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The origin of viscosity as seen through atomic level stress correlation function
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10.1063/1.4789306
/content/aip/journal/jcp/138/4/10.1063/1.4789306
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/4/10.1063/1.4789306
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Stress correlation functions for a system of 5488 particles at 2000 K. The black solid line shows correlation function for the total stress. The red dashed line shows autocorrelation correlation for atomic level stress. The blue dotted-dashed line shows the difference between the total and the autocorrelation functions. The error in the correlation function of the total stress is comparable with the line thickness. The error in the autocorrelation function is much smaller than the line thickness.

Image of FIG. 2.
FIG. 2.

Dependencies of the total and autocorrelation viscosities on the upper cutoff in integration time at 2000 K. Obtained from the data in Fig. 1 .

Image of FIG. 3.
FIG. 3.

Dependencies on temperature of the final contributions to viscosity from the auto and the cross terms.

Image of FIG. 4.
FIG. 4.

Normalized microscopic sscf's F(r, t)/F(r = 0, t = 0) for (a) 5488 particles system (L/2 ∼ 20.60 Å) at 1500 K, (b) 43 904 particles system (L/2 ∼ 41.21 Å) at 1500 K, and (c) 43 904 particles system at 2000 K. The values of the stress autocorrelation functions at zero time are ∼0.41 eV at 1500 K, ∼0.55 eV at 2000 K. Comparison of the panels (a) and (b) clearly shows that finite size of the system and the PBC affect the stress field. Comparison of panels (b) and (c) shows temperature dependence of the stress field in a moderately supercooled system.

Image of FIG. 5.
FIG. 5.

(a)–(d) Normalized microscopic sscf's F(r, t)/F(r = 0, t = 0) for the system of 5488 particles at temperatures 3000 K, 5000 K, 7000 K, 10 000 K correspondingly. The values of the stress autocorrelation functions at zero time are ∼0.86 eV at 3000 K, ∼1.52 eV at 5000 K, ∼2.18 eV at 7000 K, ∼3.13 eV at 10 000 K. (e) Normalized sscf at temperature 10 000 K for the large system of 43 904 particles. The comparison of panels (d) and (e) shows that PBC introduce some distortion into the sscf even at such a high temperature as 10 000 K.

Image of FIG. 6.
FIG. 6.

Constant time cuts of the (non-normalized) sscf from Fig. 4(b) . The legends show the cut times in femtoseconds.

Image of FIG. 7.
FIG. 7.

The black curves show zero-time cuts of the sscf's normalized to the zero-time value of the corresponding autocorrelation functions on the system of 5488 particles. The red curves show corresponding PDFs scaled in such a way that the heights of the first peaks in the scaled PDFs would match the heights of the first peaks in the sscf's. Note that the splitting of the second peak in PDF at 1500 K is also reflected in the sscf.

Image of FIG. 8.
FIG. 8.

Dependencies of the microscopic viscosities on inclusion distance R max for the intermediate (5488) and the large (43 904) systems at 1500 K for some values of t max . For all t max , the red curves correspond to the intermediate size system, while the black curves correspond to the large system.

Image of FIG. 9.
FIG. 9.

Dependencies of the microscopic viscosities on inclusion distance R max for the systems of different sizes at 2000 K for some values of t max . For all t max , the blue curves correspond to the smallest system, the red curves correspond to the intermediate size system and the black curves correspond to the large system.

Image of FIG. 10.
FIG. 10.

Dependencies of the particles mean square displacement on time at different temperatures. The results were obtained on a system of 5488 particles.

Image of FIG. 11.
FIG. 11.

Fourier transform of the pair distribution function , which is related to the normalized scattering intensity S(q). Note that the first peak occurs at q ≈ 3.05 Å−1. The inset shows the pair potential used in our MD simulations.

Image of FIG. 12.
FIG. 12.

Dependencies of the intermediate self-scattering function on time for several temperatures. The data from MD simulations (solid curves) were fitted with the dashed curves. The error bars for the MD curves are smaller than the widths of the curves. The results were obtained on a system of 5488 particles. The legends show the temperatures and the parameters of the stretched exponential A exp [−(t/τ)β] fits, i.e., A, τ, β.

Image of FIG. 13.
FIG. 13.

The total sscf and the auto contribution to it at 1325 K for the system of 5488 particles. Both panels have the same data, but the upper panel has linear scale on y-axis, while the lower panel has log  scale on y-axis (to better demonstrate quality of the fit at small values of sscf). The upper solid line (in both panels) corresponds to the total sscf, while the lower corresponds to the auto contribution. The dashed curves are the fits made with the sums of compressed and stretched exponential functions. The equation of the fit curve for the total sscf is 0.427 exp [−(t/39)2] + 0.220 exp [−(t/2350)3/4]. The equation of the fit curve for the auto term is 0.254 exp [−(t/34)2] + 0.112 exp [−(t/2500)3/4]. The size of the oscillations at very large times that could be observed in the total sscf gives some estimate of the errors. The errors at short times are smaller than the thickness of the curves.

Image of FIG. 14.
FIG. 14.

Dependencies of the different relaxation times on temperature for the system of 5488 particles. The time τα is obtained by fitting the tails of the intermediate self-scattering function with the stretched exponential. The time τ2 is obtained by fitting with stretched exponential the tails of the total sscf. Note that τα ≈ 3τ2. The time τ S was calculated from the behavior of total sscf as it was approximated with twice the value of the auto term of sscf and the auto term was fitted with the sum of the compressed and stretched exponential functions. Note that τα is longer than τ S by nearly a factor of five. The τ M curve was obtained by explicit integration of the total sscf and by calculating the values of the shear modulus G from the instant configurations. Note that there is a relatively good agreement between τ s and η/G curves, as it should be. The time τ1 corresponds to the compressed exponent that describes short time behavior of the sscf.

Image of FIG. 15.
FIG. 15.

The dependencies of the microscopic viscosities on inclusion distance from the intermediate (5488), the large (43 904) and the folded/mapped sscf at 1500 K and cutoff in time 2000 fs. The folded/mapped sscf function was produced by the mapping of the sscf obtained on the large system onto the small system as described in the text. We see that the mapped curve is rather similar to the curve obtained on the intermediate size system. The final saturated value of the mapped viscosity should be the same as the final value of the viscosity calculated on the large system (note that it is the case). The differences between the folded viscosity and the viscosity calculated on the intermediate size system directly could originate in statistical error or in the fact that our large system is not the infinite size system. However, these differences could also contain additional information about the nature of the atomic level stresses and the stress waves. Further investigation of these details might be interesting.

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/content/aip/journal/jcp/138/4/10.1063/1.4789306
2013-01-31
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The origin of viscosity as seen through atomic level stress correlation function
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/4/10.1063/1.4789306
10.1063/1.4789306
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