Abstract
The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude before the liquid freezes into a glass. The rate of the change with temperature is characterized by liquid fragility. The mechanism of such a spectacular behavior and the origin of fragility have long been discussed, but it remains unresolved because of the difficulty of carrying out experiments and constructing theories that bridge over a wide timescale from atomic (ps) to bulk (minutes). Through the x-ray diffraction measurement and molecular dynamics simulation for metallic liquids we suggest that large changes in viscosity can be caused by relatively small changes in the structural coherence which characterizes the medium-range order. Here the structural coherence does not imply that of atomic-scale structure, but it relates to the coarse-grained density fluctuations represented by the peaks in the pair-distribution function (PDF) beyond the nearest neighbors. The coherence length is related to fragility and increases with decreasing temperature, and it diverges only at a negative temperature. This analysis is compared with several current theories which predict a phase transition near the glass transition temperature.
Highlights
The viscosity of many liquids, such as water, is of the order of 10−2 poise (= 10−3 Pa.s)
Upon cooling liquid viscosity rises rather quickly, if crystallization can be avoided for instance by fast cooling
The timescale of liquid dynamics changes by as much as 15 orders of magnitude over a moderate temperature range. Such a rapid change has direct implications on glass-forming ability and other properties of glass-forming liquids, as well as on applications. The origin of this large change has long been debated without wide agreement (Debenedetti and Stillinger, 2001; March and Tosi, 2002; Dyre, 2006; Lubchenko and Wolynes, 2007; Götze, 2009; Donth, 2010; Berthier and Biroli, 2011; Edigar and Harrowell, 2012; Parisi et al, 2020), and remains one of the glass mysteries
Summary
The viscosity of many liquids, such as water, is of the order of 10−2 poise (= 10−3 Pa.s). TA, viscosity shows the Arrhenius behavior with a constant value of Ea, and below TA it becomes strongly super-Arrhenius (Angell, 1995; Kivelson et al., 1995), resulting in rapid increase in viscosity culminating to the glass transition It has been shown by simulations (Iwashita et al, 2013) and by experiments (Iwashita et al, 2017; Shinohara et al, 2019; Ashcraft et al, 2020) that above TA viscosity is determined by a bond cutting dynamics, and τM = τLC, where τLC is the timescale for an atom to lose just one neighbor. The ideal glass state defined by G0(r) has long-range density correlation without periodicity in the structure (Ryu et al, 2019). For a single-component liquid the second peak of S(Q), which is more sensitive to crystallinity, diverges at a positive temperature below Tg (Ryu et al, 2019) This suggests that the divergence of the local order just below Tg implies nanoscale crystallization. The glass theories based on exact solutions in infinite dimensions (Parisi et al, 2020) are beautiful, but the success of its application to real liquids and glasses needs to be proven
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