| 38 |
|
\begin{doublespace} |
| 39 |
|
|
| 40 |
|
\begin{abstract} |
| 41 |
< |
|
| 41 |
> |
A molecular simulation method is developed based on the previous RNEMD |
| 42 |
> |
algorithm. With scaling the velocities of molecules in specific |
| 43 |
> |
regions of a system, unphysical thermal transfer can be achieved and |
| 44 |
> |
thermal / momentum gradient can be established, as well as |
| 45 |
> |
conservation of linear momentum and translational kinetic |
| 46 |
> |
energy. Thermal conductivity calculations of Lennard-Jones fluid water |
| 47 |
> |
(SPC/E model) and crystal gold (QSC and EAM model) are performed and |
| 48 |
> |
demostrate the validity of the method. Furthermore, NIVS-RNEMD |
| 49 |
> |
improves the non-Maxwell-Boltzmann velocity distributions in previous |
| 50 |
> |
RNEMD methods, where unphysical momentum transfer occurs. NIVS-RNEMD |
| 51 |
> |
also extends its application to interfacial thermal conductivity |
| 52 |
> |
calculations, thanks to its novel means in kinetic energy transfer. |
| 53 |
|
\end{abstract} |
| 54 |
|
|
| 55 |
|
\newpage |
| 658 |
|
|
| 659 |
|
|
| 660 |
|
\subsection{Interfaciel Thermal Conductivity} |
| 661 |
< |
After valid simulations of homogeneous water and gold systems using |
| 662 |
< |
NIVS-RNEMD method, calculation of gold/water interfacial thermal |
| 663 |
< |
conductivity was followed. It is found out that the interfacial |
| 664 |
< |
conductance is low due to a hydrophobic surface in our system. Figure |
| 665 |
< |
\ref{interfaceDensity} demonstrates this observance. Consequently, our |
| 666 |
< |
approximation in $G$ calculation (Eq. \ref{interfaceCalc}) maintains |
| 667 |
< |
valid. Reported results (Table \ref{interfaceRes}) are of two orders of |
| 668 |
< |
magnitude smaller than our calculations on homogeneous systems, and |
| 669 |
< |
thus have larger relative errors than our calculation results on |
| 670 |
< |
homogeneous systems. |
| 661 |
> |
After simulations of homogeneous water and gold systems using |
| 662 |
> |
NIVS-RNEMD method were proved valid, calculation of gold/water |
| 663 |
> |
interfacial thermal conductivity was followed. It is found out that |
| 664 |
> |
the low interfacial conductance is probably due to the hydrophobic |
| 665 |
> |
surface in our system. Figure \ref{interfaceDensity} demonstrates mass |
| 666 |
> |
density change along $z$-axis, which is perpendicular to the |
| 667 |
> |
gold/water interface. It is observed that water density significantly |
| 668 |
> |
decreases when approaching the surface. Under this low thermal |
| 669 |
> |
conductance, both gold and water phase have sufficient time to |
| 670 |
> |
eliminate temperature difference inside respectively (Figure |
| 671 |
> |
\ref{interfaceGrad}). With indistinguishable temperature difference |
| 672 |
> |
within respective phase, it is valid to assume that the temperature |
| 673 |
> |
difference between gold and water on surface would be approximately |
| 674 |
> |
the same as the difference between the gold and water phase. This |
| 675 |
> |
assumption enables convenient calculation of $G$ using |
| 676 |
> |
Eq. \ref{interfaceCalc} instead of measuring temperatures of thin |
| 677 |
> |
layer of water and gold close enough to surface, which would have |
| 678 |
> |
greater fluctuation and lower accuracy. Reported results (Table |
| 679 |
> |
\ref{interfaceRes}) are of two orders of magnitude smaller than our |
| 680 |
> |
calculations on homogeneous systems, and thus have larger relative |
| 681 |
> |
errors than our calculation results on homogeneous systems. |
| 682 |
|
|
| 683 |
|
\begin{figure} |
| 684 |
|
\includegraphics[width=\linewidth]{interfaceDensity} |
| 685 |
|
\caption{Density profile for interfacial thermal conductivity |
| 686 |
< |
simulation box.} |
| 686 |
> |
simulation box. Significant water density decrease is observed on |
| 687 |
> |
gold surface.} |
| 688 |
|
\label{interfaceDensity} |
| 689 |
|
\end{figure} |
| 690 |
|
|
| 691 |
|
\begin{figure} |
| 692 |
|
\includegraphics[width=\linewidth]{interfaceGrad} |
| 693 |
|
\caption{Temperature profiles for interfacial thermal conductivity |
| 694 |
< |
simulation box.} |
| 694 |
> |
simulation box. Temperatures of different slabs in the same phase |
| 695 |
> |
show no significant difference.} |
| 696 |
|
\label{interfaceGrad} |
| 697 |
|
\end{figure} |
| 698 |
|
|
| 790 |
|
|
| 791 |
|
\section{Conclusions} |
| 792 |
|
NIVS-RNEMD simulation method is developed and tested on various |
| 793 |
< |
systems. Simulation results demonstrate its validity of thermal |
| 794 |
< |
conductivity calculations. NIVS-RNEMD improves non-Boltzmann-Maxwell |
| 795 |
< |
distributions existing in previous RNEMD methods, and extends its |
| 796 |
< |
applicability to interfacial systems. NIVS-RNEMD has also limited |
| 797 |
< |
application on shear viscosity calculations, but under high momentum |
| 798 |
< |
flux, it could cause temperature difference among different |
| 799 |
< |
dimensions. Modification is necessary to extend the applicability of |
| 800 |
< |
NIVS-RNEMD in shear viscosity calculations. |
| 793 |
> |
systems. Simulation results demonstrate its validity in thermal |
| 794 |
> |
conductivity calculations, from Lennard-Jones fluid to multi-atom |
| 795 |
> |
molecule like water and metal crystals. NIVS-RNEMD improves |
| 796 |
> |
non-Boltzmann-Maxwell distributions, which exist in previous RNEMD |
| 797 |
> |
methods. Furthermore, it develops a valid means for unphysical thermal |
| 798 |
> |
transfer between different species of molecules, and thus extends its |
| 799 |
> |
applicability to interfacial systems. Our calculation of gold/water |
| 800 |
> |
interfacial thermal conductivity demonstrates this advantage over |
| 801 |
> |
previous RNEMD methods. NIVS-RNEMD has also limited application on |
| 802 |
> |
shear viscosity calculations, but could cause temperature difference |
| 803 |
> |
among different dimensions under high momentum flux. Modification is |
| 804 |
> |
necessary to extend the applicability of NIVS-RNEMD in shear viscosity |
| 805 |
> |
calculations. |
| 806 |
|
|
| 807 |
|
\section{Acknowledgments} |
| 808 |
|
Support for this project was provided by the National Science |
