Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z45: Porous Media, Fibers, and Flow |
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Sponsoring Units: DPOLY Chair: Jonathan Brown, Ohio State University Room: 216AB |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z45.00001: Methane Adsorption and Diffusion in Nanoporous Gray Shale by QENS Bo Wang, Matthew Byran, Garfield Warren, Paul Sokol Adsorption of methane in nano-porous structures is of great important for applications such as shale gas extraction. In this work, methane adsorption and diffusion in gray shale is studied by Quasi-elastic Neutron Scattering (QENS) on the CNCS at the Spallation Neutron Source. The shale, which was characterized by nitrogen adsorption isotherms and X-ray diffraction, has an average pore size of 5.6 nm and pore volume of 0.038cm3/g. Adsorption studies were carried out at 106 K as a function of pressure between 0 and 1.5 bar. Diffusion studies were carried out at temperatures between 50 K and 190 K at several methane filling fractions. Diffusion constants obtained using several different diffusion models will be discussed. We find that the microscopic diffusion of methane depends on the filling fraction inside the pore and the jump length is comparable to the scale of confinement. This report was prepared by Indiana University under award 70NANB10H255 from the National Institute of Standards and Technology (NIST), U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of the NIST or the U.S. Department of Commerce. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z45.00002: Enhanced Methanol Diffusion in Homogeneous Isotropic and Anisotropic Silica Aerogels Jeongseop A. Lee, Yizhou Xin, A.M. Zimmerman, Yang Shen, William Halperin It has recently been shown that the ballistic mean free path of silica aerogels can be directly measured by measuring diffusion of adsorbed fluids that are in fast exchange with vapor state using nuclear magnetic resonance [1-2]. With this technique we have studied the effect of compression on the mean free path of radially shrunken and isotropic silica aerogels with 98\% porosity. We have found unusual behaviors of pore geometry as a function of compression. Our preliminary findings suggest that the average pore geometry and the corresponding anisotropy thereof do not follow classical stress-strain relations suggesting more complex mechanism is at play. \\[4pt] [1] Jeongseop A. Lee, et al., Phys Rev. B 90, 174501 (2014). \newline [2] F. D'Orazio, et al., Phys. Rev. Lett. 63, 43 (1989). [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z45.00003: Diffusive and Rotational Dynamics of Condensed $n$-H$_2$ Confined in MCM-41 Paul Sokol, Matthew Bryan, Timothy Prisk We report neutron scattering studies of the condensed phases of normal-hydrogen confined within MCM-41. This is a high surface area, mesoporous silica glass with a narrow pore size distribution. The neutron scattering data suggests a picture of condensed normal-hydrogen within small mesopores in which the adsorbed hydrogen may be conceptually divided into an interfacial layer and the inner core volume. Preferential adsorption of ortho-hydrogen makes the interfacial layer rich in ortho-hydrogen, while the inner core volume consists of a depleted mixture of ortho- and para- hydrogen. In the liquid state, the hydrogen molecules making up the interfacial layer are immobile and tightly bound to adsorption sites, unable to diffuse on picosecond time scales. Molecules within the inner core volume undergo liquid-like jump diffusion, but with residence times much longer than the bulk fluid. In the solid state, only the hydrogen molecules occupying the inner core volume show the free quantum rotor behavior characteristic of the bulk crystal. The ortho-hydrogen molecules bound to the pore walls experience an orientational hindering potential which perturbs their rotational energy levels. Comparison with Vycor suggests the pore walls of MCM-41 are smoother on the atomic-scale. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z45.00004: Molecularly thin metal organic framework (MOF) film at air-water interface: Fabrication and buckling under compression Pritam Mandal, Sahraoui Chaieb A metal organic framework (MOF) - a hybrid of inorganic (metal) nodes and organic linkers - is an emerging class of highly crystalline porous materials that provide an extremely high surface area/volume ratio making them very suitable candidates in selective separation, filtration and storage of gases. Though MOFs are usually produced as powders, for many applications such as selective gas separation and filtration, MOFs as flat membrane is the most appropriate candidate. To our knowledge no large-scale fabrication of 2D MOFs has been reported. In this work, we prepared large area MOF film at an air/water interface and employed Brewster angle microscopy (BAM) to directly image the film-formation (surface pressure during and after the film-formation was tracked, although this measurement for a solid film is not accurate due to the elasticity of the film). Metal-ligand coordination was confirmed through FTIR results. Surface morphology as seen via scanning electron microscopy (SEM) shows the film was smooth over several hundred microns. Finally, we discuss the buckling/fracture of the MOF film due to compression by two symmetrically movable barriers, which hinted to the existence of a solid molecularly thin film. [Preview Abstract] |
(Author Not Attending)
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Z45.00005: Mechanism of breakup of electrospun jet at the nozzle Kostya Kornev, Vladislav Vekselman, Gilles Mohl In the electrostatic formation of nanofibers, a high voltage is applied to a polymer solution to withdraw it through the nozzle as a jet. We report on an interesting instability leading to the jet breakup right at the supporting meniscus making the generation of sequential jets sporadic and inconsistent. The conventional electrospinning setup was modified to provide visualization of the meniscus inside the nozzle. A glass capillary was used as a nozzle. A wire connecting a high-voltage power supply was inserted into a tube and secured inside the glass capillary. The end of the wire was positioned far away from the free end of the glass capillary where meniscus was formed. In this method, the body of polymer solution was held at the same potential. Using fluorescence microscopy it was observed that meniscus is very sensitive to the applied voltage and is able to move inside the nozzle as the voltage increases. As meniscus moves inside the nozzle, it forms a liquid film leaning to the inner wall of the nozzle. This film connects meniscus and the jet. The jet breakup is caused by the thinning and rapturing of this film. A simple scaling model showing interplay between the capillary pressure and Maxwell stress is proposed and used to explain the jet instability. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z45.00006: Atomic scale images of polyvinylidene fluoride nanofibers by electron microscopy Darrell Reneker, Christian Kisielowski, George Chase, Dinesh Lolla, Joe Gorse Atomic scale electron micrographs of polyvinylidene fluoride (PVDF) molecules in thin ($\sim$ 3 nm) nanofibers revealed twist around the axes of molecular chains, small relative motions of adjacent molecular chains and many other structural and dynamical phenomena. The positions and relative motions of CF2 groups, spaced 0.25 nm apart, on (PVDF) molecules, were followed along polymer segments. Atomic scale, aberration corrected electron microscopy is presently at its best when the sample is less than about 3 nanometers thick. Conformations of segments of polymer molecules, and the relations between more ordered and less ordered segments are displayed in this thickness range. The TEAM 0.5 aberration corrected microscope at the Lawrence Berkeley Laboratory ``Molecular Foundry,'' was used to create hundreds of high magnification images of PVDF molecules in nanofibers, at electron doses much smaller than the doses that produce extensive chain scission or other such chemical changes in the molecules. A commonly held view, that useful high magnification electron micrographs of polymer molecules cannot be obtained without causing overwhelming changes to the molecule, is misleading. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z45.00007: Flow in wavy walled microchannels Raghavi Rao, Pushpavanam S Improving the efficiency of mass transfer processes in microchannels is of significance in many applications. Here the flow is primarily laminar and mass transfer occurs by diffusion. Making the surface of the channel rough allows us to induce convection in transverse directions to the flow. In this work, we will study the effect of periodically varying surface of a channel wall using a perturbation solution. The modification in the velocity profile and its effect on improving mass transfer will be analyzed. The analysis will be illustrated for the case of extraction of a solute from one liquid phase to another. The effect of various parameters like surface tension, density ratio and viscosity ratio of the fluids will be studied and the regions in parameter space where a significant improvement in performance is expected will be determined. The results of the perturbation series solution will be compared with numerical results. Two cases, that of a constant and a deformed interface between the liquid phases, will be analyzed. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z45.00008: Micropillar sequence design for inertial fluid flow sculpting Daniel Stoecklein, Baskar Ganapathysubramanian, Chueh-Yu Wu, Dino Di Carlo New methods for controlling fluid flow in microchannels make use of inertial fluid flow deformation around pillar structures spanning the height of the channel. A small set of micropillar sizes and locations has been shown to produce a rich phase space with a wide variety of flow transformations, demonstrating the untapped wealth of possibilities in this fluid flow manipulation scheme. Previous work has successfully demonstrated the potential, with experimental validation, for manual hierarchical design where sequences of pillars are stacked to create flow sculpting. But such a method is not ideal for seeking out complex sculpted flows where the search space quickly becomes too large for efficient manual discovery. This is further complicated by the non-uniqueness of pathways to a desired fluid transformation, and the effect of diffusion as the number of micropillars increases. We formulate the inertial flow transformation as a set of state transition matrix operations. This allows rapid simulation of different design configurations, enabling efficient optimization over a large search space. We show how this framework can be used for novel fluid sculpting targets with application in microelectronics and medical diagnostics, and also show validation via confocal imaging. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z45.00009: Three-dimensional microstreaming flows Bhargav Rallabandi, Alvaro Gomez-Marin, Massimiliano Rossi, Cheng Wang, Christian Kaehler, Sascha Hilgenfeldt Streaming due to acoustically excited bubbles has been used successfully for applications such as size-sorting, trapping and focusing of particles, as well as fluid mixing. Many of these applications involve the precise control of particle trajectories, typically achieved using cylindrical bubbles, which establish planar flows. Using astigmatic particle tracking velocimetry (APTV), we show that, while this two-dimensional picture is a useful description of the flow over short times, a systematic three-dimensional flow structure is evident over long time scales. We demonstrate that this long-time three-dimensional fluid motion can be understood through asymptotic theory, superimposing secondary axial flows (induced by boundary conditions at the device walls) onto the two-dimensional description. Beyond microbubble streaming, this leads to a general framework that describes three-dimensional flows in confined microstreaming systems, guiding the design of applications that profit from minimizing or maximizing these effects. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z45.00010: Retarded desorption from porous media caused by wetting/dewetting of the external surface Thomas Lee, Benoit Coasne, Roland J.-M. Pellenq, Franz-Josef Ulm, Lyd\'eric Bocquet Despite the increasing attention devoted to nanofluidics, the role of external surfaces on transport in nanopores has not been fully investigated. Here, we report molecular dynamics simulations showing that methane recovery from a hydrophobic nanoporous membrane is retarded by the presence of liquid water at the external surface. Despite the pressure gradient used to trigger methane desorption and the hydrophobicity of the membrane, methane remains trapped for long times until water desorbs from the external surface. Using umbrella sampling calculations, we show that this retardation effect is induced by the free energy cost of dewetting the external surface as water is replaced by an adsorbed methane film. To account for this effect, we propose a simple thermodynamic model which describes the increase in free energy during extraction (dominated by the large methane-water surface tension). We also extend our approach to hybrid external surfaces made up of hydrophilic/hydrophobic regions, and consider fluids other than water (CO$_2$), as these are relevant to practical applications such as gas/oil recovery from shale. Such retarded processes also have implications for transport in nanofluidic systems and adsorption/transport in solid catalysts, chromatographic devices, etc.. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z45.00011: Particle Migration and Sorting in Microbubble Streaming Flow Sascha Hilgenfeldt, Raqeeb Thameem, Bhargav Rallabandi, Rui Yang Ultrasonic driving of sessile semicylindrical bubbles results in powerful steady streaming flows that are robust over a wide range of driving frequencies. In a microchannel, this flow field pattern can be fine-tuned to achieve size-sensitive sorting and trapping of particles at scales much smaller than the bubble itself. The sorting process is passive and relies on large forces induced by the streaming flow that lead to particle migration across streamlines. While a thresholded size sorting can be understood from simple arguments about the steady streaming flow geometry, we show that more sophisticated size separation is achievable through a better understanding of the flow at short time scales. We present experimental data (high-speed videography) and theoretical analysis (asymptotic description of the flow fields) that show how particles undergo significant migration and separation on time scales ranging from milliseconds to microseconds, and on length scales of about $10\mu$m. The highly tunable experimental set-up and the accurate analytical description of the flow field make this system an extremely fast and versatile device for applications ranging from flow cytometry to lab-on-a-chip micromanipulation. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z45.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z45.00013: Effects of the uniaxial elongation of a polymer/CNT fiber on its electrical properties Hyun Woo Cho, Bong June Sung We elucidate the effects of the uniaxial elongation of a polymer/CNT fiber on its electrical properties. Polymer fibers containing conductive nanofillers (such as carbon nanotubes (CNTs), and silver nanoparticles) have been utilized extensively for fabricating various forms of stretchable electronics including artificial muscles or electric conductive fabric. The electric conductivity of the polymer fiber usually decreases when it is stretched along the fiber axis, which would limit the scope of application. In addition, the reason and mechanism of decrease in the electrical conductivity remain elusive. In this work, we employ a coarse-grained model for the polymer/CNT fiber, and obtain the configurations of the fiber with respect to the strain via dynamic Monte Carlo (MC) simulations. Using global tunneling network (GTN) model, we calculate the electric conductivity as a function of strain. We find that the electric conductivity decreases during the elongation of the polymer/CNT fiber as was in experiments. We also find from tunneling network diagrams and critical path approximation (CPA) that the topological structure of the electrical network of the CNTs changes collectively during the elongation, which is responsible for the reduction of the electrical conductivity. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z45.00014: High thermal conductivity polymers Mortaza Saeidijavash, Jivtesh Garg In this work we investigate the effect of mechanical stretching on polymer thermal conductivity. Polymer materials are used as electrical insulators, but their poor thermal conductivity also makes them thermal insulators making removal of heat generated in such electronic systems challenging. Enhancement of polymer thermal conductivity can allow for better thermal management including design of low cost heat sinks and compliant thermal interface materials. Stretching is known to induce alignment of molecular chains in a polymer system increasing thermal conductivity. In this work we explore this idea by mechanically stretching ultra-high molecular weight (UHMW) polyethylene bars using a tensile load cell. The in-plane thermal conductivity of stretched polymer is measured using laser-flash method. We have measured thermal conductivity enhancement of almost 100{\%} for stretch ratios of 6 to 10. These result are consistent with previous studies of thermal conductivity enhancement through such stretching. Ways to chemically achieve this molecular alignment are being explored using techniques of spin coating and electrospinning. [Preview Abstract] |
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