We study the linear response in different models of driven granular gases. In some situations, even if the the velocity statistics can be strongly non-Gaussian, we do not observe appreciable violations of the Einstein formula for diffusion versus mobility. The situation changes when strong correlations between velocities and density are present: in this case, although a form of fluctuation-dissipation relation holds, the differential velocity response of a particle and its velocity self-correlation are no more proportional. This happens at high densities and strong inelasticities, but still in the fluid-like (and ergodic) regime.
Molecular dynamics simulations are performed on a system of model linear polymers to look at the violations of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations near the mode coupling theory transition temperature $T_c$ at three (one higher and two lower) densities. At low temperatures, both lower density systems show stable gas-supercooled-liquid coexistence whereas the higher density system is homogeneous. We show that monomer density relaxation exhibits SE violation for all three densities, whereas molecular density relaxation shows a weak violation of the SE relation near $T_c…
最近的计算机模拟结果[Barrat { \em et al.},Physica A 334(2004)513]已经显示了当气体$T$的温度被通常爱因斯坦关系中的杂质$T_0$的温度代替时,扩散$D$和迁移率$λ$系数之间的爱因斯坦关系$ε\equiv D/(T_0λ)=1$的有效性。本文用Chapman-Enskog方法解析求解Boltzmann-Lorentz方程来分析这个问题。气体在外部驱动力(恒温器)的作用下被加热,该驱动力确实起作用以补偿碰撞损失的能量。考虑两种类型的恒温器:(a)与粒子速度成比例的确定性力(高斯恒温器),以及(b)白噪声外力(随机恒温器)。的扩散和迁移率系数给出的两个线性积分方程,这是近似解决的Sonine多项式展开的二阶的解决方案。结果表明,爱因斯坦关系($ε\neq 1$)的破坏仅仅是由于杂质速度分布函数的非麦克斯韦行为(吉布斯态的缺失)。在定量水平上,动力学理论的结果还表明,$ε$从1的偏差是更显着的高斯恒温器的情况下比在随机的情况下,在这种情况下的爱因斯坦关系的偏差一般小于1%。这一结论与计算机模拟的结果吻合得很好。
Molecular dynamics simulations have been performed on TIP4P/2005 supercooled water to investigate the molecular diffusion and shear viscosity at various timescales and assess the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations. For this purpose, we calculated various time correlation functions, such as the mean-squared displacement, stress relaxation function, density correlation function, hydrogen-bond correlation function, rotational correlation function of molecular orientation, non-Gaussian parameter, and four-point correlation function. Our study of the SE and SED relations…