Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S30: Monolayers, Membranes & Microemulsions |
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Sponsoring Units: DFD Chair: Alex Levine, University of California, Los Angeles Room: Colorado Convention Center 304 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S30.00001: Structure, Wrinkling, and Reversibility of Langmuir Monolayers of Gold Nanoparticles Binhua Lin, David Schultz, Xiao-Ming Lin, Dongxu Li, Mati Meron, Jeff Gebhardt, P. James Viccaro The assembly of nanoparticles into large, two-dimensional structures provides a route for the exploration of collective phenomena among mesoscopic building blocks. We characterize the structure of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with \textit{in situ }optical microscopy and X-ray scattering. The interparticle spacing increases with thiol concentration and does not depend on surface pressure. The correlation lengths of the Langmuir monolayer crystalline domains are on the order of five to six particle diameters. Further compression of the monolayers causes wrinkling; however, we find that wrinkled monolayers with excess thiol can relax to an unwrinkled state following a reduction of surface pressure. A theoretical model based on van der Waals attraction and tunable steric repulsion is adopted to explain this reversibility. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S30.00002: Experimental and theoretical studies of collapsed fatty-acids Langmuir monolayers Wei Bu, Chris Lorenz, Alex Travesset, David Vaknin, Sushil K. Satija Long-chain Langmuir monolayers collapse by exploring the third dimension after being compressed beyond the point of densely packed chains. Recent experimental investigations using surface sensitive X-ray and neutron techniques have shown that arachidic acid (AA) monolayers, spread on pure water surfaces, collapse by forming a trilayer structure that exhibits a remarkable degree of crystalline order. Similar experiments of AA spread on CaCl$_2$ solutions show that the collapsed film consists of a mixture of hydrophobic bilayer domains (where hydrocarbon chains are in contact with water) and trilayer domains. Under suitable experimental conditions, monolayer collapse on CaCl$_2$ solution can produce an almost pure bilayer phase. We present atomistic simulations that account for the role of water, ion binding, and hydrocarbon chain conformations to better understand these experimental results. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S30.00003: Dynamic spiral patterns in Langmuir monolayers of chiral molecules Lena Lopatina, Jonathan V. Selinger Experiments with Langmuir monolayers of chiral molecules on a water surface report a collective propeller-like precession of the molecules due to the evaporation of water [1]. If the molecular orientation is pinned along an edge, the precession leads to a series of stripes along the edge. This pattern formation has been explained by a dynamic equation due to the Lehmann effect [2]. Here, we consider how the patterns change if the monolayer contains vortices, topological defects which pin the molecular orientation. We model an annular ring with a single vortex at its center, and show that the director field forms a spiral centered at the defect, which reverses handedness between the inner and outer boundaries. We also simulate a system with one vortex and one anti-vortex on a lattice, and find that the defects form spirals with opposite handedness. These analytic and computational results are in good agreement with preliminary experiments [3]. \newline [1] Y. Tabe, H. Yokoyama, Nat. Mater. \textbf{2}, 806 (2003). \newline [2] D. Svensek, H. Pleiner, H. R. Brand, Phys. Rev. Lett. \textbf{96}, 140601 (2006). \newline [3] K. A. Suresh, private communication. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S30.00004: Effects of topology and curvature on the hydrodynamics of membranes and interfaces Mark L. Henle, A.J. Levine, Ryan McGorty, A.D. Dinsmore Understanding membrane and interfacial hydrodynamics is vital for a variety of biological systems and technological applications. Within the cell membrane, for example, the diffusion of proteins is essential for cell-cell signaling. For many of these applications, the membrane/interface is spherical. Such a geometry imposes a \textit{global} topological constraint that, for instance, forces the velocity field on an incompressible membrane to have two vortices. In addition, the \textit{local} membrane curvature strongly modifies particulate transport when it is comparable to the Saffman-Delbr\"{u}ck length (the ratio of the membrane viscosity to the viscosity of the surrounding fluid). In this talk, we present both experimental and theoretical results on the motion of extended objects (rods) in spherical membranes. The experiments investigate the motion of colloidal rods trapped on the surface of a water-in-oil droplet decorated with nanoparticles; the analytic theory solves for the rod mobility as well as the flows in the membrane and the surrounding fluids caused by the motion of such rods. We find that the topology of the membrane can indeed have a significant effect on the dynamics of the rod, and that our theoretical description agrees quantitatively with the experimental results. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S30.00005: Langmuir-Gibbs Surface Phases and Transitions Benjamin Ocko, Eli Sloutskin, Zvi Sapir, Lilach Tamam, Moshe Deutsch, Colin Bain Recent synchrotron x-ray measurements reveal surface ordering transitions in films of medium-length linear hydrocarbons (alkanes), spread on the water surface. Alkanes longer than hexane do not spread on the free surface of water. However, sub-mM concentrations of some anionic surfactants (e.g. CTAB) induce formation of thermodynamically stable alkane monolayers, through a ``pseudo-partial wetting'' phenomenon[1]. The monolayers, incorporating both water-insoluble alkanes (Langmuir) and water-soluble CTAB molecules (Gibbs) are called Langmuir-Gibbs (LG) films. The films formed by alkanes with $n \leq 17$ exhibit ordering transition upon cooling [2], below which the molecules are normal to the water surface and hexagonally packed, with CTAB molecules randomly mixed inside the quasi-2D crystal. Alkanes with $n>17$ can not form ordered LG monolayers, due to the repulsion from the $n=16$ tails of CTAB. This repulsion arises from the two chains' length mismatch. A demixing transition occurs upon ordering, with a pure alkane quasi-2D crystal forming on top of disordered alkyl tails of CTAB molecules. [1] K.M. Wilkinson \emph{et al.,} \emph{Chem. Phys. Phys. Chem.} \textbf{6}, 547 (2005). [2] E. Sloutskin, Z. Sapir, L. Tamam, B.M. Ocko, C.D. Bain, and M. Deutsch, \emph{Thin Solid Films}, in press; K.M. Wilkinson, L. Qunfang, and C.D. Bain, \emph{Soft Matter} \textbf{2}, 66 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S30.00006: Electrostatics of planar interfaces in salt solution William Kung, A.W.C. Lau, Monica Olvera de la Cruz We present an exact field-theoretic formulation for a fluctuating, generally asymmetric, salt density in the presence of a charged plate. The non-linear Poisson-Boltzmann equation is obtained as the saddle-point of our field theory action. Focussing on the case of symmetric salts, we systematically compute, in the weak-coupling limit, first-order correction to the free energy density, arising from electrolyte fluctuation, which can be explicitly obtained in closed form. We find that for systems with moderate salt density, fluctuation corrections to the free energy depends sensitively on the salt concentration as well as their charge valency. Further, we find that electrolyte fluctuation leads to a reduced electrostatic repulsion between two point charges when they are close to the plate. We also consider the application to interfaces separating two semi-infinite regions of different dielectric media. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S30.00007: Crystallography on Curved Surfaces Vincenzo Vitelli, Julius Lucks, David Nelson We present a theoretical and numerical study of the static and dynamical properties that distinguish two dimensional curved crystals from their flat space counterparts. Experimental realizations include block copolymer mono-layers on lithographically patterned substrates and self-assembled colloidal particles on a curved interface. At the heart of our approach lies a simple observation: the packing of interacting spheres constrained to lie on a curved surface is necessarily frustrated even in the absence of defects. As a result, whenever lattice imperfections or topological defects are introduced in the curved crystal they couple to the pre-stress of geometric frustration giving rise to elastic potentials. These geometric potentials are non-local functions of the Gaussian curvature and depend on the position of the defects. They play an important role in stress relaxation dynamics, elastic instabilities and melting. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S30.00008: The use of specular reflectivity of neutrons for the investigation of polymeric membranes Guedioura Bouzid, Bendjaballah Noueddine, Hamdi Maamar, Kerdjoudj Hacene A polymeric thin film membrane 5$\mu $m and 0.5$\mu $m thickness was prepared in order to study its physical characteristics with the neutron reflectivity method. The membrane is principally made of cellulose acetate (CA) matrix in within we fix the carrier. The neutron reflectivity profiles versus the momentum transfer measurements are done using Nur reflectometer. The vertical neutron reflectometer at Nur Reactor Algeria utilizes a fixed wavelength of 0.47nm and $^{3}$He gas detector; neutrons from the radial beam are reflected by one pyrolytic graphite crystals. Maximum thermal flux, after the monochromator, amounts to approximately 4.0*10$^{4}$ neutrons/cm$^{2}$s. The fit of the experimental data was made using the parett32 software program developed at H.M.I. This program computes optical reflectivity; both for neutron and X- ray, using different models based on momentum transfer Q values or fit measured sets of data. Calculations are carried out by the dynamical approach. The information, not only on layers thicknesses, but also on detailed shape of the scattering density profile is obtained. In a second part, the thin film membrane is submerged in an ionic solution of ZnCl$_{2}$ at 0.1 mole, this investigation revealed a modification of some internal layers of the membrane. Results are presented for the characterization of a membrane. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S30.00009: Coexistence of two colloidal crystals at the nematic liquid crystal-air interface A. Nych, V. Pergamenshchik, U. Ognysta, B. Lev, V. Nazarenko, M. Skarabot, I. Musevic, O. Lavrentovich Glycerol droplets at a nematic liquid crystal - air interface form two different lattices -- hexagonal and dense quasihexagonal -- which are separated by the energy barrier and can coexist. The director distortions around each droplet form an elastic dipole. The first order transition between the two lattices is driven by a reduction of the dipole-dipole repulsion through reorientation of these dipoles. The elastic-capillary attraction is essential for the both lattices. The effect has a collective origin. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S30.00010: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S30.00011: Nanoparticle-coated liquid-metal droplets: interfacial tension and electron transport across the interface Kan Du, B. Samanta, L. Glogowski, V. Rotello, M. Tuominen, T. Emrick, T. Russell, A. Dinsmore We form stable droplets of molten metal, investigate their stability, and demonstrate their potential for forming electronic devices. Droplets of liquid Ga, 0.1-100 microns in diameter, are suspended in water and stabilized by Au or Fe$_{3}$O$_{4}$ nanoparticles. We measure a large reduction of the surface tension of the Ga droplets when nanoparticles assemble at the interface. To investigate electron transport through the Ga-nanoparticle-Ga junction, we deposit coated droplets on substrates with patterned electrodes. We apply a bias voltage to the electrodes and measure the current after evaporation of the solvent. The nonlinear I-V curve shifts with a gate voltage and indicates a transistor is formed in the junction. Improved understanding of the electrical characteristics may allow inexpensive assembly of a large number of functional devices. We acknowledge support from the Center for UMass/Industry Research on Polymers (CUMIRP) and from NSF NIRT program (CTS-0609107). [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S30.00012: Strong Attractions with Controllable Size between Hydrophilic Inorganic Macroanions and Reversible Supramolecular Formations Melissa Kistler, Anish Bhatt, Guang Liu, Tianbo Liu The polyoxometalate (POM) hydrophilic macroionic solutions, offer a direct connection between traditional fields of simple inorganic ions, colloidal suspensions, polyelectrolytes, particularly proteins and DNAs. Many types of POM macroanions are highly soluble, but undergo reversible self-assembly to form uniform, stable, soft, single-layer vesicle-like ``blackberry'' structures containing $>$1000 individual POMs in dilute solutions. Blackberry structures represent a new state of soluble inorganic ions. The driving forces of the POM self-assembly are unlike those of surfactant micelles or colloid aggregates. The POM driving forces are most likely counterion-mediated attraction (like-charge attraction). Blackberry size is controlled by the solvent quality, or the charge density of macroions. Blackberry structures may be analogous to virus shell structures formed by capsid proteins. Unexpected phenomena have been observed in the novel POM systems. References: JACS. 2005, 127, 6942; 2003, 125, 312; 2002, 124, 10942. Nature, 2003, 426, 59. J. Clust. Sci, 2006, 17, 427. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S30.00013: Imaging the structure of water near hydrophobic solutes Gerard C. L. Wong, Robert H. Coridan, Ghee Hwee Lai, Nathan S. Schmidt, Michael Krisch, Peter Abbamonte Theoretical studies of the structure of interfacial water on the surface of hydrophobic solutes show a strong dependence on the radius of the solute itself. At small radii, a hydrogen-bond network is still capable of forming around the solute, generally forbidding association between the solute molecules. At large radii water can no longer form a hydrogen-bond network around the solute molecule, resulting in the ``drying'' of the surface and a strong attraction between solute molecules. The crossover length between the two regimes is on the order of a nanometer. We will show that it is possible to make movies of water around hydrophobic solutes of varying size by extracting the density propagator from the dynamical structure factor measured via high-resolution inelastic x-ray scattering spectra at 3rd generation synchrotron sources. [Preview Abstract] |
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