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\usepackage{graphicx}% Include figure files |
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%\usepackage{bm}% bold math |
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\usepackage{times} |
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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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\usepackage{url} |
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\begin{document} |
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|
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\title{Friction at Water / Ice-I$_\mathrm{h}$ interfaces: Do the |
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Facets of Ice Have Different Hydrophilicity?} |
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|
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\author{Patrick B. Louden} |
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|
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\author{J. Daniel Gezelter} |
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\email{gezelter@nd.edu.} |
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\affiliation{Department of Chemistry and Biochemistry, University |
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of Notre Dame, Notre Dame, IN 46556} |
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|
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\date{\today} |
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|
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\begin{abstract} |
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Abstract abstract abstract... |
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\end{abstract} |
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|
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\maketitle |
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|
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\section{Introduction} |
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Explain a little bit about ice Ih, point group stuff. |
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|
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Mention previous work done / on going work by other people. Haymet and Rick |
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seem to be investigating how the interfaces is perturbed by the presence of |
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ions. This is the conlcusion of a recent publication of the basal and |
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prismatic facets of ice Ih, now presenting the pyramidal and secondary |
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prism facets under shear. |
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|
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\section{Methodology} |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{SP_comic_strip} |
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\caption{\label{fig:spComic} The secondary prism interface with a shear |
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rate of 3.5 ms\textsuperscript{-1}. Lower panel: the local tetrahedral order |
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parameter, $q(z)$, (black circles) and the hyperbolic tangent fit (red line). |
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Middle panel: the imposed thermal gradient required to maintain a fixed |
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interfacial temperature. Upper panel: the transverse velocity gradient that |
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develops in response to an imposed momentum flux. The vertical dotted lines |
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indicate the locations of the midpoints of the two interfaces.} |
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\end{figure} |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{Pyr_comic_strip} |
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\caption{\label{fig:pyrComic} The pyramidal interface with a shear rate of 3.8 \ |
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ms\textsuperscript{-1}. Panel descriptions match those in figure \ref{fig:spComic}.} |
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\end{figure} |
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|
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\subsection{Pyramidal and secondary prism system construction} |
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|
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The construction of the pyramidal and secondary prism systems follows that of |
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the basal and prismatic systems presented elsewhere\cite{Louden13}, however |
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the ice crystals and water boxes were equilibrated and combined at 50K |
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instead of 225K. The ice / water systems generated were then equilibrated |
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to 225K. The resulting pyramidal system was |
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$37.47 \times 29.50 \times 93.02$ \AA\ with 1216 |
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SPC/E molecules in the ice slab, and 2203 in the liquid phase. The secondary |
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prism system generated was $71.87 \times 31.66 \times 161.55$ \AA\ with 3840 |
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SPC/E molecules in the ice slab and 8176 molecules in the liquid phase. |
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|
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\subsection{Computational details} |
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% Do we need to justify the sims at 225K? |
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% No crystal growth or shrinkage over 2 successive 1 ns NVT simulations for |
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% either the pyramidal or sec. prism ice/water systems. |
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|
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The computational details performed here were equivalent to those reported |
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in the previous publication\cite{Louden13}. The only changes made to the |
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previously reported procedure were the following. VSS-RNEMD moves were |
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attempted every 2 fs instead of every 50 fs. This was done to minimize |
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the magnitude of each individual VSS-RNEMD perturbation to the system. |
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|
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All pyramidal simulations were performed under the NVT ensamble except those |
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during which statistics were accumulated for the orientational correlation |
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function, which were performed under the NVE ensamble. All secondary prism |
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simulations were performed under the NVE ensamble. |
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|
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\section{Results and discussion} |
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\subsection{Interfacial width} |
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In the literature there is good agreement that between the solid ice and |
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the bulk water, there exists a region of 'slush-like' water molecules. |
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In this region, the water molecules are structured differently and |
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behave differently than those of the solid ice or the bulk water. |
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The characteristics of this region have been defined by both structural |
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and dynamic properties; and its width has been measured by the change of these |
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properties from their bulk liquid values to those of the solid ice. |
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Examples of these properties include the density, the diffusion constant, and |
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the translational order profile. \cite{Bryk02,Karim90,Gay02,Hayward01,Hayward02,Karim88} |
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|
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Since the VSS-RNEMD moves used to impose the thermal and velocity gradients |
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perturb the momenta of the water molecules in |
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the systems, parameters that depend on translational motion may give |
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faulty results. A stuructural parameter will be less effected by the |
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VSS-RNEMD perturbations to the system. Due to this, we have used the |
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local order tetrahedral parameter to quantify the width of the interface, |
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which was originally described by Kumar\cite{Kumar09} and |
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Errington\cite{Errington01} and explained in our |
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previous publication\cite{Louden13} in relation to an ice/water system. |
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|
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Paragraph and eq. for tetrahedrality here. |
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|
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To determine the width of the interfaces, each of the systems were |
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divided into 100 artificial bins along the |
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$z$-dimension, and the local tetrahedral order parameter, $q(z)$, was |
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time-averaged for each of the bins, resulting in a tetrahedrality profile of |
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the system. These profiles are shown across the $z$-dimension of the systems |
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in panel $a$ of Figures \ref{fig:spComic} |
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and \ref{fig:pyrComic} (black circles). The $q(z)$ function has a range of |
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(0,1), where a larger value indicates a more tetrahedral environment. |
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The $q(z)$ for the bulk liquid was found to be $\approx $ 0.77, while values of |
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$\approx $0.92 were more common for the ice. The tetrahedrality profiles were |
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fit using a hyperbolic tangent\cite{Louden13} designed to smoothly fit the |
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bulk to ice |
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transition, while accounting for the thermal influence on the profile by the |
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kinetic energy exchanges of the VSS-RNEMD moves. In panels $b$ and $c$, the |
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imposed thermal and velocity gradients can be seen. The verticle dotted |
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lines traversing all three panels indicate the midpoints of the interface |
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as determined by the hyperbolic tangent fit of the tetrahedrality profiles. |
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|
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From fitting the tetrahedrality profiles for each of the 0.5 nanosecond |
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simulations (panel c of Figures \ref{fig:spComic} and \ref{fig:pyrComic}) |
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by Eq. 6\cite{Louden13},we find the interfacial width to be |
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$3.2 \pm 0.2$ and $3.2 \pm 0.2$ \AA\ for the control system with no applied |
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momentum flux for both the pyramidal and secondary prism systems. |
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Over the range of shear rates investigated, |
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$0.6 \pm 0.2 \mathrm{ms}^{-1} \rightarrow 5.6 \pm 0.4 \mathrm{ms}^{-1}$ for |
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the pyramidal system and $0.9 \pm 0.3 \mathrm{ms}^{-1} \rightarrow 5.4 \pm 0.1 |
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\mathrm{ms}^{-1}$ for the secondary prism, we found no significant change in |
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the interfacial width. This follows our previous findings of the basal and |
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prismatic systems, in which the interfacial width was invarient of the |
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shear rate of the ice. The interfacial width of the quiescent basal and |
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prismatic systems was found to be $3.2 \pm 0.4$ \AA\ and $3.6 \pm 0.2$ \AA\ |
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respectively. Over the range of shear rates investigated, $0.6 \pm 0.3 |
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\mathrm{ms}^{-1} \rightarrow 5.3 \pm 0.5 \mathrm{ms}^{-1}$ for the basal |
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system and $0.9 \pm 0.2 \mathrm{ms}^{-1} \rightarrow 4.5 \pm 0.1 |
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\mathrm{ms}^{-1}$ for the prismatic. |
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|
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These results indicate that the surface structure of the exposed ice crystal |
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has little to no effect on how far into the bulk the ice-like structural |
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ordering is. Also, it appears that the interface is not structurally effected |
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by shearing the ice through water. |
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|
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|
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\subsection{Orientational dynamics} |
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%Should we include the math here? |
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The orientational time correlation function, |
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\begin{equation}\label{C(t)1} |
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C_{2}(t)=\langle P_{2}(\mathbf{u}(0)\cdot \mathbf{u}(t))\rangle, |
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\end{equation} |
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helps indicate the local environment around the water molecules. The function |
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begins with an initial value of unity, and decays to zero as the water molecule |
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loses memory of its former orientation. Observing the rate at which this decay |
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occurs can provide insight to the mechanism and timescales for the relaxation. |
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In eq. \eqref{C(t)1}, $P_{2}$ is the second-order Legendre polynomial, and |
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$\mathbf{u}$ is the the bisecting HOH vector. The angle brackets indicate |
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an ensemble average over all the water molecules in a given spatial region. |
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|
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To investigate the dynamics of the water molecules across the interface, the |
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systems were divided in the $z$-dimension into bins, each $\approx$ 3 \AA\ |
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wide, and \eqref{C(t)1} was computed for each of the bins. A water |
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molecule was allocated to a particular bin if it was initially in the bin |
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at time zero. To compute \eqref{C(t)1}, each 0.5 ns simulation was followed |
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by an additional 200 ps microcanonical (NVE) simulation during which the |
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position and orientations of each molecule were recorded every 0.1 ps. |
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|
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The data obtained for each bin was then fit to a triexponential decay given by |
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\begin{equation}\label{C(t)_fit} |
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C_{2}(t) \approx a e^{-t/\tau_\mathrm{short}} + b e^{-t/\tau_\mathrm{middle}} +\ |
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c |
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e^{-t/\tau_\mathrm{long}} + (1-a-b-c) |
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\end{equation} |
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where $\tau_{short}$ corresponds to the librational motion of the water |
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molecules, $\tau_{middle}$ corresponds to jumps between the breaking and |
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making of hydrogen bonds, and $\tau_{long}$ corresponds to the translational |
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motion of the water molecules. The last term in \eqref{C(t)_fit} accounts |
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for the water molecules trapped in the ice which do not experience any |
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long-time orientational decay. |
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|
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In Figures \ref{fig:PyrOrient} and \ref{fig:SPorient} we see the $z$-coordinate |
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profiles for the three decay constants, $\tau_{short}$ (panel a), |
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$\tau_{middle}$ (panel b), |
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and $\tau_{long}$ (panel c) for the pyramidal and secondary prismatic systems |
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respectively. The control experiments (no shear) are shown in black, and |
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an experiment with an imposed momentum flux is shown in red. The vertical |
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dotted line traversing all three panels denotes the midpoint of the |
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interface as determined by the local tetrahedral order parameter fitting. |
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In the liquid regions of both systems, we see that $\tau_{middle}$ and |
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$\tau_{long}$ have approximately consistent values of $3-6$ ps and $30-40$ ps, |
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resepctively, and increase in value as we approach the interface. Conversely, |
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in panel a, we see that $\tau_{short}$ decreases from the liquid value |
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of $72-76$ fs as we approach the interface. We believe this speed up is due to |
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the constrained motion of librations closer to the interface. Both the |
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approximate values for the decays and relative trends match those reported |
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previously for the basal and prismatic interfaces. |
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|
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As done previously, we have attempted to quantify the distance, $d_{pyramidal}$ |
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and $d_{secondary prism}$, from the |
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interface that the deviations from the bulk liquid values begin. This was done |
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by fitting the orientational decay constant $z$-profiles by |
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\begin{equation}\label{tauFit} |
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\tau(z)\approx\tau_{liquid}+(\tau_{solid}-\tau_{liquid})e^{-(z-z_{wall})/d} |
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\end{equation} |
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where $\tau_{liquid}$ and $\tau_{solid}$ are the liquid and projected solid |
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values of the decay constants, $z_{wall}$ is the location of the interface, |
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and $d$ is the displacement from the interface at which these deviations |
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occur. The values for $d_{pyramidal}$ and $d_{secondary prismatic}$ were |
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determined |
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for each of the decay constants, and then averaged for better statistics |
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($\tau_{middle}$ was ommitted for secondary prism). For the pyramidal system, |
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$d_{pyramidal}$ was found to be 2.7 \AA\ for both the control and the sheared |
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system. We found $d_{secondary prismatic}$ to be slightly larger than |
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$d_{pyramidal}$ for both the control and with an applied shear, with |
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displacements of $4$ \AA\ for the control system and $3$ \AA\ for the |
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experiment with the imposed momentum flux. These values are consistent with |
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those found for the basal ($d_{basal}\approx2.9$ \AA\ ) and prismatic |
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($d_{prismatic}\approx3.5$ \AA\ ) systems. |
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|
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\subsection{Coefficient of friction of the interfaces} |
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While investigating the kinetic coefficient of friction for the larger |
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prismatic system, there was found to be a dependence for $\mu_k$ |
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on the temperature of the liquid water in the system. We believe this |
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dependence |
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arrises from the sharp discontinuity of the viscosity for the SPC/E model |
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at temperatures approaching 200 K\cite{kuang12}. Due to this, we propose |
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a weighting to the structural interfacial parameter, $\kappa$ by the |
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viscosity at $225$ K, the temperature of the interface. $\kappa$ is |
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traditionally defined as |
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\begin{equation}\label{kappa} |
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\kappa = \eta/\delta |
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\end{equation} |
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where $\eta$ is the viscosity and $\delta$ is the slip length. |
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In our ice/water shearing simulations, the system has reached a steady state |
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when the applied force, |
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|
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\begin{equation} |
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f_{applied} = \mathbf{j}_z(\mathbf{p})L_x L_y |
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\end{equation} |
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is equal to the frictional force resisting the motion of the ice block |
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\begin{equation} |
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f_{friction} = \frac{\eta \mathbf{v} L_x L_y}{\delta} |
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\end{equation} |
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where $\mathbf{v}$ is the relative velocity of the liquid from the ice. |
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When this condition is met, we are able to solve the resulting expression to |
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obtain, |
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\begin{equation}\label{force_equality} |
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\frac{\eta}{\delta} = \frac{\mathbf{j}_z(\mathbf{p})}{\mathbf{v}} |
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\end{equation} |
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From \eqref{kappa}, \eqref{force_equality} becomes |
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\begin{equation} |
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\kappa = \frac{\mathbf{j}_z(\mathbf{p})}{\mathbf{v}} |
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\end{equation} |
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which we will multiply by a viscosity weighting term to reach |
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\begin{equation} \label{kappa2} |
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\kappa = \frac{\mathbf{j}_z(\mathbf{p})}{\mathbf{v}} \frac{\eta(225)}{\eta(T)} |
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\end{equation} |
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Assuming linear response theory is valid, an expression for ($\eta$) can |
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be found from the imposed momentum flux and the measured velocity gradient. |
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\begin{equation}\label{eta_eq} |
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\eta = \frac{\mathbf{j}_z(\mathbf{p})}{\frac{\partial v_x}{\partial z}} |
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\end{equation} |
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Substituting eq \eqref{eta_eq} into eq \eqref{kappa2} we arrive at |
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\begin{equation} |
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\kappa = \frac{\frac{\partial v_x}{\partial z}}{\mathbf{v}}\eta(225) |
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\end{equation} |
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|
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To obtain the value of $\eta(225)$ for the SPC/E model, a $31.09 \times 29.38 |
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\times 124.39$ \AA\ box with 3744 water molecules was equilibrated to 225K, |
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and 6 unique shearing experiments were performed. Each experiment was |
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conducted in the microcanonical ensemble (NVE) and were 1 ns in |
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length. The VSS were attempted every timestep, which was set to 2 fs. |
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|
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|
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\begin{table}[h] |
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\centering |
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\caption{Solid-liquid friction coefficients (measured in amu~fs\textsuperscript\ |
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{-1}). \\ |
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\textsuperscript{a} See ref. \onlinecite{Louden13}. } |
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\label{tab:lambda} |
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\begin{tabular}{|ccc|} \hline |
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& \multicolumn{2}{c|}{Drag direction} \\ |
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Interface & $x$ & $y$ \\ \hline |
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basal\textsuperscript{a} & $0.08 \pm 0.02$ & $0.09 \pm 0.03$ \\ |
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prismatic (T = 225)\textsuperscript{a} & $0.037 \pm 0.008$ & $0.04 \pm 0.01$ \\ |
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prismatic (T = 230) & $0.10 \pm 0.01$ & $0.070 \pm 0.006$\\ |
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pyramidal & $0.13 \pm 0.03$ & $0.14 \pm 0.03$ \\ |
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secondary prism & $0.13 \pm 0.02$ & $0.12 \pm 0.03$ \\ \hline |
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\end{tabular} |
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\end{table} |
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|
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{Pyr-orient} |
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\caption{\label{fig:PyrOrient} The three decay constants of the |
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orientational time correlation function, $C_2(t)$, for water as a function |
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of distance from the center of the ice slab. The vertical dashed line |
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indicates the edge of the pyramidal ice slab determined by the local order |
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tetrahedral parameter. The control (black circles) and sheared (red squares) |
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experiments were fit by a shifted exponential decay (Eq. 9\cite{Louden13}) |
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shown by the black and red lines respectively. The upper two panels show that |
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translational and hydrogen bond making and breaking events slow down |
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through the interface while approaching the ice slab. The bottom most panel |
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shows the librational motion of the water molecules speeding up approaching |
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the ice block due to the confined region of space allowed for the molecules |
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to move in.} |
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\end{figure} |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{SP-orient-less} |
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\caption{\label{fig:SPorient} Decay constants for $C_2(t)$ at the secondary |
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prism face. Panel descriptions match those in \ref{fig:PyrOrient}.} |
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\end{figure} |
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|
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|
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|
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\section{Conclusion} |
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Conclude conclude conclude... |
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|
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|
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\begin{acknowledgments} |
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Support for this project was provided by the National |
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Science Foundation under grant CHE-1362211. Computational time was |
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provided by the Center for Research Computing (CRC) at the |
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University of Notre Dame. |
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\end{acknowledgments} |
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|
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\newpage |
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|
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\bibliography{iceWater} |
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|
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\end{document} |
