Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session BN: Nano Fluids II |
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Chair: Richard Craster, Imperial College London Room: Salt Palace Convention Center 251 B |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BN.00001: Molecular dynamics simulation of water inside and outside carbon nanotubes John Thomas, Alan McGaughey The behavior of liquid water inside and outside 1.1 nm, 2.8 nm, 6.9 nm, and 10 nm diameter armchair carbon nanotubes (CNT) is predicted using molecular dynamics simulation. The effect of CNT diameter on mass density and distribution, molecular orientation, and the self-diffusion coefficient tensor are identified for both the confined and unconfined fluids. Within 1 nm of the CNT surface, unconfined water molecules assume a spatially varying density profile. The molecules distribute non-uniformly around the carbon surface and align parallel to the CNT centerline. This results in a non-uniform self-diffusion coefficient tensor near the solid. The behavior of the unconfined water molecules is invariant with CNT diameter. The behavior of the confined water is correlated to CNT diameter. Inside the 10 nm tube, the molecular behavior is indistinguishable from that of the unconfined fluid. Within the smaller tubes, surface curvature effects force water molecules away from the surface and extend the influence of the solid. These spatially varying static properties influence the self-diffusion tensor within the tube. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BN.00002: Aspect-ratio effects on the electrorheology of dilute carbon-nanotube suspensions Chen Lin, Jerry W. Shan The electrorheology of a dilute, single-wall-carbon- nanotube(SWNT)/alpha-terpineol suspension under external electric fields was experimentally investigated. The apparent viscosity of the suspension at SWNT volume fraction $\phi = 1.5 \times 10^{-5}$ was found to more than double at moderate shear rates under a field of strength 160 V/mm. The electrorheological response is interpreted in terms of an electrostatic-polarization model, where the governing parameter is a modified Mason number giving the ratio of viscous to dipole-dipole forces. A scaling analysis suggests that the magnitude of the electrorheological response in the dilute SWNT suspension, which is much higher than conventional electrorheological fluids at comparable volume fractions, is due to the high aspect ratio of the nanotubes. Comparison is made to a suspension of glassy carbon spheres, in which a three- order-of-magnitude-higher volume fraction is required to achieve similar increases in the apparent viscosity under the same conditions. [Preview Abstract] |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BN.00003: Hydrogen Transport And Storage Inside Carbon Nanotubes In Presence Of Encapsulated Metal Ions. Soumik Banerjee, Ishwar Puri We investigate the hydrogen storage potential of carbon nanotubes (CNTS) through fundamental molecular dynamics (MD) simulations. We suggest possible changes in the structure and conditions of CNTS that could enhance their storage capacity. Our parametric investigation involves the variation of crucial parameters that influence hydrogen storage in carbon nanostructures, i.e., (1) pressure; (2) temperature; and (3) metal particle encapsulation. Our MD simulation results suggest that increased pressure and low temperature improve hydrogen storage inside carbon nanotubes, which is intuitive. We also obtain a more novel result, i.e., that the presence of encapsulated metal ions inside CNTS is a vital factor that considerably modifies their adsorption characteristics, enhancing storage. In addition, we determine that an open-ended nanotube stores more hydrogen than a closed nanotube. [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BN.00004: Quantifying the Dynamics of Thermoelastically Driven Nanoscale Beams in Fluid. Margarita Smith, Matthew Clark, Mark Paul A current technological challenge is the efficient on-chip actuation and detection of nanoscale beams in liquid that retain a high frequency response and quality factor. A promising approach is the use of thermoelastically driven doubly-clamped beams. ~As the beam dimensions decrease the dominance of viscous drag lowers performance resulting in reductions in the resonant frequency and quality factor. However, the quality factor increases with increasing frequency parameter. We explore several strategies to exploit this for nanoscale doubly-clamped beams. First, we tailor the beam geometry to maintain a large resonant frequency while minimizing fluid dissipation and therefore increasing the quality factor. Second, we include pre-tension in the beam to increase the frequency of oscillation. We discuss results from full numerical simulations for the precise conditions of interest. These results are compared with approximate analytical theory valid for long and thin beams. We use our findings to present a quantitative description of the dynamics and to suggest the parameter range of interest for future experiment. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BN.00005: A Technique for Estimating the Surface Conductivity of Single Molecules Haim Bau, Mark Arsenault, Hui Zhao, Prashant Purohit, Yale Goldman When an AC electric field at 2MHz was applied across a small gap between two metal electrodes elevated above a surface, rhodamine-phalloidin-labeled actin filaments were attracted to the gap and became suspended between the two electrodes. The variance of each filament's horizontal, lateral displacement was measured as a function of electric field intensity and position along the filament. The variance significantly decreased as the electric field intensity increased. Hypothesizing that the electric field induces electroosmotic flow around the filament that, in turn, induces drag on the filament, which appears as effective tension, we estimated the tension using a linear, Brownian dynamic model. Based on the tension, we estimated the filament's surface conductivity. Our experimental method provides a novel means for trapping and manipulating biological filaments and for probing the surface conductance and mechanical properties of single polymers. [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BN.00006: Nano-Rheology: Stress Shielding and Stick-Slip Dynamics Xinguang Cheng, Hsueh-Chia Chang A molecular Langevin theory explains the rich and nonlinear viscoelastic rheology exhibited by monolayers and bilayers of water confined between two charged mica surfaces. Elastic storage endowed by asymmetric water-surface and water-water interaction is shown to produce a curious stress shielding phenomenon. Noise-sensitive stick-slip dynamics occurs when the surface speed is comparable to the molecular equilibration speed, with distinct hopping statistics between surface sites captured by a Fokker-Planck analysis. At large displacement, two-time asymptotics shows that sliding dynamics over multiple sites is responsible for the viscous properties but the elastic component is due to slow near-equilibrium dynamics at the slow intervals. Scaling theories for the rheological moduli are favorably compared to literature data. Both stress shielding and slip at large amplitudes are responsible for the 1e4-1e5 order difference in reported viscosity. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BN.00007: The spreading of droplets of nanoparticulate suspensions Richard Craster, Omar Matar, Khellil Sefiane The spreading or thinning of even simple fluids, micellar- or particle-laden solutions are often accompanied by terracing in the free surface at the advancing contact line or stepwise thinning. Several notable examples appear in the literature: the terraced spreading of nanodroplets showing the advance of the droplet edge as molecular layers, the stepwise thinning of liquid films of micellar solutions And, more recently, in the detachment of oil droplets by nanoparticle laden solutions. This talk explores the dynamics of this latter example by generating a dynamical model exploiting structural disjoining pressures. We show that a distinct step (of the diameter of a nanoparticle) emanates from the contact line and advances with constant velocity; this is broadly in line with experimental observations. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BN.00008: STM-Controlled Capillary Based Non-Contact Fluid Deposition Nanolithography Artur Lutfurakhmanov, Rob Sailer, Doug Schulz, Iskander Akhatov A new method of fluid deposition based on scanning tunneling microscopy (STM) is presented. STM-Controlled Capillary Based Non-Contact Fluid Deposition Nanolithography consists of a Au-coated glass nanocapillary tip integrated into a commercial STM scanner platform where the tip serves the dual purpose of imaging and deposition. The small diameter hollow fiber (O.D. less than 500 nm) coupled with a conducting coating allows sub-angstrom-level z-resolution imaging using standard STM methodology. For fluid deposition, the tip is first located within 10 nm of the substrate before the nanocapillary is pressurized with a fluid (P = 50-500 KPa) leading to the formation of a small meniscus that then interacts with the underlying surface to give small spot of fluid deposition. Initial results show the ability to form features less than 500 nm in diameter using alpha-terpineol as the model fluid and highly-oriented pyrolytic graphite as the substrate. In addition to non-contact deposition, this technology also allows non-contact imaging using the constant height STM mode thereby eliminating the difficulties associated with finding nanometer-sized features. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BN.00009: DNA translocation through nanopores: effect of salt concentration Sandip Ghosal Recent experiments on the detection of single molecules of linear polyelectrolytes through nanopores could lead to an ultrafast and inexpensive method of rapidly sequencing linear polymer chains such as DNA and RNA. In earlier work (see Ghosal, S. in APS DFD06 abstracts) a hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through an axially symmetric slowly varying nanopore was presented in the limit of vanishingly small Debye length. Here the case of a finite Debye layer thickness is considered within the Debye-H\"{u}ckel and Stokes flow approximations while restricting the pore geometry to that of a cylinder of length much larger than the diameter. Further, the possibility of a uniform surface charge on the walls of the nanopore is taken into account. The model admits an exact analytical solution from which translocation times are calculated and found to be consistent with recent measurements in solidstate nanopores. It is suggested, based on the solution to the model problem, that the translocation speed can be greatly reduced if the $\zeta$-potential of the nanopore walls could be fine tuned to match closely the $\zeta$-potential of the polyelectrolyte. Physically this amounts to balancing the net electrical force on the polyelectrolyte inside the pore by the viscous drag of the electroosmotic flow inside the nanopore. Reduction of the translocation speed by several orders of magnitude is essential for achieving single base resolution. [Preview Abstract] |
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