Singh Ankit, Bhattacharyya Sarika Maitra, Singh Yashwant
Department of Physics, Banaras Hindu University, Varanasi 221 005, India.
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune 411008, India.
Phys Rev E. 2021 Mar;103(3-1):032611. doi: 10.1103/PhysRevE.103.032611.
In this paper, we develop a theory to calculate the structural relaxation time τ_{α} of fragile supercooled liquids. Using the information of the configurational entropy and structure, we calculate the number of dynamically free, metastable, and stable neighbors around a central particle. In supercooled liquids, the cooperatively reorganizing clusters (CRCs) in which the stable neighbors form "stable" nonchemical bonds with the central particle emerge. For an event of relaxation to take place, these bonds have to reorganize irreversibly; the energy involved in the processes is the effective activation energy of relaxation. The theory brings forth a temperature T_{a} and a temperature-dependent parameter ψ(T) which characterize slowing down of dynamics on cooling. It is shown that the value of ψ(T) is equal to 1 for T>T_{a}, indicating that the underlying microscopic mechanism of relaxation is dominated by the entropy-driven processes, while for T<T_{a}, ψ(T) decreases on cooling, indicating the emergence of the energy-driven processes. This crossover of ψ(T) from high to low temperatures explains the crossover seen in τ_{α}. The dynamics of systems that may have similar static structure but very different dynamics can be understood in terms of ψ(T). We present results for the Kob-Anderson model for three densities and show that the calculated values of τ_{α} are in excellent agreement with simulation values for all densities. We also show that when ψ(T), τ_{α}, and other quantities are plotted as a function of T/T_{a} (or T_{a}/T), the data collapse on master curves.
在本文中，我们发展了一种理论来计算脆性过冷液体的结构弛豫时间τα。利用构型熵和结构的信息，我们计算了中心粒子周围动态自由、亚稳和稳定邻居的数量。在过冷液体中，出现了稳定邻居与中心粒子形成“稳定”非化学键的协同重组团簇（CRC）。为了发生弛豫事件，这些键必须不可逆地重新组织；该过程中涉及的能量是弛豫的有效活化能。该理论提出了一个温度Ta和一个与温度有关的参数ψ(T)，它们表征了冷却过程中动力学的减慢。结果表明，当T>Ta时，ψ(T)的值等于1，这表明弛豫的潜在微观机制由熵驱动过程主导，而当T<Ta时，ψ(T)在冷却时减小，这表明能量驱动过程的出现。ψ(T)从高温到低温的这种转变解释了在τα中看到的转变。对于具有相似静态结构但动力学非常不同的系统，可以根据ψ(T)来理解其动力学。我们给出了三种密度下Kob-Anderson模型的结果，并表明τα的计算值与所有密度下的模拟值都非常吻合。我们还表明，当将ψ(T)、τα和其他量作为T/Ta（或Ta/T）的函数绘制时，数据会汇聚在主曲线上。
