Rusciano Francesco, Pastore Raffaele, Greco Francesco
Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Piazzale Tecchio 80, Napoli 80125, Italy.
Phys Rev Lett. 2022 Apr 22;128(16):168001. doi: 10.1103/PhysRevLett.128.168001.
Fickian yet non-Gaussian diffusion (FnGD), a most intriguing open issue in soft matter, is generically associated with some dynamical and/or structural heterogeneity of the environment. Here we investigate the features of FnGD in glass-forming liquids, the epitome of dynamical heterogeneity, drawing on experiments on hard-sphere colloidal suspensions and simulations of a simple model of molecular liquid. We demonstrate that FnGD strengthens on approaching the glass transition, by identifying distinct timescales for Fickianity, τ_{F}, and for restoring of Gaussianity, τ_{G}>τ_{F}, as well as their associated length scales, ξ_{F} and ξ_{G}. We find τ_{G}∝τ_{F}^{γ} with γ≃1.8 for both systems. In the deep FnGD regime, the displacement distributions display exponential tails. We show that, in simulations, the time-dependent decay lengths l(t) at different temperatures all collapse onto a power-law master curve [l(t)/(ξ_{G})]∝(t/τ_{G})^{α}, with α=0.33. A similar collapse, if less sharp, is also found in experiments, seemingly with the same exponent α. We further discuss the connections of the timescales and length scales characterizing FnGD with structural relaxation and dynamic heterogeneity.
菲克型非高斯扩散（FnGD）是软物质领域中一个极具吸引力的开放性问题，通常与环境的某些动力学和/或结构异质性相关。在此，我们利用硬球胶体悬浮液的实验以及分子液体简单模型的模拟，研究玻璃形成液体中FnGD的特征，玻璃形成液体是动力学异质性的典型代表。我们通过确定菲克型的不同时间尺度τ_F以及高斯性恢复的时间尺度τ_G>τ_F，以及它们相关的长度尺度ξ_F和ξ_G，证明了在接近玻璃化转变时FnGD会增强。我们发现两个系统中τ_G∝τ_F^γ，γ≃1.8。在深度FnGD区域，位移分布呈现指数尾部。我们表明，在模拟中，不同温度下随时间变化的衰减长度l(t)都塌缩到一条幂律主曲线上，即[l(t)/(ξ_G)]∝(t/τ_G)^α，α = 0.33。在实验中也发现了类似的塌缩现象，尽管不太明显，且指数α似乎相同。我们进一步讨论了表征FnGD的时间尺度和长度尺度与结构弛豫和动力学异质性之间的联系。
