Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U21: Surfaces and Interfaces I |
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Sponsoring Units: DCP Chair: Arthur Reber, Virginia Commonwealth University Room: Morial Convention Center 213 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U21.00001: Automated Analysis of Nanocar Molecules as Observed by VT-STM. A. J. Osgood, T. Sasaki, J. M. Tour, K. F. Kelly The observation and measurement of individual nanocar molecules by variable temperature scanning tunneling microscopy (STM) has uncovered a great deal of information regarding their electronic properties and dynamic abilities. While STM is particularly powerful in measuring the properties of individual molecules, it is often desirable and enlightening to obtain information of the ensemble as well. Many groups have previously worked on the automatic detection and recognition of molecules in STM images, however, the complex 4-lobed nature of the nanocar introduces additional challenges.~Therefore, we have developed an automated image processing routine that is more robust and able to overcome these problems.~We then apply this to the analysis of nanocars imaged by STM at various temperatures and demonstrate the recognition of spinning vs. stationary fullerene wheels on the nanocar molecules by correlating the rotational state with observed changes in their electronic properties. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U21.00002: Study of p-diaminobenzene Adsorption on Au(111) by Scanning Tunneling Microscopy Hui Zhou, Zonghai Hu, Daejin Eom, Kwang Rim, Li Liu, George Flynn, Latha Venkataraman, Alberto Morgante, Tony Heinz From the well-defined conductivity obtained for various individual diamino-substituted molecules spanning two gold contacts, as well as from theoretical analysis [1], researchers have suggested that amines adsorb preferentially to coordinatively unsaturated surface Au atoms through the N lone pair. To understand the nature of the amine binding, we have applied ultrahigh vacuum scanning tunneling microscope (STM) to investigate the adsorption of p-diaminobenzene molecules on the reconstructed Au(111) surface. The STM topography images (taken at 4 K) show that the molecules adsorb preferentially to step edges, corresponding to sites of reduced Au atom coordination. The adsorbed molecules are found to display a distinctive orientation along the step edges. The two-lobe topographic structure of each molecule seen by STM is compatible with the previously calculated charge density of the HOMO level. [1] L. Venkataraman at el., Nano Lett. 7, 502 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U21.00003: Adsorption of colloids with Gaussian-size distribution on clean and pre-patterned substrates Joao F. Marques, A.B. Lima, Nuno A.M. Araujo, Antonio Cadilhe We performed extensive Monte Carlo simulations to study the influence of Gaussian size-distributed colloidal particles on film morphology. Also, we take the limit of irreversible adsorption, which leads to the study of a generalized random sequential adsorption model. We considered size dispersions ranging from $1\%$ up to $20\%$. The study concerns not only the jammed state but also the full kinetic dependence. Moreover, we also considered the influence of a patterned substrate, consisting of equal sized squares regularly distributed on the surface. Colloids can adsorb solely inside these squares. Results make understandable why colloids with size dispersions up to $4\%$ are considered monodisperse. Finally, we consider in the patterned substrate case cells with a prescribed number of colloids and characterize these deposits. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U21.00004: Binary mixture study of CF$_{4}$ and CF$_{3}$Cl on graphite Petros Thomas, Daniel Velazquez, George Hess In a binary mixture adsorption study of CF$_{4}$ and CF$_{3}$Cl on graphite from 60 K to 105 K, both the CF$_{3}$Cl - \textit{$\nu $}$_{4}$ and the CF$_{4}$ - \textit{$\nu $}$_{3}$ frequency shifts are measured using IRAS as the spreading pressure (chemical potential) of CF$_{4}$ is increased. Even though CF$_{3}$Cl has a much lower saturation vapor pressure (SVP) compared to CF$_{4}$ (at 80 K, SVP of CF$_{4}$ is $\sim $ 70 mT and that of CF$_{3}$Cl is $\sim $ 0.1 mT), the CF$_{4}$ either continuously displaces or adsorbs on top of CF$_{3}$Cl depending on the initial coverage of CF$_{3}$Cl on the graphite surface. For temperatures between 70 K and 105 K and lower coverage of CF$_{3}$Cl, where the molecules lie with their C -- Cl axis nearly parallel with the surface, CF$_{4}$ continuously displaces CF$_{3}$Cl from the surface. For saturated monolayer coverage of CF$_{3}$Cl, where the C -- Cl axis of the molecules are tilted relative to the surface, the CF$_{4}$ molecules adsorb on top of the CF$_{3}$Cl -- HOPG template. At 60 K, the displacement of the low-coverage CF$_{3}$Cl is only partial and the orientation of the remaining CF$_{3}$Cl is tilted relative to the surface from a nearly flat position. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U21.00005: DFT Estimation of Lateral Interactions in Lattice-gas Models of Br and Cl on Ag(100) T. Juwono, P.A. Rikvold We studied Br and Cl chemisorbed on a Ag(100) surface, using a latice-gas model and the Density Functional Theory (DFT) method. In this model the Br and Cl ions adsorb at the fourfold hollow sites of the Ag(100) surface, which yields a square lattice of adsorption sites. Five different coverages for each kind of adsorbate were calculated. For each adsorbate and coverage, we obtained the minimum-energy configuration, its energy, and its charge distribution. From these data we calculated dipole moments, lateral interaction energies, and binding energies. Our results showed that for Br the lattice-gas model obtained by fitting to the adsorption energies from the DFT calculation is consistent with long-range dipole-dipole lateral interactions using the dipole moments calculated from DFT charge distribution. For Cl we found less consistency, which indicates that long-range dipole-dipole interactions are not sufficient to describe the Chlorine system. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U21.00006: Modelling Interfaces in Liquid Crystal/isotropic fluid mixtures Colin Denniston, Dan Vriesinga We use all-atom molecular dynamics simulations of mixtures of a real liquid crystal (5CB) and water to study the 5CB/water interface. Properties of the director anchoring at the interface are studied in detail. We map our results onto a continuum model implemented using lattice Boltzmann simulations. We examine anchoring as a function of interface shape and droplet size and discuss the impact on liquid crystal colloidal suspensions. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U21.00007: Extended Analysis of a Fluid Configuration Experiment on the Space Shuttle Eric Barnett, Marcus Dejmek Glass cylinders, partially filled with water, were exposed to the near free-fall environment. In at least two of the cylinders, the liquid-vapour interface adopted a two-interface configuration, as previously predicted. An initial analysis was conducted on 20 images for one cylinder, resulting in contact angles of 6.7$\pm $2.7\r{ } at the upper three-phase line and 26.5$\pm $6.2\r{ } at the lower. Herein, the analysis has been extended to include all 12538 images recorded for each of two cylinders, in addition to correcting for optical distortion. An automated procedure to calculate the contact angles was developed, resulting in values of 2.7$\pm $2.8\r{ } and 16.5$\pm $5.3\r{ } for the same cylinder previously analyzed. The effective gravity (g$_{e})$ based on this analysis was inferred to be 3.3$\pm $2.1 x 10$^{-4}$ g/g$_{0}$, which differs from that previously reported. However, the standard deviation of g$_{e}$ is of the same order of magnitude as the RMS accelerations recorded. The difference in pressures between the two liquid phases was calculated to be 0.21$\pm $0.14 Pa. A Fourier analysis was conducted and no significant frequencies could be distinguished. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U21.00008: Dissipation at Moving Contact Lines: Effect of Interface Width and Slip Mark Robbins, Shengfeng Cheng, Colin Denniston Continuum mechanics predicts a diverging stress and total dissipation when the contact line between a fluid interface and a solid substrate is advanced. Several models for removing this divergence have been advanced. One is that the divergence is cutoff by a finite slip length. Another is that diffusion can remove the singularity for fluid interfaces of finite width. Extensive molecular dynamics simulations of partially miscible binary fluids were used to test these two pictures. The interfacial tension was changed by a factor of 20 and the interfacial width by an order of magnitude. The interface width had no direct effect on the dissipation and diffusion was orthogonal to the predicted direction. The dissipation only depended on system size, the dimensionless capillary number, and the slip length S associated with the flow boundary condition in the fluid far from the contact line. The divergence in stress is cut off at the sum of S and a distance of order the molecular diameter. The dissipation rises rapidly as the amount of slip is decreased. In all cases there is a first order transition where the advancing contact line becomes unstable and a film is entrained. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U21.00009: Counterintuitive connection between layering and mobility in confined fluids Gaurav Goel, William Krekelberg, Jeffrey Errington, Thomas Truskett Fluids confined to narrow spaces adopt a spatially inhomogeneous distribution of density due to the interactions between the fluid particles and the boundaries. This ``density profile'' is the most common measure of inhomogeneous structure in confined fluids, but its connection to fluid transport coefficients is poorly understood. We explore via molecular simulations how tuning particle-wall interactions to flatten or enhance the particle layering of a model Weeks-Chandler-Andersen (WCA) confined fluid impacts its self-diffusivity, viscosity, and entropy. Counterintuitively, interactions that eliminate particle layering significantly reduce confined fluid mobility, while those that enhance layering have the opposite effect. Excess entropy helps to both understand and predict these trends. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U21.00010: Effective interfacial tension between miscible fluids John Pojman, Nick Bessonov, Gloria Viner, Vitaly Volpert Isobutyric acid (IBA) and water have an Upper Critical Solution Temperature of 27 C. Using spinning drop tensiometry, we were able to demonstrate the existence of an effective interfacial tension by preparing a drop of isobutyric acid-rich phase below the UCST and then raising the temperature above the UCST. The capillary instability was also observed by rapidly reducing the rotation rate. We also demonstrated that such an effective interfacial tension is not unique to the IBA-water systems but can also occur in the cyclohexane -- aniline, which has a Lower Critical Solution Temperature. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U21.00011: The Au(111) electrolyte interface: A DFT investigation Timo Jacob, Sudha Venkatachalam, Felice Simeone, Dieter Kolb Density functional theory calculations have been performed to derive a detailed model of the electric double layer for Au(111) in contact with an aqueous H$_2$SO$_4$ electrolyte. At potentials of $E\ge +0.8$\,V vs. SCE various surface sensitive techniques found evidence for a ($\sqrt{3}\times \sqrt{7}$)R19.1$^\circ$ (bi)sulfate structure, but the nature of coadsorbates remains still unclear. Focusing on a sulfate adlayer, the coadsorption of H$_3$O$^+$ and/or H$_2$O has been studied [1]. The calculated binding energies show that the coadsorption of a single H$_3$O$^+$ per sulfate (stabilizing the adlayer by hydrogen bonds) is the most stable configuration. In addition, the charge density distribution within the adlayer well agrees with effective barrier heights deduced from recent distance tunnelling spectroscopy measurements [2]. Afterwards we studied the interfacial structure that forms at negative electrode potentials and found that water arranges near the electrode in an ice-like hexagonal structure with hydronium ions being located in the second water layer and non-specifically adsorbed. Again the calculated charge density distribution shows a perfect correspondence to distance tunnelling spectroscopy measurements. [1] S. Venkatachalam and T. Jacob, Z. Phys. Chem., {\bf 221}, 1393 (2007). [2] S. Venkatachalam {\it et al.}, Angew. Chem. Int. Ed., {\bf 46}, 8903 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U21.00012: NMR Study of Organic Counterion Binding to Perfluorinated Micellar Structures Dobrin Bossev, Mustuo Matsumoto, Masaru Nakahara In this study we have applied our previously developed NMR method to study the adsorption of tetramethylammonium (TMA$^{+})$ and tetraethylammonium (TEA$^{+})$ counterions to micelles formed by perfluorooctylsulfonate (FOS$^{-})$ surfactant in water at 30 \r{ }C. These two counterions induce formation of threadlike surfactant structures that result in well pronounced viscoelastic properties of the solution. To selectively probe the degree of counterion binding we have used $^{1}$H and $^{19}$F NMR chemical shifts and self-diffusion coefficients that are sensitive to the \textit{Stern} and \textit{diffuse double layers}, respectively. The competitive adsorption of TMA$^{+}$ and TEA$^{+}$ was examined as a function of the TMA$^{+}$/TEA$^{+}$ ratio at a constant FOS$^{-}$ concentration of 100 mM. The two counterions were found to form Stern layer around the FOS$^{-}$ micelles with comparable packing; about one counterion per two micellized FOS molecules. When mixed at intermediate proportions, however, the TEA$^{+}$ counterion shows preferential binding; the concentration of TEA$^{+}$ in the \textit{Stern layer} is found to be twice higher than that of TMA$^{+}$ at equal total respective concentrations in the solution. These results are discussed in terms of counterion size and hydrophobicity and presented in parallel with those that involved the smaller and more hydrophilic lithium counterion. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U21.00013: Design and Fabrication of Micro-textures for Inducing a Superhydrophobic Behavior on Hydrophilic Materials Di Gao, Liangliang Cao, Anmin Cao, Hsin-Hua Hu Artificial superhydrophobic surfaces are typically fabricated by tuning the surface roughness of intrinsically hydrophobic surfaces. We here report the design and fabrication of micro-textures for inducing a superhydrophobic behavior on intrinsically hydrophilic hydrogen-terminated Si surfaces with an intrinsic water contact angle of about 74 degree. The micro-textures consist of overhang structures with well-defined geometries fabricated by microfabrication technologies, which provide positions to support the liquid and prevent the liquid from entering the indents between the micro-textures. As a result, water is in contact with a composite surface of solid and air, which induces the observed macroscopic superhydrophobic behavior. The principle is applied to fabricate non-aging superhydrophobic surfaces by packing flower-like micrometer-sized hematite particles. The as-fabricated superhydrophobic surfaces do not age even in extremely oxidative environments---they retain the superhydrophobicity after being stored in ambient laboratory air for 4 months, heated to 800 degree C in air for 10 hours, and exposed to ultraviolet ozone for 10 hours. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U21.00014: Heterogeneity and Fluctuations in Electrochemical Sensors Jean-Luc Fraikin, Michael Requa, Michael Stanton, Andrew Cleland Metal electrodes submerged in aqueous electrolytes biased with very small voltages frequently display a capacitive low-frequency electrical impedance, which is primarily imaginary but typically displays a 1/$f ^{\alpha }$ frequency dependence, with 0.7 $\le \quad \alpha \quad \le $ 1. This electrode-electrolyte interface is phenomenologically modeled as a constant phase element (CPE). There are a number of explanations for the observed frequency dependence, including geometric arguments based on the assumption of fractal surface geometries, but it is difficult to quantitatively match such models to experiment. We propose a new model to explain this phenomenon, as well as other low frequency electrical characteristics of the electrode-electrolyte interface, using a model that relies on microscopic heterogeneity, allowing for local variations in capacitance and diffusion coefficients. We will present the basic aspects of our model, and describe measurements under way to validate this model, using a combination of impedance measurements and electrochemical noise spectroscopy. [Preview Abstract] |
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