Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W10: Adsorption, Wetting, and Complex Interfaces |
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Sponsoring Units: DCMP Chair: Milton Cole, Penn State University Room: D221 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W10.00001: Surface Tension Anisotropy of Lennard-Jones Systems Emre Esenturk Anisotropy in the surface tension plays a significant role in the evolution of interfaces and in determining the equilibrium shapes of materials (dendritic growth, motion of grain boundaries). We present a discrete version of integral Phase Field Model with non-local potential for the crystal-melt interfaces of Lennard-Jones Systems. The model provides a methodology to understand the process of transfer of microscopic anisotropy to macroscopic scale. We calculate the surface tension and the anisotropy of the crystal-melt interface in the [100] and [110] directions and compare our results with recent simulations. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W10.00002: Adsorption of Methane and Propane on a LJ Wall and on Molybdenum Surface: A Simulation Study Ali Abu Nada, Gary Leuty, Mesfin Tsige Atomic-scale MD simulations were used to study multilayer adsorption as a function of temperature for two different alkanes (CH$_{4}$ and C$_{3}$H$_{8})$ on a fixed Lennard-Jones wall and on the (001) surface of molybdenum. In sets of simulations on the molybdenum surface, the substrate atoms were made to interact via a Lennard-Jones potential in one study and via an embedded-atom model (EAM) potential in the next. The results show that CH$_{4}$ and C$_{3}$H$_{8}$ on the flat wall possess a highly ordered packing arrangement exhibiting a higher degree of order than films adsorbed on the molybdenum substrate. Additionally, the number of ordered layers seen in the case of adsorption of CH$_{4}$ was noted to be greater than the number of ordered layers in the case of adsorption of C$_{3}$H$_{8}$. In each case, the first layer appears frozen, implying there is no translational motion for the molecules in this layer, and we can confidently surmise that the first adsorbed layer exists in the solid phase far above the bulk melting temperature. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W10.00003: Evaporation kinetics of CO$_{2}$ laser heated fused silica Selim Elhadj, M.J. Matthews, S.T. Yang, D. Cooke, J.S. Stolken, M.D. Feit Laser-based machining strategies of optical surfaces remain mostly empirical, yet, systematic and controlled studies that relate gas chemistry and surface temperature to evaporation kinetics are limited, especially at extreme temperatures ($>$2800K) reached during laser irradiation. We present experimental results of CO$_{2}$ laser heating of silica in oxidizing and non-oxidizing environments, along with analysis of surface shape from which a near-equilibrium evaporation model is derived. Based on this model, temperature dependent enthalpies of evaporation are determined and compared to published results. This model reproduces experimental laser-etch rates, while still accounting for laser, mass transport, and gas chemistry parameters. Although heat and mass transport processes are complex and tightly coupled, general conditions for which such an approach can be used to guide laser-based evaporation will be presented. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W10.00004: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W10.00005: Simulation study on structure of water in aqueous solutions confined between graphene electrodes under very high applied electric field Gary Leuty, Mesfin Tsige, Saikat Talapatra Arising from questions regarding electric double-layer capacitors utilizing graphene electrodes and aqueous electrolyte (KOH solution), atomistic MD simulations of electrolyte confined between graphene electrodes were performed to understand the behavior of electrolyte as a function of electric field strength and solution concentration, from pure water to 6M KOH. It was noted that the strength of the electric field had a demonstrable effect on the structure of pure water between the electrodes (as has previously been seen in highly confined multilayer water systems), creating regularly spaced channels and densely packed sheets of highly ordered molecules. We also saw a clear effect due to the presence of electrolyte ions and their separation from the water due to the action of the field; different field strengths appear to greatly alter the distribution of ions, which in turn affects the structure and ordering of the water. Time dependence in the strength of the electric field was also studied to determine what effect, if any, it has on induced structure. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W10.00006: Thermodynamics and kinetics of wetting transition of an oily fluid on surfaces with nanoscale roughness: A molecular dynamics study Elizabeth Savoy, Fernando Escobedo Surface wettability has garnered significant interest in recent years, as design and manufacture of nanoscale features allows fabrication of highly non-wetting surfaces. Such behavior is more difficult to achieve for low surface-tension fluids such as oils, and requires novel approaches. One approach is to create roughness features that provide an energy barrier to the fluid's transition from the composite to fully wetted state. We use molecular dynamics of small droplets in combination with various simulation techniques, such as umbrella sampling and forward flux sampling, to probe the energy landscape associated with the wetting transition and compute transition rates and their dependence on key topological parameters such as feature height. We find that the drop does not transition with a flat liquid-vapor interface when it penetrates and wets the subsurface features (as is often assumed in continuum treatments) and that the hysteresis in the wetting and dewetting transitions is associated with differences in the evolution of that interface. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W10.00007: Understanding and designing of steam-phobic surfaces Ila Badge, Sunny Sethi, Ali Dhinojwala The wetting behavior of a surface under steam condensation depends on its intrinsic wettability and micron or nanoscale surface roughness. A typical superhydrophobic surface may not be suitable as a steam-phobic surface due to nucleation and growth of water inside the valleys and thus, failure to form air-liquid- solid composite interface. Here, we present the results of steam condensation on chemically modified nano-structured carbon nanotube carpets. The combination of surface chemistry and surface roughness provides a mechanism to retain superhydrophobicity of the nanotube surfaces under steam condensation. Ability of withstand steam temperature and pressure also implies improved hydrostatic stability of the surface. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W10.00008: Effects of host relaxation on gas uptake in porous media Milton Cole, Annie Grosman, Susana Hernandez, Angela Lueking We have recently predicted [1] an \textit{imbibition transition} as a prototype of a general phenomenon-substrate relaxation due to adsorption. That transition is exemplified by a graphene sheet's lifting off of a surface in order to intercalate gas. Other relevant phenomena include the expansion of nanotube bundles or MOFs to accommodate imbibed gases. In our new work, we first analyze the relaxation problem, in general, and then address infinite cylindrical and slit pore geometries, for which simplifications occur because there is just one finite dimension. Research supported by DOE. \\[4pt] [1] K. E. Noa, A. D. Lueking and M. W. Cole, \textit{Imbibition transition: gas intercalation between graphene and silica}, submitted to J. Low Temp. Phys. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W10.00009: Surface metal-oxygen bond length on hydrated rutile(110) and cassiterite(110) surface - A measure of the local environment Nitin Kumar, Paul Kent, Andrei Bandura, David Wesolowski, James Kubicki, Jorge Sofo We study the dynamics of water on the surface of rutile (110) and cassiterite (110) using ab-initio molecular dynamics simulation. The water molecule covalently attach with the fivefold coordinated metal atoms on the surface. It can remain in a molecular form or it can dissociate to form hydroxyls on the surface. The distance between the metal and the oxygen depends on the protonation state of the latter. Moreover, we find that the local environment is not only limited to the number of covalently bonded hydrogen but it also depends on number of hydrogen bonds and the species participating in it. In general, the metal oxygen distance shows much larger fluctuations in rutile compared with cassiterite. The half width half maximum (HWHM) of the metal oxygen distance histogram, for the terminal oxygen, is 0.27 Angstrom for rutile and 0.16 Angstrom for cassiterite. Also, for bridging oxygen HWHM is 0.18 and 0.12 Angstrom for rutile and cassiterite, respectively. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W10.00010: Water-Thin-Film Adsorption on Alpha-Quartz (0001) Surface Yun-Wen Chen, Yan Wang, Hai-Ping Cheng We investigated thin water films adsorbed on quartz (0001) surfaces using first-principles density functional theory calculations. Interfacial structure and energetics were studied through a layer-by-layer deposition. From monolayer to multilayer, the low energy state configurations and adsorption sites show a transition due to formation of a highly stable bilayer membranelike structure. The water adsorption energy on a quartz surface coated by this membrane is of typical hydrogen bond strength for both dry and fully hydroxylated surfaces. The interactions between the surface and the water films are short-ranged due to shielding of the bilayer. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W10.00011: Mn Adsorption on MgO/Ag(100): DFT and DFT+U Calculations Hossein Hashemi, Barbara Jones The adsorption properties of a Mn adatom on MgO ultrathin films deposited on a Ag(100) substrate are determined from first principles DFT calculations and compared with the corresponding adsorption characteristics from DFT+U calculations. First, we investigate the properties of a pure Ag (100) surface and a MgO/Ag(100) system. The structural relaxation, work function and surface energy for the Ag(100) surface, as a function of thickness dependence, is discussed. Next, we discuss the addition of a Mn adatom. Regular (U=0) DFT calculations show that the most stable site for Mn adsorption on MgO/Ag(100) is the bridge site, followed closely by the Oxygen site, and a very unlikely position, the Mg site. We have also investigated the role of strong electron correlations in the substrate on the chemisorption properties of a Mn adatom. DFT+U calculations predict the Oxygen site to be the most stable site, instead of the bridge site, in contrast to U=0 DFT calculations. The energies, geometry, and magnetic properties of the Mn adatom are all influenced by adding a Coulomb energy. Altogether our results show that the on-site Coulomb repulsion in the Mn d band plays an important role in the description of adsorption on MgO/Ag(100). [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W10.00012: Stabilizing Graphitic Thin Films of Wurtzite Materials by Epitaxial Strain Dangxin Wu, Feng Liu Recent theoretical and experimental work showed that (0001) ultrathin films of wurtzite materials transform into a stable graphite-like structure if their thickness is reduced to only a few atomic layers. Using first-principles calculations of both freestanding and substrate-supported thin films, we predict that the thickness range of stable graphitic films can be greatly extended by epitaxial tensile strain but reduced by compressive strain. The band gap of the resulting graphitic films can be tuned by strain and film thickness either above or below that of the bulk wurtzite phase. Our prediction suggests a plausible physical mechanism to be explored by future experiments for strain engineering of graphitic films from wurtzite materials with a wide range of potential applications. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W10.00013: Simulating phase formation during exothermic reactions in Al/Ni and Al/Zr multilayered foils Rong-Guang Xu, Michael L. Falk, Hong-Wei Sheng Reactive multilayered foils are composed of thousands of alternating micro- to nano- scale layers of elements which have a large negative enthalpy of mixing. When a small pulse of energy (such as an electric spark or a thermal pulse) is provided, highly exothermic, self-propagating chemical reactions can be triggered. Both theoretical models and experimental data indicate that even a relatively small amount of premixing can have a dramatic effect on the heats and velocities of the propagating reaction front. We have implemented molecular dynamics simulation to study the phase transformation sequence during multilayered reactions and to elucidate how premixing can affect the sequence of phase formation during such reactions. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W10.00014: Chemical Reactivity at the Ti/CuO Interface A. Chourasia, J. Edmondson The chemical reactivity between titanium and copper oxide at the Ti/CuO interface has been investigated using x-ray photoelectron spectroscopy. About 15 nm thick copper film was deposited on silicon substrates by the e-beam method. Such samples were oxidized in an oxygen environment in a quartz tube furnace at 400\r{ }C. The formation of CuO was checked by the XPS spectral data. Thin films of titanium were then deposited on these CuO samples. The titanium 2p, oxygen 1s and copper 2p regions were investigated by XPS. The magnesium anode (energy = 1253.6 eV) has been used for this purpose. The spectral data show chemical reactivity at the Ti/CuO interface. The samples were annealed afterwards in air at 400\r{ }C. The spectral data were recorded at different take-off angles. Comparison of the data with the pre-annealed samples shows diffusion of Cu through the titanium overlayer alongwith the formation of CuO. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W10.00015: Composite TiO2-Carbon nano films with enhanced photocatalytic activity Dinko Chakarov, Raja Sellappan Composite TiO2-carbon thin films prepared by physical vapor deposition techniques on fused silica substrates show enhanced photocatalytic activity, as compared to pure TiO2 films of similar thickness, towards decomposition of methanol to CO2 and water. Raman and XRD measurements confirm that annealed TiO2 films exhibit anatase structure while the carbon layer becomes graphitic. Characteristic for the composite films is an enhanced optical absorption in the visible range. The presence of the carbon film causes a shift of the TiO2 absorption edge and modifies its grain size to be smaller. The observed enhancement is attributed to synergy effects at the carbon-TiO2 interface, resulting in smaller crystallite size and anisotropic charge carrier transport, which in turn reduces their recombination probability. [Preview Abstract] |
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