Molecular Dynamics Technology Co. Ltd., Hsinchu County 30265, Taiwan.
J Chem Phys. 2011 Jan 28;134(4):044511. doi: 10.1063/1.3541825.
Extensive computer experiments have been conducted in order to shed light on the macroscopic shear flow behavior of liquid n-hexadecane fluid under isobaric-isothermal conditions through the nonequilibrium molecular dynamic methodology. With respect to shear rates, the accompanying variations in structural properties of the fluid span the microscopic range of understanding from the intrinsic to extrinsic characteristics. As drawn from the average value of bond length and bond angle, the distribution of dihedral angle, and the radius distribution function of intramolecular and intermolecular van der Waals distances, these intrinsic structures change with hardness, except in the situation of extreme shear rates. The shear-induced variation of thermodynamic state curve along with the shear rate studied is shown to consist of both the quasiequilibrium state plateau and the nonequilibrium-thermodynamic state slope. Significantly, the occurrence of nonequilibrium-thermodynamic state behavior is attributed to variations in molecular potential energies, which include bond stretching, bond bending, bond torsion, and intra- and intermolecular van der Waals interactions. To unfold the physical representation of extrinsic structural deformation, under the aggressive influence of a shear flow field, the molecular dimension and appearance can be directly described via the squared radius of gyration and the sphericity angle, R(g)(2) and ϕ, respectively. In addition, a specific orientational order S(x) defines the alignment of the molecules with the flow direction of the x-axis. As a result, at low shear rates, the overall molecules are slightly stretched and shaped in a manner that is increasingly ellipsoidal. Simultaneously, there is an obvious enhancement in the order. In contrast to high shear rates, the molecules spontaneously shrink themselves with a decreased value of R(g)(2), while their shape and order barely vary with an infinite value of ϕ and S(x). It is important to note that under different temperatures and pressures, these three parameters are integrated within a molecular description in response to thermodynamic state variable of density and rheological material function of shear viscosity.
为了通过非平衡分子动力学方法阐明等压-恒温条件下液体正十六烷的宏观剪切流动行为，进行了广泛的计算机实验。在剪切速率方面，流体的结构性质的伴随变化跨越了从内在特性到外在特性的微观理解范围。从键长和键角的平均值、二面角的分布以及分子内和分子间范德华距离的半径分布函数来看，这些内在结构随硬度而变化，除了在极端剪切速率的情况下。研究中，随着剪切速率的变化，热力学状态曲线的剪切诱导变化包括准平衡状态平台和非平衡热力学状态斜率。值得注意的是，非平衡热力学状态行为的发生归因于分子势能的变化，包括键拉伸、键弯曲、键扭转以及分子内和分子间的范德华相互作用。为了揭示外在结构变形的物理表现，在剪切流场的强烈影响下，分子的尺寸和外观可以通过转动惯量的平方半径和球形度角 R(g)(2) 和 ϕ 直接描述。此外，特定的取向序 S(x) 定义了分子与 x 轴流动方向的对齐方式。因此，在低剪切速率下，整个分子稍微伸展并呈逐渐椭圆形状。同时，有序性明显增强。与高剪切速率相比，分子自发收缩，R(g)(2) 值降低，而其形状和有序性几乎不变，ϕ 和 S(x) 值为无穷大。需要注意的是，在不同的温度和压力下，这三个参数在分子描述中是整合的，以响应密度这一热力学状态变量和剪切粘度这一流变材料函数。
