伴随玻璃化的协同性的出现：密度波动动力学的见解
The emergence of cooperativity accompanying vitrification: Insights from density fluctuation dynamics
摘要：
基于脆性玻璃形成液体的密度涨落动力学，讨论了玻璃化过程中协同效应的产生和发展。(i)密度涨落的弛豫是通过粒子（密度）交换过程进行的，并且是扩散的;也就是说，允许的动力学路径受到局部守恒定律的强烈限制。(ii)在正常的液体状态下，这种交换过程是不太合作的，并且密度波动的扩散系数$D_c$被给出为$D_c\sim λ^2/τ_α$，其中$λ$是颗粒尺寸并且$τ_α$是结构弛豫时间。另一方面，在过冷状态下，随着过冷度的增加，对动力学路径的限制更加严重，这使得交换过程更加合作，导致$D_c \sim ξ_{\rm d}^2/τ_α$，其中$ξ_{\rm d}$是合作长度尺度。(iii)分子动力学模拟结果表明，标记粒子的自扩散系数$D_s$与$D_c$几乎一致，表明集体密度扩散与粒子扩散具有相同的机制：在正常态下，$D_s$决定$D_c$，而在过冷态下，$D_c$决定集体密度扩散.这立即导致了这样的想法，即斯托克斯-爱因斯坦关系的破裂不是单粒子动力学中的异常，而是反映了在$ξ_{\rm d}$的长度尺度上密度扩散中协同性的增加。
英文摘要：
We discuss the emergence and growth of the cooperativity accompanying vitrification based on the density fluctuation dynamics for fragile glass-forming liquids. (i) The relaxation of density fluctuations proceeds by the particle (density) exchange process, and is diffusive; that is, the allowable kinetic paths are strongly restricted by the local conservation law. (ii) In normal liquid states, this exchange process is less cooperative, and the diffusion coefficient of density fluctuations $D_c$ is given as $D_c\sim λ^2/τ_α$, where $λ$ is the particle size and $τ_α$ is the structural relaxation time. On the other hand, in supercooled states the restriction on the kinetic path is more severe with increasing the degree of supercooling, which makes the exchange process more cooperative, resulting in $D_c \sim ξ_{\rm d}^2/τ_α$ with $ξ_{\rm d}$ being the cooperative length scale. (iii) The molecular dynamics simulation results show that the self-diffusion coefficient of the tagged particle, $D_s$, almost coincides with $D_c$, suggesting that the collective density diffusion and the particle diffusion closely share the same mechanism: in normal states $D_s$ determines $D_c$, but vice versa in supercooled states. This immediately leads to the idea that the breakdown of the Stokes-Einstein relation is not the anomaly in the single-particle dynamics but reflects the increase in the cooperativity in the density diffusion at the length scale of $ξ_{\rm d}$.
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