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\begin{document} |
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\title{The different facets of ice have different hydrophilicities: \\ |
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Friction at water / ice-I$_\mathrm{h}$ interfaces} |
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\author{Patrick B. Louden} |
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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 of |
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Notre Dame, Notre Dame, IN 46556} |
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|
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\date{\today} |
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|
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|
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\begin{abstract} |
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We present evidence that the prismatic and secondary prism facets of |
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ice-I$_\mathrm{h}$ crystals possess structural features that can |
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reduce the effective hydrophilicity of the ice/water interface. The |
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spreading dynamics of liquid water droplets on ice facets exhibits |
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long-time behavior that differs for the prismatic |
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$\{10\bar{1}0\}$ and secondary prism $\{11\bar{2}0\}$ facets |
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when compared with the basal $\{0001\}$ and pyramidal |
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$\{20\bar{2}1\}$ facets. We also present the results of |
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simulations of solid-liquid friction of the same four crystal facets |
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being drawn through liquid water, and find that the two prismatic |
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facets exhibit roughly half the solid-liquid friction of the basal |
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and pyramidal facets. These simulations provide evidence that the |
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two prismatic faces have a significantly smaller effective surface |
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area in contact with the liquid water. The ice / water interfacial |
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widths for all four crystal facets are similar (using both |
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structural and dynamic measures), and were found to be independent |
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of the shear rate. Additionally, decomposition of orientational |
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time correlation functions show position-dependence for the short- |
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and longer-time decay components close to the interface. |
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\end{abstract} |
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|
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\pacs{68.08.Bc, 68.08.De, 66.20.Cy} |
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\keywords{ice; water; interfaces; hydrophobicity} |
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\maketitle |
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|
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\section{Introduction} |
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Surfaces can be characterized as hydrophobic or hydrophilic |
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based on the strength of the interactions with water. Hydrophobic |
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surfaces do not have strong enough interactions with water to overcome |
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the internal attraction between molecules in the liquid phase, and the |
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degree of hydrophilicity of a surface can be described by the extent a |
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droplet can spread out over the surface. The contact angle, $\theta$, |
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formed between the solid and the liquid depends on the free energies |
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of the three interfaces involved, and is given by Young's |
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equation~\cite{Young05}, |
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\begin{equation}\label{young} |
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\cos\theta = (\gamma_{sv} - \gamma_{sl})/\gamma_{lv} . |
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\end{equation} |
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Here $\gamma_{sv}$, $\gamma_{sl}$, and $\gamma_{lv}$ are the free |
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energies of the solid/vapor, solid/liquid, and liquid/vapor interfaces, |
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respectively. Large contact angles, $\theta > 90^{\circ}$, correspond |
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to hydrophobic surfaces with low wettability, while small contact |
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angles, $\theta < 90^{\circ}$, correspond to hydrophilic surfaces. |
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Experimentally, measurements of the contact angle of sessile drops is |
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often used to quantify the extent of wetting on surfaces with |
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thermally selective wetting |
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characteristics~\cite{Tadanaga00,Liu04,Sun04}. |
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|
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Nanometer-scale structural features of a solid surface can influence |
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the hydrophilicity to a surprising degree. Small changes in the |
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heights and widths of nano-pillars can change a surface from |
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superhydrophobic, $\theta \ge 150^{\circ}$, to hydrophilic, $\theta |
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\sim 0^{\circ}$~\cite{Koishi09}. This is often referred to as the |
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Cassie-Baxter to Wenzel transition. Nano-pillared surfaces with |
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electrically tunable Cassie-Baxter and Wenzel states have also been |
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observed~\cite{Herbertson06,Dhindsa06,Verplanck07,Ahuja08,Manukyan11}. |
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Luzar and coworkers have modeled these transitions on nano-patterned |
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surfaces~\cite{Daub07,Daub10,Daub11,Ritchie12}, and have found the |
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change in contact angle is due to the field-induced perturbation of |
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hydrogen bonding at the liquid/vapor interface~\cite{Daub07}. |
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|
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One would expect the interfaces of ice to be highly hydrophilic (and |
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possibly the most hydrophilic of all solid surfaces). In this paper we |
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present evidence that some of the crystal facets of ice-I$_\mathrm{h}$ |
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have structural features that can reduce the effective hydrophilicity. |
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Our evidence for this comes from molecular dynamics (MD) simulations |
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of the spreading dynamics of liquid droplets on these facets, as well |
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as reverse non-equilibrium molecular dynamics (RNEMD) simulations of |
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solid-liquid friction. |
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|
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Quiescent ice-I$_\mathrm{h}$/water interfaces have been studied |
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extensively using computer simulations. Hayward and Haymet |
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characterized and measured the widths of these |
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interfaces~\cite{Hayward01,Hayward02}. Nada and Furukawa have also |
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modeled the width of basal/water and prismatic/water |
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interfaces~\cite{Nada95} as well as crystal restructuring at |
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temperatures approaching the melting point~\cite{Nada00}. |
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|
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The surface of ice exhibits a pre-melting layer, often called a |
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quasi-liquid layer (QLL), at temperatures near the melting point. MD |
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simulations of the facets of ice-I$_\mathrm{h}$ exposed to vacuum have |
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found QLL widths of approximately 10 \AA\ at 3 K below the melting |
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point~\cite{Conde08}. Similarly, Limmer and Chandler have used the mW |
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water model~\cite{Molinero09} and statistical field theory to estimate |
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QLL widths at similar temperatures to be about 3 nm~\cite{Limmer14}. |
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|
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Recently, Sazaki and Furukawa have developed a technique using laser |
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confocal microscopy combined with differential interference contrast |
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microscopy that has sufficient spatial and temporal resolution to |
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visualize and quantitatively analyze QLLs on ice crystals at |
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temperatures near melting~\cite{Sazaki10}. They have found the width of |
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the QLLs perpendicular to the surface at -2.2$^{o}$C to be 3-4 \AA\ |
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wide. They have also seen the formation of two immiscible QLLs, which |
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displayed different dynamics on the crystal surface~\cite{Sazaki12}. |
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|
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% There is now significant interest in the \textit{tribological} |
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% properties of ice/ice and ice/water interfaces in the geophysics |
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% community. Understanding the dynamics of solid-solid shearing that is |
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% mediated by a liquid layer~\cite{Cuffey99, Bell08} will aid in |
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% understanding the macroscopic motion of large ice |
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% masses~\cite{Casassa91, Sukhorukov13, Pritchard12, Lishman13}. |
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|
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Using molecular dynamics simulations, Samadashvili has recently shown |
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that when two smooth ice slabs slide past one another, a stable |
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liquid-like layer develops between them~\cite{Samadashvili13}. In a |
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previous study, our RNEMD simulations of ice-I$_\mathrm{h}$ shearing |
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through liquid water have provided quantitative estimates of the |
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solid-liquid kinetic friction coefficients~\cite{Louden13}. These |
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displayed a factor of two difference between the basal and prismatic |
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facets. The friction was found to be independent of shear direction |
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relative to the surface orientation. We attributed facet-based |
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difference in liquid-solid friction to the 6.5 \AA\ corrugation of the |
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prismatic face which reduces the effective surface area of the ice |
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that is in direct contact with liquid water. |
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|
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In the sections that follow, we describe the simulations of |
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droplet-spreading dynamics using standard MD as well as simulations of |
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tribological properties using RNEMD. These simulations give |
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complementary results that point to the prismatic and secondary prism |
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facets having roughly half of their surface area in direct contact |
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with the liquid. |
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|
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\section{Methodology} |
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\subsection{Construction of the Ice / Water interfaces} |
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Ice I$_\mathrm{h}$ crystallizes in the hexagonal space group |
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P$6_3/mmc$, and common ice crystals form hexagonal plates with the |
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basal face, $\{0001\}$, forming the top and bottom of each plate, and |
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the prismatic facet, $\{10\bar{1}0\}$, forming the sides. In extreme |
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temperatures or low water saturation conditions, ice crystals can |
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easily form as hollow columns, needles and dendrites. These are |
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structures that expose other crystalline facets of the ice to the |
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surroundings. Among the more common facets are the secondary prism, |
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$\{11\bar{2}0\}$, and pyramidal, $\{20\bar{2}1\}$, faces. |
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|
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We found it most useful to work with proton-ordered, zero-dipole |
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crystals that expose strips of dangling H-atoms and lone |
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pairs~\cite{Buch:2008fk}. Our structures were created starting from |
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Structure 6 of Hirsch and Ojam\"{a}e's set of orthorhombic |
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representations for ice-I$_{h}$~\cite{Hirsch04}. This crystal |
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structure was cleaved along the four different facets. The exposed |
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face was reoriented normal to the $z$-axis of the simulation cell, and |
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the structures were extended to form large exposed facets in |
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rectangular box geometries. Liquid water boxes were created with |
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identical dimensions (in $x$ and $y$) as the ice, with a $z$ dimension |
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of three times that of the ice block, and a density corresponding to 1 |
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g / cm$^3$. Each of the ice slabs and water boxes were independently |
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equilibrated at a pressure of 1 atm, and the resulting systems were |
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merged by carving out any liquid water molecules within 3 \AA\ of any |
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atoms in the ice slabs. Each of the combined ice/water systems were |
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then equilibrated at 225K, which is the liquid-ice coexistence |
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temperature for SPC/E water~\cite{Bryk02}. Reference |
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\citealp{Louden13} contains a more detailed explanation of the |
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construction of similar ice/water interfaces. The resulting dimensions |
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as well as the number of ice and liquid water molecules contained in |
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each of these systems are shown in Table \ref{tab:method}. |
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|
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The SPC/E water model~\cite{Berendsen87} has been extensively |
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characterized over a wide range of liquid |
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conditions~\cite{Arbuckle02,Kuang12}, and its phase diagram has been |
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well studied~\cite{Baez95,Bryk04b,Sanz04b,Fennell:2005fk}. With longer |
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cutoff radii and careful treatment of electrostatics, SPC/E mostly |
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avoids metastable crystalline morphologies like |
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ice-\textit{i}~\cite{Fennell:2005fk} and ice-B~\cite{Baez95}. The |
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free energies and melting |
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points~\cite{Baez95,Arbuckle02,Gay02,Bryk02,Bryk04b,Sanz04b,Fennell:2005fk,Fernandez06,Abascal07,Vrbka07} |
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of various other crystalline polymorphs have also been calculated. |
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Haymet \textit{et al.} have studied quiescent Ice-I$_\mathrm{h}$/water |
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interfaces using the SPC/E water model, and have seen structural and |
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dynamic measurements of the interfacial width that agree well with |
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more expensive water models, although the coexistence temperature for |
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SPC/E is still well below the experimental melting point of real |
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water~\cite{Bryk02}. Given the extensive data and speed of this model, |
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it is a reasonable choice even though the temperatures required are |
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somewhat lower than real ice / water interfaces. |
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|
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\section{Droplet Simulations} |
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Ice surfaces with a thickness of $\sim$~20~\AA\ were created as |
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described above, but were not solvated in a liquid box. The crystals |
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were then replicated along the $x$ and $y$ axes (parallel to the |
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surface) until a large surface ($>$ 126 nm\textsuperscript{2}) had |
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been created. The sizes and numbers of molecules in each of the |
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surfaces is given in Table S1. Weak translational restraining |
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potentials with spring constants of 1.5~$\mathrm{kcal\ |
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mol}^{-1}\mathrm{~\AA}^{-2}$ (prismatic and pyramidal facets) or |
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4.0~$\mathrm{kcal\ mol}^{-1}\mathrm{~\AA}^{-2}$ (basal facet) were |
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applied to the centers of mass of each molecule in order to prevent |
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surface melting, although the molecules were allowed to reorient |
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freely. A water droplet containing 2048 SPC/E molecules was created |
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separately. Droplets of this size can produce agreement with the Young |
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contact angle extrapolated to an infinite drop size~\cite{Daub10}. The |
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surfaces and droplet were independently equilibrated to 225~K, at |
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which time the droplet was placed 3-5~\AA\ above the surface. Five |
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statistically independent simulations were carried out for each facet, |
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and the droplet was placed at unique $x$ and $y$ locations for each of |
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these simulations. Each simulation was 5~ns in length and was |
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conducted in the microcanonical (NVE) ensemble. Representative |
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configurations for the droplet on the prismatic facet are shown in |
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figure \ref{fig:Droplet}. |
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|
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\section{Shearing Simulations (Interfaces in Bulk Water)} |
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To perform the shearing simulations, the velocity shearing and scaling |
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variant of reverse non-equilibrium molecular dynamics (VSS-RNEMD) was |
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employed \cite{Kuang12}. This method performs a series of simultaneous |
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non-equilibrium exchanges of linear momentum and kinetic energy |
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between two physically-separated regions of the simulation cell. The |
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system responds to this unphysical flux with velocity and temperature |
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gradients. When VSS-RNEMD is applied to bulk liquids, transport |
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properties like the thermal conductivity and the shear viscosity are |
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easily extracted assuming a linear response between the flux and the |
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gradient. At the interfaces between dissimilar materials, the same |
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method can be used to extract \textit{interfacial} transport |
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properties (e.g. the interfacial thermal conductance and the |
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hydrodynamic slip length). |
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|
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The kinetic energy flux (producing a thermal gradient) is necessary |
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when performing shearing simulations at the ice-water interface in |
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order to prevent the frictional heating due to the shear from melting |
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the crystal. Reference \citealp{Louden13} provides more details on the |
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VSS-RNEMD method as applied to ice-water interfaces. A representative |
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configuration of the solvated prismatic facet being sheared through |
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liquid water is shown in figure \ref{fig:Shearing}. |
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|
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All simulations were performed using OpenMD~\cite{OOPSE,openmd}, with |
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a time step of 2 fs and periodic boundary conditions in all three |
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dimensions. Electrostatics were handled using the damped-shifted |
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force real-space electrostatic kernel~\cite{Ewald}. |
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|
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The interfaces were equilibrated for a total of 10 ns at equilibrium |
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conditions before being exposed to either a shear or thermal gradient. |
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This consisted of 5 ns under a constant temperature (NVT) integrator |
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set to 225~K followed by 5 ns under a microcanonical (NVE) integrator. |
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Weak thermal gradients were allowed to develop using the VSS-RNEMD |
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(NVE) integrator using a small thermal flux ($-2.0\times 10^{-6}$ |
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kcal/mol/\AA$^2$/fs) for a duration of 5 ns to allow the gradient to |
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stabilize. The resulting temperature gradient was $\approx$ 10K over |
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the entire box length, which was sufficient to keep the temperature at |
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the interface within $\pm 1$ K of the 225~K target. |
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|
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Velocity gradients were then imposed using the VSS-RNEMD (NVE) |
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integrator with a range of momentum fluxes. The systems were divided |
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into 100 bins along the $z$-axis for the VSS-RNEMD moves, which were |
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attempted every time step. Although computationally expensive, this |
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was done to minimize the magnitude of each individual momentum |
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exchange. Because individual VSS-RNEMD exchange moves conserve both |
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total energy and linear momentum, the method can be ``bolted-on'' to |
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simulations in any ensemble. The simulations of the pyramidal |
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interface were performed under the canonical (NVT) ensemble. When |
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time correlation functions were computed, the RNEMD simulations were |
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done in the microcanonical (NVE) ensemble. All simulations of the |
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other interfaces were carried out in the microcanonical ensemble. |
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|
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These gradients were allowed to stabilize for 1~ns before data |
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collection started. Once established, four successive 0.5~ns runs were |
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performed for each shear rate. During these simulations, |
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configurations of the system were stored every 1~ps, and statistics on |
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the structure and dynamics in each bin were accumulated throughout the |
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simulations. Although there was some small variation in the measured |
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interfacial width between succcessive runs, no indication of bulk |
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melting or crystallization was observed. That is, no large scale |
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changes in the positions of the top and bottom interfaces occurred |
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during the simulations. |
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|
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\section{Results} |
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\subsection{Ice - Water Contact Angles} |
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To determine the extent of wetting for each of the four crystal |
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facets, contact angles for liquid droplets on the ice surfaces were |
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computed using two methods. In the first method, the droplet is |
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assumed to form a spherical cap, and the contact angle is estimated |
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from the $z$-axis location of the droplet's center of mass |
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($z_\mathrm{cm}$). This procedure was first described by Hautman and |
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Klein~\cite{Hautman91}, and was utilized by Hirvi and Pakkanen in |
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their investigation of water droplets on polyethylene and poly(vinyl |
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chloride) surfaces~\cite{Hirvi06}. For each stored configuration, the |
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contact angle, $\theta$, was found by inverting the expression for the |
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location of the droplet center of mass, |
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\begin{equation}\label{contact_1} |
328 |
\langle z_\mathrm{cm}\rangle = 2^{-4/3}R_{0}\bigg(\frac{1-cos\theta}{2+cos\theta}\bigg)^{1/3}\frac{3+cos\theta}{2+cos\theta} , |
329 |
\end{equation} |
330 |
where $R_{0}$ is the radius of the free water droplet. |
331 |
|
332 |
In addition to the spherical cap method outlined above, a second |
333 |
method for obtaining the contact angle was described by Ruijter, |
334 |
Blake, and Coninck~\cite{Ruijter99}. This method uses a cylindrical |
335 |
averaging of the droplet's density profile. A threshold density of |
336 |
0.5 g cm\textsuperscript{-3} is used to estimate the location of the |
337 |
edge of the droplet. The $r$ and $z$-dependence of the droplet's edge |
338 |
is then fit to a circle, and the contact angle is computed from the |
339 |
intersection of the fit circle with the $z$-axis location of the solid |
340 |
surface. Again, for each stored configuration, the density profile in |
341 |
a set of annular shells was computed. Due to large density |
342 |
fluctuations close to the ice, all shells located within 2 \AA\ of the |
343 |
ice surface were left out of the circular fits. The height of the |
344 |
solid surface ($z_\mathrm{suface}$) along with the best fitting origin |
345 |
($z_\mathrm{droplet}$) and radius ($r_\mathrm{droplet}$) of the |
346 |
droplet can then be used to compute the contact angle, |
347 |
\begin{equation} |
348 |
\theta = 90 + \frac{180}{\pi} \sin^{-1}\left(\frac{z_\mathrm{droplet} - |
349 |
z_\mathrm{surface}}{r_\mathrm{droplet}} \right). |
350 |
\end{equation} |
351 |
Both methods provided similar estimates of the dynamic contact angle, |
352 |
although the spherical cap method is significantly less prone to |
353 |
noise, and is the method used to compute the contact angles in table |
354 |
\ref{tab:kappa}. |
355 |
|
356 |
Because the initial droplet was placed above the surface, the initial |
357 |
value of 180$^{\circ}$ decayed over time (See figure 1 in the |
358 |
SI). Each of these profiles were fit to a |
359 |
biexponential decay, with a short-time contribution ($\tau_c$) that |
360 |
describes the initial contact with the surface, a long time |
361 |
contribution ($\tau_s$) that describes the spread of the droplet over |
362 |
the surface, and a constant ($\theta_\infty$) to capture the |
363 |
infinite-time estimate of the equilibrium contact angle, |
364 |
\begin{equation} |
365 |
\theta(t) = \theta_\infty + (180-\theta_\infty) \left[ a e^{-t/\tau_c} + |
366 |
(1-a) e^{-t/\tau_s} \right] |
367 |
\end{equation} |
368 |
We have found that the rate for water droplet spreading across all |
369 |
four crystal facets, $k_\mathrm{spread} = 1/\tau_s \approx$ 0.7 |
370 |
ns$^{-1}$. However, the basal and pyramidal facets produced estimated |
371 |
equilibrium contact angles, $\theta_\infty \approx$ 35$^{\circ}$, while |
372 |
prismatic and secondary prismatic had values for $\theta_\infty$ near |
373 |
43$^{\circ}$ as seen in Table \ref{tab:kappa}. |
374 |
|
375 |
These results indicate that by traditional measures, the basal and |
376 |
pyramidal facets are more hydrophilic than the prismatic and secondary |
377 |
prism facets, and surprisingly, that the differential hydrophilicities |
378 |
of the crystal facets is not reflected in the spreading rate of the |
379 |
droplet. |
380 |
|
381 |
% This is in good agreement with our calculations of friction |
382 |
% coefficients, in which the basal |
383 |
% and pyramidal had a higher coefficient of kinetic friction than the |
384 |
% prismatic and secondary prismatic. Due to this, we believe that the |
385 |
% differences in friction coefficients can be attributed to the varying |
386 |
% hydrophilicities of the facets. |
387 |
|
388 |
\subsection{Solid-liquid friction of the interfaces} |
389 |
In a bulk fluid, the shear viscosity, $\eta$, can be determined |
390 |
assuming a linear response to a shear stress, |
391 |
\begin{equation}\label{Shenyu-11} |
392 |
j_{z}(p_{x}) = \eta \frac{\partial v_{x}}{\partial z}. |
393 |
\end{equation} |
394 |
Here $j_{z}(p_{x})$ is the flux (in $x$-momentum) that is transferred |
395 |
in the $z$ direction (i.e. the shear stress). The RNEMD simulations |
396 |
impose an artificial momentum flux between two regions of the |
397 |
simulation, and the velocity gradient is the fluid's response. This |
398 |
technique has now been applied quite widely to determine the |
399 |
viscosities of a number of bulk fluids~\cite{Muller99,Bordat02,Cavalcanti07}. |
400 |
|
401 |
At the interface between two phases (e.g. liquid / solid) the same |
402 |
momentum flux creates a velocity difference between the two materials, |
403 |
and this can be used to define an interfacial friction coefficient |
404 |
($\kappa$), |
405 |
\begin{equation}\label{Shenyu-13} |
406 |
j_{z}(p_{x}) = \kappa \left[ v_{x}(liquid) - v_{x}(solid) \right] |
407 |
\end{equation} |
408 |
where $v_{x}(solid)$ is the velocity of the solid and $v_{x}(liquid)$ |
409 |
is the velocity of the liquid measured at the hydrodynamic boundary |
410 |
layer. |
411 |
|
412 |
The simulations described here contain significant quantities of both |
413 |
liquid and solid phases, and the momentum flux must traverse a region |
414 |
of the liquid that is simultaneously under a thermal gradient. Since |
415 |
the liquid has a temperature-dependent shear viscosity, $\eta(T)$, |
416 |
estimates of the solid-liquid friction coefficient can be obtained if |
417 |
one knows the viscosity of the liquid at the interface (i.e. at the |
418 |
melting temperature, $T_m$), |
419 |
\begin{equation}\label{kappa-2} |
420 |
\kappa = \frac{\eta(T_{m})}{\left[v_{x}(fluid)-v_{x}(solid)\right]}\left(\frac{\partial v_{x}}{\partial z}\right). |
421 |
\end{equation} |
422 |
For SPC/E, the melting temperature of Ice-I$_\mathrm{h}$ is estimated |
423 |
to be 225~K~\cite{Bryk02}. To obtain the value of |
424 |
$\eta(225\mathrm{~K})$ for the SPC/E model, a $31.09 \times 29.38 |
425 |
\times 124.39$ \AA\ box with 3744 water molecules in a disordered |
426 |
configuration was equilibrated to 225~K, and five |
427 |
statistically-independent shearing simulations were performed (with |
428 |
imposed fluxes that spanned a range of $3 \rightarrow 13 |
429 |
\mathrm{~m~s}^{-1}$ ). Each simulation was conducted in the |
430 |
microcanonical ensemble with total simulation times of 5 ns. The |
431 |
VSS-RNEMD exchanges were carried out every 2 fs. We estimate |
432 |
$\eta(225\mathrm{~K})$ to be 0.0148 $\pm$ 0.0007 Pa s for SPC/E, |
433 |
roughly ten times larger than the shear viscosity previously computed |
434 |
at 280~K~\cite{Kuang12}. |
435 |
|
436 |
The interfacial friction coefficient can equivalently be expressed as |
437 |
the ratio of the viscosity of the fluid to the hydrodynamic slip |
438 |
length, $\kappa = \eta / \delta$. The slip length is an indication of |
439 |
strength of the interactions between the solid and liquid phases, |
440 |
although the connection between slip length and surface hydrophobicity |
441 |
is not yet clear. In some simulations, the slip length has been found |
442 |
to have a link to the effective surface |
443 |
hydrophobicity~\cite{Sendner:2009uq}, although Ho \textit{et al.} have |
444 |
found that liquid water can also slip on hydrophilic |
445 |
surfaces~\cite{Ho:2011zr}. Experimental evidence for a direct tie |
446 |
between slip length and hydrophobicity is also not |
447 |
definitive. Total-internal reflection particle image velocimetry |
448 |
(TIR-PIV) studies have suggested that there is a link between slip |
449 |
length and effective |
450 |
hydrophobicity~\cite{Lasne:2008vn,Bouzigues:2008ys}. However, recent |
451 |
surface sensitive cross-correlation spectroscopy (TIR-FCCS) |
452 |
measurements have seen similar slip behavior for both hydrophobic and |
453 |
hydrophilic surfaces~\cite{Schaeffel:2013kx}. |
454 |
|
455 |
In each of the systems studied here, the interfacial temperature was |
456 |
kept fixed to 225~K, which ensured the viscosity of the fluid at the |
457 |
interace was identical. Thus, any significant variation in $\kappa$ |
458 |
between the systems is a direct indicator of the slip length and the |
459 |
effective interaction strength between the solid and liquid phases. |
460 |
|
461 |
The calculated $\kappa$ values found for the four crystal facets of |
462 |
Ice-I$_\mathrm{h}$ are shown in Table \ref{tab:kappa}. The basal and |
463 |
pyramidal facets were found to have similar values of $\kappa \approx |
464 |
6$ ($\times 10^{-4} \mathrm{amu~\AA}^{-2}\mathrm{fs}^{-1}$), while the |
465 |
prismatic and secondary prism facets exhibited $\kappa \approx 3$ |
466 |
($\times 10^{-4} \mathrm{amu~\AA}^{-2}\mathrm{fs}^{-1}$). These |
467 |
results are also essentially independent of the direction of the shear |
468 |
relative to channels on the surfaces of the facets. The friction |
469 |
coefficients indicate that the basal and pyramidal facets have |
470 |
significantly stronger interactions with liquid water than either of |
471 |
the two prismatic facets. This is in agreement with the contact angle |
472 |
results above - both of the high-friction facets exhibited smaller |
473 |
contact angles, suggesting that the solid-liquid friction (and inverse |
474 |
slip length) is correlated with the hydrophilicity of these facets. |
475 |
|
476 |
\subsection{Structural measures of interfacial width under shear} |
477 |
One of the open questions about ice/water interfaces is whether the |
478 |
thickness of the 'slush-like' quasi-liquid layer (QLL) depends on the |
479 |
facet of ice presented to the water. In the QLL region, the water |
480 |
molecules are ordered differently than in either the solid or liquid |
481 |
phases, and also exhibit distinct dynamical behavior. The width of |
482 |
this quasi-liquid layer has been estimated by finding the distance |
483 |
over which structural order parameters or dynamic properties change |
484 |
from their bulk liquid values to those of the solid ice. The |
485 |
properties used to find interfacial widths have included the local |
486 |
density, the diffusion constant, and both translational and |
487 |
orientational order |
488 |
parameters~\cite{Karim88,Karim90,Hayward01,Hayward02,Bryk02,Gay02,Louden13}. |
489 |
|
490 |
The VSS-RNEMD simulations impose thermal and velocity gradients. |
491 |
These gradients perturb the momenta of the water molecules, so |
492 |
parameters that depend on translational motion are often measuring the |
493 |
momentum exchange, and not physical properties of the interface. As a |
494 |
structural measure of the interface, we have used the local |
495 |
tetrahedral order parameter, which measures the match of the local |
496 |
molecular environments (e.g. the angles between nearest neighbor |
497 |
molecules) to perfect tetrahedral ordering. This quantity was |
498 |
originally described by Errington and Debenedetti~\cite{Errington01} |
499 |
and has been used in bulk simulations by Kumar \textit{et |
500 |
al.}~\cite{Kumar09} It has previously been used in ice/water |
501 |
interfaces by by Bryk and Haymet~\cite{Bryk04b}. |
502 |
|
503 |
To determine the structural widths of the interfaces under shear, each |
504 |
of the systems was divided into 100 bins along the $z$-dimension, and |
505 |
the local tetrahedral order parameter (Eq. 5 in Reference |
506 |
\citealp{Louden13}) was time-averaged in each bin for the duration of |
507 |
the shearing simulation. The spatial dependence of this order |
508 |
parameter, $q(z)$, is the tetrahedrality profile of the interface. |
509 |
The lower panels in figures 2-5 in the supporting information show |
510 |
tetrahedrality profiles (in circles) for each of the four interfaces. |
511 |
The $q(z)$ function has a range of $(0,1)$, where a value of unity |
512 |
indicates a perfectly tetrahedral environment. The $q(z)$ for the |
513 |
bulk liquid was found to be $\approx~0.77$, while values of |
514 |
$\approx~0.92$ were more common in the ice. The tetrahedrality |
515 |
profiles were fit using a hyperbolic tangent function (see Eq. 6 in |
516 |
Reference \citealp{Louden13}) designed to smoothly fit the bulk to ice |
517 |
transition while accounting for the weak thermal gradient. In panels |
518 |
$b$ and $c$ of the same figures, the resulting thermal and velocity |
519 |
gradients from an imposed kinetic energy and momentum fluxes can be |
520 |
seen. The vertical dotted lines traversing these figures indicate the |
521 |
midpoints of the interfaces as determined by the tetrahedrality |
522 |
profiles. The hyperbolic tangent fit provides an estimate of |
523 |
$d_\mathrm{struct}$, the structural width of the interface. |
524 |
|
525 |
We find the interfacial width to be $3.2 \pm 0.2$ \AA\ (pyramidal) and |
526 |
$3.2 \pm 0.2$ \AA\ (secondary prism) for the control systems with no |
527 |
applied momentum flux. This is similar to our previous results for the |
528 |
interfacial widths of the quiescent basal ($3.2 \pm 0.4$ \AA) and |
529 |
prismatic systems ($3.6 \pm 0.2$ \AA). |
530 |
|
531 |
Over the range of shear rates investigated, $0.4 \rightarrow |
532 |
6.0~\mathrm{~m~s}^{-1}$ for the pyramidal system and $0.6 \rightarrow |
533 |
5.2~\mathrm{~m~s}^{-1}$ for the secondary prism, we found no |
534 |
significant change in the interfacial width. The mean interfacial |
535 |
widths are collected in table \ref{tab:kappa}. This follows our |
536 |
previous findings of the basal and prismatic systems, in which the |
537 |
interfacial widths of the basal and prismatic facets were also found |
538 |
to be insensitive to the shear rate~\cite{Louden13}. |
539 |
|
540 |
The similarity of these interfacial width estimates indicate that the |
541 |
particular facet of the exposed ice crystal has little to no effect on |
542 |
how far into the bulk the ice-like structural ordering persists. Also, |
543 |
it appears that for the shearing rates imposed in this study, the |
544 |
interfacial width is not structurally modified by the movement of |
545 |
water over the ice. |
546 |
|
547 |
\subsection{Dynamic measures of interfacial width under shear} |
548 |
The spatially-resolved orientational time correlation function, |
549 |
\begin{equation}\label{C(t)1} |
550 |
C_{2}(z,t)=\langle P_{2}(\mathbf{u}_i(0)\cdot \mathbf{u}_i(t)) |
551 |
\delta(z_i(0) - z) \rangle, |
552 |
\end{equation} |
553 |
provides local information about the decorrelation of molecular |
554 |
orientations in time. Here, $P_{2}$ is the second-order Legendre |
555 |
polynomial, and $\mathbf{u}_i$ is the molecular vector that bisects |
556 |
the HOH angle of molecule $i$. The angle brackets indicate an average |
557 |
over all the water molecules and time origins, and the delta function |
558 |
restricts the average to specific regions. In the crystal, decay of |
559 |
$C_2(z,t)$ is incomplete, while in the liquid, correlation times are |
560 |
typically measured in ps. Observing the spatial-transition between |
561 |
the decay regimes can define a dynamic measure of the interfacial |
562 |
width. |
563 |
|
564 |
To determine the dynamic widths of the interfaces under shear, each of |
565 |
the systems was divided into bins along the $z$-dimension ($\approx$ 3 |
566 |
\AA\ wide) and $C_2(z,t)$ was computed using only those molecules that |
567 |
were in the bin at the initial time. To compute these correlation |
568 |
functions, each of the 0.5 ns simulations was followed by a shorter |
569 |
200 ps microcanonical (NVE) simulation in which the positions and |
570 |
orientations of every molecule in the system were recorded every 0.1 |
571 |
ps. |
572 |
|
573 |
The time-dependence was fit to a triexponential decay, with three time |
574 |
constants: $\tau_{short}$, measuring the librational motion of the |
575 |
water molecules, $\tau_{middle}$, measuring the timescale for breaking |
576 |
and making of hydrogen bonds, and $\tau_{long}$, corresponding to the |
577 |
translational motion of the water molecules. An additional constant |
578 |
was introduced in the fits to describe molecules in the crystal which |
579 |
do not experience long-time orientational decay. |
580 |
|
581 |
In Figures 6-9 in the supporting information, the $z$-coordinate |
582 |
profiles for the three decay constants, $\tau_{short}$, |
583 |
$\tau_{middle}$, and $\tau_{long}$ for the different interfaces are |
584 |
shown. (Figures 6 \& 7 are new results, and Figures 8 \& 9 are |
585 |
updated plots from Ref \citealp{Louden13}.) In the liquid regions of |
586 |
all four interfaces, we observe $\tau_{middle}$ and $\tau_{long}$ to |
587 |
have approximately consistent values of $3-6$ ps and $30-40$ ps, |
588 |
respectively. Both of these times increase in value approaching the |
589 |
interface. Approaching the interface, we also observe that |
590 |
$\tau_{short}$ decreases from its liquid-state value of $72-76$ fs. |
591 |
The approximate values for the decay constants and the trends |
592 |
approaching the interface match those reported previously for the |
593 |
basal and prismatic interfaces. |
594 |
|
595 |
We have estimated the dynamic interfacial width $d_\mathrm{dyn}$ by |
596 |
fitting the profiles of all the three orientational time constants |
597 |
with an exponential decay to the bulk-liquid behavior, |
598 |
\begin{equation}\label{tauFit} |
599 |
\tau(z)\approx\tau_{liquid}+(\tau_{wall}-\tau_{liquid})e^{-(z-z_{wall})/d_\mathrm{dyn}} |
600 |
\end{equation} |
601 |
where $\tau_{liquid}$ and $\tau_{wall}$ are the liquid and projected |
602 |
wall values of the decay constants, $z_{wall}$ is the location of the |
603 |
interface, as measured by the structural order parameter. These |
604 |
values are shown in table \ref{tab:kappa}. Because the bins must be |
605 |
quite wide to obtain reasonable profiles of $C_2(z,t)$, the error |
606 |
estimates for the dynamic widths of the interface are significantly |
607 |
larger than for the structural widths. However, all four interfaces |
608 |
exhibit dynamic widths that are significantly below 1~nm, and are in |
609 |
reasonable agreement with the structural width above. |
610 |
|
611 |
\section{Conclusions} |
612 |
In this work, we used MD simulations to measure the advancing contact |
613 |
angles of water droplets on the basal, prismatic, pyramidal, and |
614 |
secondary prism facets of Ice-I$_\mathrm{h}$. Although we saw no |
615 |
significant change in the \textit{rate} at which the droplets spread |
616 |
over the surface, the long-time behavior predicts larger equilibrium |
617 |
contact angles for the two prismatic facets. |
618 |
|
619 |
We have also used RNEMD simulations of water interfaces with the same |
620 |
four crystal facets to compute solid-liquid friction coefficients. We |
621 |
have observed coefficients of friction that differ by a factor of two |
622 |
between the two prismatic facets and the basal and pyramidal facets. |
623 |
Because the solid-liquid friction coefficient is directly tied to the |
624 |
inverse of the hydrodynamic slip length, this suggests that there are |
625 |
significant differences in the overall interaction strengths between |
626 |
these facets and the liquid layers immediately in contact with them. |
627 |
|
628 |
The agreement between these two measures have lead us to conclude that |
629 |
the two prismatic facets have a lower hydrophilicity than either the |
630 |
basal or pyramidal facets. One possible explanation of this behavior |
631 |
is that the face presented by both prismatic facets consists of deep, |
632 |
narrow channels (i.e. stripes of adjacent rows of pairs of |
633 |
hydrogen-bound water molecules). At the surfaces of these facets, |
634 |
the channels are 6.35 \AA\ wide and the sub-surface ice layer is 2.25 |
635 |
\AA\ below (and therefore blocked from hydrogen bonding with the |
636 |
liquid). This means that only 1/2 of the surface molecules can form |
637 |
hydrogen bonds with liquid-phase molecules. |
638 |
|
639 |
In the basal plane, the surface features are shallower (1.3 \AA), with |
640 |
no blocked subsurface layer. The pyramidal face has much wider |
641 |
channels (8.65 \AA) which are also quite shallow (1.37 \AA). These |
642 |
features allow liquid phase molecules to form hydrogen bonds with all |
643 |
of the surface molecules in the basal and pyramidal facets. This |
644 |
means that for similar surface areas, the two prismatic facets have an |
645 |
effective hydrogen bonding surface area of half of the basal and |
646 |
pyramidal facets. The reduction in the effective surface area would |
647 |
explain much of the behavior observed in our simulations. |
648 |
|
649 |
\begin{acknowledgments} |
650 |
Support for this project was provided by the National |
651 |
Science Foundation under grant CHE-1362211. Computational time was |
652 |
provided by the Center for Research Computing (CRC) at the |
653 |
University of Notre Dame. |
654 |
\end{acknowledgments} |
655 |
\bibliographystyle{aip} |
656 |
\newpage |
657 |
\bibliography{iceWater} |
658 |
% ***************************************** |
659 |
% There is significant interest in the properties of ice/ice and ice/water |
660 |
% interfaces in the geophysics community. Most commonly, the results of shearing |
661 |
% two ice blocks past one |
662 |
% another\cite{Casassa91, Sukhorukov13, Pritchard12, Lishman13} or the shearing |
663 |
% of ice through water\cite{Cuffey99, Bell08}. Using molecular dynamics |
664 |
% simulations, Samadashvili has recently shown that when two smooth ice slabs |
665 |
% slide past one another, a stable liquid-like layer develops between |
666 |
% them\cite{Samadashvili13}. To fundamentally understand these processes, a |
667 |
% molecular understanding of the ice/water interfaces is needed. |
668 |
|
669 |
% Investigation of the ice/water interface is also crucial in understanding |
670 |
% processes such as nucleation, crystal |
671 |
% growth,\cite{Han92, Granasy95, Vanfleet95} and crystal |
672 |
% melting\cite{Weber83, Han92, Sakai96, Sakai96B}. Insight gained to these |
673 |
% properties can also be applied to biological systems of interest, such as |
674 |
% the behavior of the antifreeze protein found in winter |
675 |
% flounder,\cite{Wierzbicki07,Chapsky97} and certain terrestial |
676 |
% arthropods.\cite{Duman:2001qy,Meister29012013} Elucidating the properties which |
677 |
% give rise to these processes through experimental techniques can be expensive, |
678 |
% complicated, and sometimes infeasible. However, through the use of molecular |
679 |
% dynamics simulations much of the problems of investigating these properties |
680 |
% are alleviated. |
681 |
|
682 |
% Understanding ice/water interfaces inherently begins with the isolated |
683 |
% systems. There has been extensive work parameterizing models for liquid water, |
684 |
% such as the SPC\cite{Berendsen81}, SPC/E\cite{Berendsen87}, |
685 |
% TIP4P\cite{Jorgensen85}, TIP4P/2005\cite{Abascal05}, |
686 |
% ($\dots$), and more recently, models for simulating |
687 |
% the solid phases of water, such as the TIP4P/Ice\cite{Abascal05b} model. The |
688 |
% melting point of various crystal structures of ice have been calculated for |
689 |
% many of these models |
690 |
% (SPC\cite{Karim90,Abascal07}, SPC/E\cite{Baez95,Arbuckle02,Gay02,Bryk02,Bryk04b,Sanz04b,Gernandez06,Abascal07,Vrbka07}, TIP4P\cite{Karim88,Gao00,Sanz04,Sanz04b,Koyama04,Wang05,Fernandez06,Abascal07}, TIP5P\cite{Sanz04,Koyama04,Wang05,Fernandez06,Abascal07}), |
691 |
% and the partial or complete phase diagram for the model has been determined |
692 |
% (SPC/E\cite{Baez95,Bryk04b,Sanz04b}, TIP4P\cite{Sanz04,Sanz04b,Koyama04}, TIP5P\cite{Sanz04,Koyama04}). |
693 |
% Knowing the behavior and melting point for these models has enabled an initial |
694 |
% investigation of ice/water interfaces. |
695 |
|
696 |
% The Ice-I$_\mathrm{h}$/water quiescent interface has been extensively studied |
697 |
% over the past 30 years by theory and experiment. Haymet \emph{et al.} have |
698 |
% done significant work characterizing and quantifying the width of these |
699 |
% interfaces for the SPC,\cite{Karim90} SPC/E,\cite{Gay02,Bryk02}, |
700 |
% CF1,\cite{Hayward01,Hayward02} and TIP4P\cite{Karim88} models for water. In |
701 |
% recent years, Haymet has focused on investigating the effects cations and |
702 |
% anions have on crystal nucleaion and |
703 |
% melting.\cite{Bryk04,Smith05,Wilson08,Wilson10} Nada and Furukawa have studied |
704 |
% the the basal- and prismatic-water interface width\cite{Nada95}, crystal |
705 |
% surface restructuring at temperatures approaching the melting |
706 |
% point\cite{Nada00}, and the mechanism of ice growth inhibition by antifreeze |
707 |
% proteins\cite{Nada08,Nada11,Nada12}. Nada has developed a six-site water model |
708 |
% for ice/water interfaces near the melting point\cite{Nada03}, and studied the |
709 |
% dependence of crystal growth shape on applied pressure\cite{Nada11b}. Using |
710 |
% this model, Nada and Furukawa have established differential |
711 |
% growth rates for the basal, prismatic, and secondary prismatic facets of |
712 |
% Ice-I$_\mathrm{h}$ and found their origins due to a reordering of the hydrogen |
713 |
% bond network in water near the interface\cite{Nada05}. While the work |
714 |
% described so far has mainly focused on bulk water on ice, there is significant |
715 |
% interest in thin films of water on ice surfaces as well. |
716 |
|
717 |
% It is well known that the surface of ice exhibits a premelting layer at |
718 |
% temperatures near the melting point, often called a quasi-liquid layer (QLL). |
719 |
% Molecular dynamics simulations of the facets of ice-I$_\mathrm{h}$ exposed |
720 |
% to vacuum performed by Conde, Vega and Patrykiejew have found QLL widths of |
721 |
% approximately 10 \AA\ at 3 K below the melting point\cite{Conde08}. |
722 |
% Similarly, Limmer and Chandler have used course grain simulations and |
723 |
% statistical field theory to estimated QLL widths at the same temperature to |
724 |
% be about 3 nm\cite{Limmer14}. |
725 |
% Recently, Sazaki and Furukawa have developed an experimental technique with |
726 |
% sufficient spatial and temporal resolution to visulaize and quantitatively |
727 |
% analyze QLLs on ice crystals at temperatures near melting\cite{Sazaki10}. They |
728 |
% have found the width of the QLLs perpindicular to the surface at -2.2$^{o}$C |
729 |
% to be $\mathcal{O}$(\AA). They have also seen the formation of two immiscible |
730 |
% QLLs, which displayed different stabilities and dynamics on the crystal |
731 |
% surface\cite{Sazaki12}. Knowledge of the hydrophilicities of each |
732 |
% of the crystal facets would help further our understanding of the properties |
733 |
% and dynamics of the QLLs. |
734 |
|
735 |
% Presented here is the follow up to our previous paper\cite{Louden13}, in which |
736 |
% the basal and prismatic facets of an ice-I$_\mathrm{h}$/water interface were |
737 |
% investigated where the ice was sheared relative to the liquid. By using a |
738 |
% recently developed velocity shearing and scaling approach to reverse |
739 |
% non-equilibrium molecular dynamics (VSS-RNEMD), simultaneous temperature and |
740 |
% velocity gradients can be applied to the system, which allows for measurment |
741 |
% of friction and thermal transport properties while maintaining a stable |
742 |
% interfacial temperature\cite{Kuang12}. Structural analysis and dynamic |
743 |
% correlation functions were used to probe the interfacial response to a shear, |
744 |
% and the resulting solid/liquid kinetic friction coefficients were reported. |
745 |
% In this paper we present the same analysis for the pyramidal and secondary |
746 |
% prismatic facets, and show that the differential interfacial friction |
747 |
% coefficients for the four facets of ice-I$_\mathrm{h}$ are determined by their |
748 |
% relative hydrophilicity by means of dynamics water contact angle |
749 |
% simulations. |
750 |
|
751 |
% The local tetrahedral order parameter, $q(z)$, is given by |
752 |
% \begin{equation} |
753 |
% q(z) = \int_0^L \sum_{k=1}^{N} \Bigg(1 -\frac{3}{8}\sum_{i=1}^{3} |
754 |
% \sum_{j=i+1}^{4} \bigg(\cos\psi_{ikj}+\frac{1}{3}\bigg)^2\Bigg) |
755 |
% \delta(z_{k}-z)\mathrm{d}z \Bigg/ N_z |
756 |
% \label{eq:qz} |
757 |
% \end{equation} |
758 |
% where $\psi_{ikj}$ is the angle formed between the oxygen sites of molecules |
759 |
% $i$,$k$, and $j$, where the centeral oxygen is located within molecule $k$ and |
760 |
% molecules $i$ and $j$ are two of the closest four water molecules |
761 |
% around molecule $k$. All four closest neighbors of molecule $k$ are also |
762 |
% required to reside within the first peak of the pair distribution function |
763 |
% for molecule $k$ (typically $<$ 3.41 \AA\ for water). |
764 |
% $N_z = \int\delta(z_k - z) \mathrm{d}z$ is a normalization factor to account |
765 |
% for the varying population of molecules within each finite-width bin. |
766 |
|
767 |
|
768 |
% The hydrophobicity or hydrophilicity of a surface can be described by the |
769 |
% extent a droplet of water wets the surface. The contact angle formed between |
770 |
% the solid and the liquid, $\theta$, which relates the free energies of the |
771 |
% three interfaces involved, is given by Young's equation. |
772 |
% \begin{equation}\label{young} |
773 |
% \cos\theta = (\gamma_{sv} - \gamma_{sl})/\gamma_{lv} |
774 |
% \end{equation} |
775 |
% Here $\gamma_{sv}$, $\gamma_{sl}$, and $\gamma_{lv}$ are the free energies |
776 |
% of the solid/vapor, solid/liquid, and liquid/vapor interfaces respectively. |
777 |
% Large contact angles ($\theta$ $\gg$ 90\textsuperscript{o}) correspond to low |
778 |
% wettability and hydrophobic surfaces, while small contact angles |
779 |
% ($\theta$ $\ll$ 90\textsuperscript{o}) correspond to high wettability and |
780 |
% hydrophilic surfaces. Experimentally, measurements of the contact angle |
781 |
% of sessile drops has been used to quantify the extent of wetting on surfaces |
782 |
% with thermally selective wetting charactaristics\cite{Tadanaga00,Liu04,Sun04}, |
783 |
% as well as nano-pillared surfaces with electrically tunable Cassie-Baxter and |
784 |
% Wenzel states\cite{Herbertson06,Dhindsa06,Verplanck07,Ahuja08,Manukyan11}. |
785 |
% Luzar and coworkers have done significant work modeling these transitions on |
786 |
% nano-patterned surfaces\cite{Daub07,Daub10,Daub11,Ritchie12}, and have found |
787 |
% the change in contact angle to be due to the external field perturbing the |
788 |
% hydrogen bonding of the liquid/vapor interface\cite{Daub07}. |
789 |
|
790 |
% SI stuff: |
791 |
|
792 |
% Correlation functions: |
793 |
% To compute eq. \eqref{C(t)1}, each 0.5 ns simulation was |
794 |
% followed by an additional 200 ps NVE simulation during which the |
795 |
% position and orientations of each molecule were recorded every 0.1 ps. |
796 |
|
797 |
|
798 |
%\end{article} |
799 |
\newpage |
800 |
\begin{figure} |
801 |
\includegraphics[width=\linewidth]{Droplet} |
802 |
\caption{\label{fig:Droplet} Computational model of a droplet of |
803 |
liquid water spreading over the prismatic $\{10\bar{1}0\}$ facet |
804 |
of ice, before (left) and 2.6 ns after (right) being introduced to the |
805 |
surface. The contact angle ($\theta$) shrinks as the simulation |
806 |
proceeds, and the long-time behavior of this angle is used to |
807 |
estimate the hydrophilicity of the facet.} |
808 |
\end{figure} |
809 |
\newpage |
810 |
\begin{figure} |
811 |
\includegraphics[width=1.9in]{Shearing} |
812 |
\caption{\label{fig:Shearing} Computational model of a slab of ice |
813 |
being sheared through liquid water. In this figure, the ice is |
814 |
presenting two copies of the prismatic $\{10\bar{1}0\}$ facet |
815 |
towards the liquid phase. The RNEMD simulation exchanges both |
816 |
linear momentum (indicated with arrows) and kinetic energy between |
817 |
the central box and the box that spans the cell boundary. The |
818 |
system responds with weak thermal gradient and a velocity profile |
819 |
that shears the ice relative to the surrounding liquid.} |
820 |
\end{figure} |
821 |
|
822 |
% \begin{figure} |
823 |
% \includegraphics[width=\linewidth]{ContactAngle} |
824 |
% \caption{\label{fig:ContactAngle} The dynamic contact angle of a |
825 |
% droplet after approaching each of the four ice facets. The decay to |
826 |
% an equilibrium contact angle displays similar dynamics. Although |
827 |
% all the surfaces are hydrophilic, the long-time behavior stabilizes |
828 |
% to significantly flatter droplets for the basal and pyramidal |
829 |
% facets. This suggests a difference in hydrophilicity for these |
830 |
% facets compared with the two prismatic facets.} |
831 |
% \end{figure} |
832 |
|
833 |
% \begin{figure} |
834 |
% \includegraphics[width=\linewidth]{Pyr_comic_strip} |
835 |
% \caption{\label{fig:pyrComic} Properties of the pyramidal interface |
836 |
% being sheared through water at 3.8 ms\textsuperscript{-1}. Lower |
837 |
% panel: the local tetrahedral order parameter, $q(z)$, (circles) and |
838 |
% the hyperbolic tangent fit (turquoise line). Middle panel: the |
839 |
% imposed thermal gradient required to maintain a fixed interfacial |
840 |
% temperature of 225 K. Upper panel: the transverse velocity gradient |
841 |
% that develops in response to an imposed momentum flux. The vertical |
842 |
% dotted lines indicate the locations of the midpoints of the two |
843 |
% interfaces.} |
844 |
% \end{figure} |
845 |
|
846 |
% \begin{figure} |
847 |
% \includegraphics[width=\linewidth]{SP_comic_strip} |
848 |
% \caption{\label{fig:spComic} The secondary prismatic interface with a shear |
849 |
% rate of 3.5 \ |
850 |
% ms\textsuperscript{-1}. Panel descriptions match those in figure \ref{fig:pyrComic}.} |
851 |
% \end{figure} |
852 |
|
853 |
% \begin{figure} |
854 |
% \includegraphics[width=\linewidth]{Pyr-orient} |
855 |
% \caption{\label{fig:PyrOrient} The three decay constants of the |
856 |
% orientational time correlation function, $C_2(z,t)$, for water as a |
857 |
% function of distance from the center of the ice slab. The vertical |
858 |
% dashed line indicates the edge of the pyramidal ice slab determined |
859 |
% by the local order tetrahedral parameter. The control (circles) and |
860 |
% sheared (squares) simulations were fit using shifted-exponential |
861 |
% decay (see Eq. 9 in Ref. \citealp{Louden13}).} |
862 |
% \end{figure} |
863 |
|
864 |
% \begin{figure} |
865 |
% \includegraphics[width=\linewidth]{SP-orient-less} |
866 |
% \caption{\label{fig:SPorient} Decay constants for $C_2(z,t)$ at the secondary |
867 |
% prismatic face. Panel descriptions match those in \ref{fig:PyrOrient}.} |
868 |
% \end{figure} |
869 |
|
870 |
\newpage |
871 |
\begin{table}[h] |
872 |
\centering |
873 |
\caption{Sizes of the droplet and shearing simulations. Cell |
874 |
dimensions are measured in \AA. \label{tab:method}} |
875 |
\begin{tabular}{r|cccc|ccccc} |
876 |
\toprule |
877 |
\multirow{2}{*}{Interface} & \multicolumn{4}{c|}{Droplet} & \multicolumn{5}{c}{ |
878 |
Shearing} \\ |
879 |
& $N_\mathrm{ice}$ & $N_\mathrm{droplet}$ & $L_x$ & $L_y$ & $N_\mathrm{ice}$ & |
880 |
$N_\mathrm{liquid}$ & $L_x$ & $L_y$ & $L_z$ \\ |
881 |
\colrule |
882 |
Basal $\{0001\}$ & 12960 & 2048 & 134.70 & 140.04 & 900 & 1846 & 23.87 & 35.83 & 98.64 \\ |
883 |
Pyramidal $\{20\bar{2}1\}$ & 11136 & 2048 & 143.75 & 121.41 & 1216 & 2203 & 37.47 & 29.50 & 93.02 \\ |
884 |
Prismatic $\{10\bar{1}0\}$ & 9900 & 2048 & 110.04 & 115.00 & 3000 & 5464 & 35.95 & 35.65 & 205.77 \\ |
885 |
Secondary Prism $\{11\bar{2}0\}$ & 11520 & 2048 & 146.72 & 124.48 & 3840 & 8176 & 71.87 & 31.66 & 161.55 \\ |
886 |
\botrule |
887 |
\end{tabular} |
888 |
\end{table} |
889 |
|
890 |
\newpage |
891 |
\begin{table}[h] |
892 |
\centering |
893 |
\caption{Structural and dynamic properties of the interfaces of |
894 |
Ice-I$_\mathrm{h}$ with water.\label{tab:kappa}} |
895 |
\begin{tabular}{r|cc|cc|cccc} |
896 |
\toprule |
897 |
\multirow{2}{*}{Interface} & \multicolumn{2}{c|}{Channel Size} &\multicolumn{2}{c|}{Droplet} & \multicolumn{4}{c}{Shearing\footnotemark[1]}\\ |
898 |
& Width (\AA) & Depth (\AA) & $\theta_{\infty}$ ($^\circ$) & $k_\mathrm{spread}$ (ns\textsuperscript{-1}) & |
899 |
$\kappa_{x}$ & $\kappa_{y}$ & $d_\mathrm{struct}$ (\AA) & $d_\mathrm{dyn}$ (\AA) \\ |
900 |
\colrule |
901 |
Basal $\{0001\}$ & 4.49 & 1.30 & $34.1(9)$ &$0.60(7)$ |
902 |
& $5.9(3)$ & $6.5(8)$ & $3.2(4)$ & $2(1)$ \\ |
903 |
Pyramidal $\{20\bar{2}1\}$ & 8.65 & 1.37 & $35(3)$ & $0.7(1)$ & |
904 |
$5.8(4)$ & $6.1(5)$ & $3.2(2)$ & $2.5(3)$\\ |
905 |
Prismatic $\{10\bar{1}0\}$ & 6.35 & 2.25 & $45(3)$ & $0.75(9)$ & |
906 |
$3.0(2)$ & $3.0(1)$ & $3.6(2)$ & $4(2)$ \\ |
907 |
Secondary Prism $\{11\bar{2}0\}$ & 6.35 & 2.25 & $43(2)$ & $0.69(3)$ & |
908 |
$3.5(1)$ & $3.3(2)$ & $3.2(2)$ & $5(3)$ \\ |
909 |
\botrule |
910 |
\end{tabular} |
911 |
\begin{flushleft} |
912 |
\footnotemark[1]\footnotesize{Liquid-solid friction coefficients ($\kappa_x$ and |
913 |
$\kappa_y$) are expressed in 10\textsuperscript{-4} amu |
914 |
\AA\textsuperscript{-2} fs\textsuperscript{-1}.} \\ |
915 |
\footnotemark[2]\footnotesize{Uncertainties in |
916 |
the last digits are given in parentheses.} |
917 |
\end{flushleft} |
918 |
\end{table} |
919 |
|
920 |
% Basal $\{0001\}$ & 4.49 & 1.30 & $34.1 \pm 0.9$ &$0.60 \pm 0.07$ |
921 |
% & $5.9 \pm 0.3$ & $6.5 \pm 0.8$ & $3.2 \pm 0.4$ & $2 \pm 1$ \\ |
922 |
% Pyramidal $\{2~0~\bar{2}~1\}$ & 8.65 & 1.37 & $35 \pm 3$ & $0.7 \pm 0.1$ & |
923 |
% $5.8 \pm 0.4$ & $6.1 \pm 0.5$ & $3.2 \pm 0.2$ & $2.5 \pm 0.3$\\ |
924 |
% Prismatic $\{1~0~\bar{1}~0\}$ & 6.35 & 2.25 & $45 \pm 3$ & $0.75 \pm 0.09$ & |
925 |
% $3.0 \pm 0.2$ & $3.0 \pm 0.1$ & $3.6 \pm 0.2$ & $4 \pm 2$ \\ |
926 |
% Secondary Prism $\{1~1~\bar{2}~0\}$ & 6.35 & 2.25 & $43 \pm 2$ & $0.69 \pm 0.03$ & |
927 |
% $3.5 \pm 0.1$ & $3.3 \pm 0.2$ & $3.2 \pm 0.2$ & $5 \pm 3$ \\ |
928 |
|
929 |
|
930 |
\end{document} |