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Evaluation of Viscosity by Using MD

Viscosity can be evaluated by calculating the shear deformation using Lees-Edwards boundary conditions.

Results for n-decane and n-hexadecane are shown below. Using the full atomistic model created with J-OCTA, the calculations were carried out with a density of 0.613 g/cm3 and a temperature of 480 K for n-decane and a density of 0.753 g/cm3 and a temperature of 323 K for n-hexadecane. The numbers of molecules in the system for the two substances were respectively 200 and 100. The viscosity was evaluated while varying the shear rate from 1e+12 [1/s] to 1e+10 [1/s]. VSOP was used for the MD calculations (Fig. 1).

Fig. 2 shows a double logarithmic plot of the calculated viscosity versus the shear rate. It can be seen that the viscosity increases with decreasing shear rate; however, for n-decane, the viscosity approaches a constant value when the shear rate is at 1e+10 [1/s] or below.

In the viscosity evaluation using the Green-Kubo method in Reference [1], the viscosity of n-decane under similar conditions is 0.178 [e-3 Pa·s] and the viscosity of n-hexadecane is 1.17 [e-3 Pa·s].

Fig. 1. Simulation system with the Lees-Edwards boundary condition. Fig. 1. Simulation system with the Lees-Edwards boundary condition.

Fig. 2. Evaluated viscosity. Fig. 2. Evaluated viscosity.

  • Reference
  • [1] S. T. Cui, P. T. CUMMINGS and H. D. COCHRAN, Molecular Physics, 1998, 93, 1, 117-121

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