Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y38: Surfaces, Interfaces, and Colloids I |
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Sponsoring Units: DCP Chair: Sunil Sainis, Rowland Institute, Harvard University Room: 410 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y38.00001: Aqueous Solutions on Silica Surfaces: Structure and Dynamics from Simulations Alberto Striolo, Dimitrios Argyris, Naga Rajesh Tummala Our group is interested in understanding the properties of aqueous electrolyte solutions at interfaces. The fundamental questions we seek to answer include: (A) how does a solid structure perturb interfacial water? (B) How far from the solid does this perturbation persist? (C) What is the rate of water reorientation and exchange in the perturbed layer? (D) What happens in the presence of simple electrolytes? To address such topics we implemented atomistic molecular dynamics simulations. Recent results for water and simple electrolytes near silicon dioxide surfaces of various degrees of hydroxylation will be presented. The data suggest the formation of a layered aqueous structure near the interface. The density profile of interfacial water seems to dictate the density profiles of aqueous solutions containing NaCl, CaCl2, CsCl, and SrCl2 near the solid surfaces. These results suggest that ion-ion and ion-water correlations are extremely important factors that should be considered when it is desired to predict the distribution of electrolytes near a charged surface. Our results will benefit a number of practical applications including water desalination, exploitation of the oil shale in the Green River Basin, nuclear waste sites remediation, and design of nanofluidic devices. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y38.00002: Formation of Organic Peroxides and Ethers at Post-Discharge Plasma Plume-Liquid Interfaces Milan Begliarbekov, Steven Kotowich, Vladimir Tarnovsky A direct current (DC) micro-hollow cathode plasma source operating in a mixed glow-streamer regime was used to generate an atmospheric pressure N$_{2}$ discharge. The post-discharge plume / afterglow was interfaced with a target liquid-phase solution, and caused a change in the chemistry of the target solution. In the present work we study the interaction of an N$_{2}$ plume with a mixture of 2-methyl-1-propanol and hexane, which results in the formation of organic peroxides and ethers at the plume-liquid interface. The presence of the peroxide and ether functional groups is established by $^{1}$H-NMR, FTIR, and Raman spectra of the reaction products. Fast Atom Bombardment (FAB) mass spectrometry is also used to further characterize the reaction products. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y38.00003: Deducing 2D Crystal Structure at the Solid/Liquid Interface with Atomic Resolution by Combined STM and SFG Study Arthur McClelland, Seokhoon Ahn, Adam J. Matzger, Zhan Chen Supplemented by computed models, Scanning Tunneling Microscopy (STM) can provide detailed structure of 2D crystals formed at the liquid/solid interface with atomic resolution. However, some structural information such as functional group orientations in such 2D crystals needs to be tested experimentally to ensure the accuracy of the deduced structures. Due to the limited sensitivity, many other experimental techniques such as Raman and infrared spectroscopy have not been allowed to provide such structural information of 2D crystals. Here we showed that Sum Frequency Generation Vibrational Spectroscopy (SFG) can measure average orientation of functional groups in such 2D crystals, or physisorbed monolayers, providing key experimental data to aid in the modeling and interpretation of the STM images. The usefulness of combining these two techniques is demonstrated with a phthalate diesters monolayer formed at the 1-phenyloctane/ highly oriented pyrolytic graphite (HOPG) interface. The spatial orientation of the ester C=O of the monolayer was successfully determined using SFG. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y38.00004: Curved space crystallography at an oil-water interface William Irvine, Stefano Sacanna, Yael Roichman, Andrew Hollingsworth, Mark Elsesser, Mark Bowick, David Grier, Paul Chaikin We study two-dimensional crystallography on a curved oil-water interface. Charged hydrophobic (PMMA) colloids in an oil phase (cyclohexyl bromide) are attracted, without wetting, by image charge effects to an oil-water interface. The micron size spheres form a monolayer on the interface and interact via screened coulomb interactions to form a crystalline lattice. We create a curved oil-water interface by controlling wetting conditions between a water droplet and a substrate or support, to produce interfaces of both constant and varying gaussian curvature with boundary. We simultaneously image and manipulate the full crystal on the curved surface using a setup capable of simultaneous holographic optical tweezing and confocal imaging. We study the resulting dynamics of topological defects. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y38.00005: On the nanometer Gold projectile - surface interaction in SIMS experiments. Francisco A. Fernandez-Lima, Veronica Pinnick, Michael Eller, Stanislav Verkhoturov, Emille Schweikert In an effort to increase the secondary molecular ion signal under ion bombardment, a series of cluster sources have been developed with sputtering yields that deviate from the linear cascade prediction due to the collective cluster beam - surface interaction. In the present talk, the variation of the massive gold Au$_{n}^{q+}$ projectile size (n=1-400) and velocity on the interaction volume and number of desorbed/sputtered particles per impact will be discussed for alkali halide targets. As the projectile size increases, a larger number of cluster ions is observed, where the secondary ion yield can be describe as a decreasing exponential function of the cluster size. Theoretical ab initio calculations show that the relative MS abundances are related to the cluster structure stabilities, defined by a ``fine'' ratio of short and long range interactions between the cluster counterparts. Angular distribution measurements of the secondary ions suggest that under keV bombardment emission normal to the target surface is favored, independent of the cluster ion size. Applications of the massive gold Au$_{n}^{q+}$ projectiles as nanometric imaging probes ($<$ 10$^{4}$ nm$^{2})$ will be presented. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y38.00006: Binding energy of adsorbates on a noble-metal surface: Exchange and correlation effects Michael Rohlfing, Thomas Bredow We discuss the physisorption of atoms (xenon) and molecules (PTCDA) to a noble-metal surface (silver) within a first-principles approach, focusing on the adsorption energy as a function of distance. Instead of density-functional theory (which fails to describe physisorption) we employ a combination of exact exchange and correlation energies, which we evaluate within the adiabatic-connection fluctuation-dissipation theorem. Correlation accounts for non-local dispersion energy, which is crucial in the present cases. At large distance {$Z$} from the surface the correlation causes van der Waals attraction with a characteristic $-C_3/(Z$$-$$Z_0)^3$ asymptotic form. At closer distance the attraction deviates from the asymptotic form and, in combination with the repulsive exact-exchange energy, yields an equilibrium of xenon and of PTCDA on the Ag(111) surface in close agreement with experiment. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y38.00007: Bonding of adenine on Cu(110) Geoffrey Stenuit, Oksana Plekan, Vitaliy Feyer, Kevin Prince, Paolo Umari We present a density functional study of the adsorption of adenine molecules on the Cu(110) surface. In agreement with experimental core level photoemission and x-ray absorption data, our calculations predict the existence of two phases: a parallel one at low coverage and a perpendicular one at high coverage. These findings resolve contradictions between calculated geometries and published vibrational spectra, and illustrate the complexity of the interaction between a relatively simple bio-molecule and a metal. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y38.00008: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y38.00009: Two-photon photoemission spectroscopy of thiophene/Au(111). Jing Zhou, Nicholas Camillone, Michael White The electronic structure of thiophene adsorbed on Au(111) is investigated by two-photon photoemission (2PPE) spectroscopy and density functional theory (DFT) calculation. The adsorption of thiophene lowers the work function from 5.50 eV for clean Au(111) to 4.62 eV for Au(111) exposed to 4.0 L thiophene, due to the electron donation from the thiophene to the substrate. With thiophene adsorbed on Au(111), the surface state of Au(111) attenuates and a localized $\sigma $* starts to form with increasing thiophene exposure on Au(111). This $\sigma $* state is attributed to the $\sigma $* antibonding orbital of a Au-S bond and is evidence of an orientational phase transition of adsorbed thiophene. Preliminary 2PPE results will also be presented for aromatic molecules bound to the Au surface via sulfur or other functional goups (e.g., isocyanide). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y38.00010: Interfacial Structure imaging of Pentacene/Si(111) by model- independent method Songtao Wo, Hua Zhou, Randall Headrick, Alexander Kazimirov Synchrotron x-ray reflectivity is utilized to study the Pentacene/Si(111) interfacial structure in the direction normal to the surface. Model-independent algorithm is used to analyze the reflectivity data to extract the electron density profile. It indicates two partially ordered layers along the interfacial normal with thickness $\sim $0.6 nm and an interfacial water layer $\sim $0.9 nm as we reported in our previous work. A pentacene monolayer $\sim $1.6 nm can also be revealed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y38.00011: Optical Trapping of Colloidal Nanoparticles by a Weakly Focused Laser Beam Chun-Yu Lin, Hsia-Yu Lin, Shean-Jen Chen, Steven M.T. Wei, H.D. Ou-Yang We present an analysis of the behavior of an ensemble of colloidal nanoparticles in the focal region of a weakly focused laser beam. Using a mechanical balance of the laser radiation pressure that causes particle migration into the light field and the osmotic pressure of these particles opposing migration, we propose a new method for quantifying the optical trapping potential of individual particles by measuring the increase of the particle concentration as a function of the laser intensity. We find comparable results for the optical trapping potential from this method with values obtained by single particle trapping methods, indicating that radiation-induced particle convection from a weakly focused laser beam does not affect the steady state distribution of the particles in the light field. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y38.00012: Colloidal Crosstalk: Brownian Diffusion of Hydrodynamically Coupled Colloids Stephen Anthony, Minsu Kim, Steve Granick Except at dilute concentrations, the Stokes-Einstein Equation inadequately describes the thermal motion of colloids, due to hydrodynamic interactions between nearby particles. Using single-particle tracking, the rotational and translational motion of hydrodynamically interacting colloids is observed, and deviations from the Stokes-Einstein Equation are readily apparent. The thermal motion of nearby colloids is found to couple in a non-additive fashion, with increasing degrees of non-linearity as particle separation decreases. Similar coupling is observed for colloids near surfaces. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y38.00013: Depletion-Driven Selective Optical Trapping in Nanoparticle Suspensions Yi Hu, Joseph Junio, H.D. Ou-Yang We report results of an optical trapping study that demonstrates the effects of size-asymmetric particles in suspension have on optical trapping efficiency. Using a model colloid system with selective fluorescent dying and particle sizing, we show that the trapping efficiency of nanoparticles can be effectively tuned by adding different sized particles, promoting the use of optical trapping for particle sorting. A variable power IR laser coupled into a high NA objective was used for trapping. For particle detection, we used a 532nm excitation laser aligned to be parfocal with the IR trapping beam through the same objective lens. Fluorescent signals emanating from the focal region common to both beams was band-passed to a pinhole set to be conjugate to the focal region for confocal detection. In a system composed of 160nm and 63nm particles we demonstrated the synergistic effect of size mixing. Experimental results are also shown for fluorescent particles being driven out of the region by size selective trapping of undyed 160nm particles. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y38.00014: Colloid diffusion on phospholipid membranes is anomalous Bo Wang, Stephen Anthony, Sung Chul Bae, Steve Granick We demonstrate experimentally a simple system in which mean-squared displacement is proportional to time yet the distribution function of displacement probability is exponential, not Gaussian as expected for a classical random walk. This is the case of fluorescent submicron-sized beads that diffuse in one dimension in the smooth potential presented by tubes composed of phospholipid bilayer.~ A discussion of possible physical origins suggests that a family of physical systems whose few degrees of freedom couple to slow environmental fluctuations may behave analogously. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y38.00015: Determination of Charge Interactions of Nanoparticles by Optical Trapping Joseph Junio, H.D. Ou-Yang We report an experimental study of interactions in colloidal nanoparticles through optical trapping. Using an optical trap with a size much larger than the trapped particles, we were able to create an optical bottle to confine and concentrate the nanoparticles. We measured the highly focused light-induced particle density fluctuation with confocal fluorescent detection. A theory based on a balance between the optical trapping radiation pressure and the osmotic pressure has been developed to calculate the isothermal osmotic compressibility from the forced density fluctuation. The measured osmotic compressibilities of colloidal crystals are then used to determine the surface charge density of the colloidal particle(1). Comparison of the experimentally determined charge density is compared to t determined by zeta potential measurements. (1)S. Alexander, P. M. Chaikin, P. Grant, G. J. Morales, and P. Pincus, D. Hone, Charge renormalization, osmotic pressure, and bulk modulus of colloidal crystals: Theory, J. Chem. Phys. 80, 5776 (1984) [Preview Abstract] |
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