangement processes. This SE
preservation was revealed to be due to the activated jumping of mobile
water molecules during the HB breakages, which characterizes the
diffusion constant. We have revealed that local rearrangements are
directly connected with those jumping motions in other glass forming
liquids such as soft-sphere mixtures (fragile liquids) [2] and
silica-like network-forming liquids (strong liquids) [3]. Thus, our
comprehensive results suggest that the SE violation observed in various
glassy systems can be completely resolved through a unified description
of local rearrangement events that cause the activated jump motions to
exceed an energy barrier between local metabasins.
[1] T. Kawasaki and K. Kim, Science Advances 3, e1700399 (2017).
[2] T. Kawasaki and A. Onuki, Phys. Rev. E 87 012312(2013).
[3] T. Kawasaki, K. Kim, and A. Onuki, J. Chem. Phys. 140 184502
(2014).
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Hajime Yoshino
Associate Professor
Cybermedia Center, Osaka University
1-32 Machikaneyama, Toyonaka
560-0043 Japan
Tel: +81-(0)6-6850-6841
FAX: +81-(0)6-6850-6842
E-mail: yoshino@cmc.osaka-u.ac.jp
http://www.cp.cmc.osaka-u.ac.jp/~yoshino/