LJ流体非牛顿现象的分子动力学模拟

使用非平衡分子动力学模拟方法研究了单原子LJ流体的非牛顿流变行为，并在系统中分别施加稳态Couette流场和振荡剪切流场.在Couette流场的模拟中，流体出现剪切变稀和法向应力差效应，不同剪切率下的径向分布函数反映了流体分子由于剪切所导致的微观结构变化，通过分析势能函数发现当剪切率增大时，分子间排斥作用增强，吸引作用减弱.在振荡剪切流场的模拟中，发现剪切应力和剪切率之间的相位差随频率增加而增加，随频率增加复数粘度的实部先增大再减小，虚部单调增加，导致虚部粘度相对实部粘度比例增大，弹性模量和粘性模量之比也随频率增加而增加.这三点现象表明LJ流体出现粘弹性行为，且在高频率下，弹性所占比重增大.

Study on Non-Newtonian Behaviors of Lennard-Jones Fluids via Molecular Dynamics Simulations

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic structure of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here:First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase monotonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.