Charge Transport Transitions and Scaling in Disordered Arrays of Metallic Dots

We examine the charge transport through disordered arrays of metallic dots using numerical simulations. We find power law scaling in the current-voltage curves for arrays containing no voids, while for void-filled arrays charge bottlenecks form and a single scaling is absent, in agreement with recent experiments. In the void-free case we also show that the scaling exponent depends on the effective dimensionality of the system. For increasing applied drives we find a transition from 2D disordered filamentary flow near threshold to a 1D smectic flow which can be identified experimentally using characteristics in the transport curves and conduction noise.Comment: 4 pages, 4 postscript figure

Moving walls accelerate mixing

Mixing in viscous fluids is challenging, but chaotic advection in principle allows efficient mixing. In the best possible scenario,the decay rate of the concentration profile of a passive scalar should be exponential in time. In practice, several authors have found that the no-slip boundary condition at the walls of a vessel can slow down mixing considerably, turning an exponential decay into a power law. This slowdown affects the whole mixing region, and not just the vicinity of the wall. The reason is that when the chaotic mixing region extends to the wall, a separatrix connects to it. The approach to the wall along that separatrix is polynomial in time and dominates the long-time decay. However, if the walls are moved or rotated, closed orbits appear, separated from the central mixing region by a hyperbolic fixed point with a homoclinic orbit. The long-time approach to the fixed point is exponential, so an overall exponential decay is recovered, albeit with a thin unmixed region near the wall.Comment: 17 pages, 13 figures. PDFLaTeX with RevTeX 4-1 styl

Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications

Long-term solutions for the disposal of toxic wastes usually involve isolation of the wastes in a deep subsurface geologic environment. In the case of spent nuclear fuel, if radionuclide leakage occurs from the engineered barrier, the geological medium represents the ultimate barrier that is relied upon to ensure safety. Consequently, an evaluation of radionuclide travel times from a repository to the biosphere is critically important in a performance assessment analysis. In this study, we develop a travel time framework based on the concept of groundwater lifetime expectancy as a safety indicator. Lifetime expectancy characterizes the time that radionuclides will spend in the subsurface after their release from the repository and prior to discharging into the biosphere. The probability density function of lifetime expectancy is computed throughout the host rock by solving the backward-in-time solute transport adjoint equation subject to a properly posed set of boundary conditions. It can then be used to define optimal repository locations. The risk associated with selected sites can be evaluated by simulating an appropriate contaminant release history. The utility of the method is illustrated by means of analytical and numerical examples, which focus on the effect of fracture networks on the uncertainty of evaluated lifetime expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200

Robust plasmon waveguides in strongly-interacting nanowire arrays

Arrays of parallel metallic nanowires are shown to provide a tunable, robust, and versatile platform for plasmon interconnects, including high-curvature turns with minimum signal loss. The proposed guiding mechanism relies on gap plasmons existing in the region between adjacent nanowires of dimers and multi-wire arrays. We focus on square and circular silver nanowires in silica, for which excellent agreement between both boundary element method and multiple multipolar expansion calculations is obtained. Our work provides the tools for designing plasmon-based interconnects and achieving high degree of integration with minimum cross talk between adjacent plasmon guides.Comment: 4 pages, 5 figure

Unstationary film model for the determination of absolute gas-liquid kinetic rate constants: ozonation of Acid Red 27, Acid Orange 7, and Acid Blue 129

A method for the determination of absolute kinetic rate constants is proposed using an unstationary film model. This methodology avoids the experimental determination of parameters like the enhancement factor or the Hatta number which are usually model-dependent. The mathematical model is general for gas-liquid systems with irreversible second order reactions. An optimization procedure based on artificial neural networks is used to estimate the initial guess of the parameters and the subsequent application of Gauss-Newton algorithm for the final nonlinear parameter estimation. The model is tested with the ozonation reaction of Acid Red 27, Acid Orange 7 and Acid Blue 129. The second-order kinetic rate constants for the direct reaction with O3 are 1615±93, 609±83, and 49±2M−1s−1, respectivelyJF acknowledges the support of the doctoral fellowship from the Universitat Politecnica de Valencia (UPV-PAID-FPI-2010-04).Ferre Aracil, J.; Cardona Navarrete, SC.; López Pérez, MF.; Abad Sempere, A.; Navarro-Laboulais, J. (2013). Unstationary film model for the determination of absolute gas-liquid kinetic rate constants: ozonation of Acid Red 27, Acid Orange 7, and Acid Blue 129. Ozone: Science and Engineering. 35(6):423-437. https://doi.org/10.1080/01919512.2013.815104S423437356Biń, A. K. (2006). Ozone Solubility in Liquids. Ozone: Science & Engineering, 28(2), 67-75. doi:10.1080/01919510600558635Cardona, S. C., López, F., Abad, A., & Navarro-Laboulais, J. (2010). On bubble column reactor design for the determination of kinetic rate constants in gas-liquid systems. The Canadian Journal of Chemical Engineering, 88(4), 491-502. doi:10.1002/cjce.20327Chang, C. 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Flow Measurement and Instrumentation, 11(2), 123-131. doi:10.1016/s0955-5986(99)00025-4Hoigné, J., & Bader, H. (1983). Rate constants of reactions of ozone with organic and inorganic compounds in water—I. Water Research, 17(2), 173-183. doi:10.1016/0043-1354(83)90098-2Jamialahmadi, M., Zehtaban, M. R., Müller-Steinhagen, H., Sarrafi, A., & Smith, J. M. (2001). Study of Bubble Formation Under Constant Flow Conditions. Chemical Engineering Research and Design, 79(5), 523-532. doi:10.1205/02638760152424299Johnson, P. N., & Davis, R. A. (1996). Diffusivity of Ozone in Water. Journal of Chemical & Engineering Data, 41(6), 1485-1487. doi:10.1021/je9602125King, C. J. (1966). Turbulent Liquid Phase Mass Transfer at Free Gas-Liquid Interface. Industrial & Engineering Chemistry Fundamentals, 5(1), 1-8. doi:10.1021/i160017a001Ledakowicz, S., Maciejewska, R., Perkowski, J., & Bin, A. (2001). Ozonation of Reactive Blue 81 in the bubble column. Water Science and Technology, 44(5), 47-52. doi:10.2166/wst.2001.0248Lewis, W. K., & Whitman, W. G. (1924). Principles of Gas Absorption. Industrial & Engineering Chemistry, 16(12), 1215-1220. doi:10.1021/ie50180a002Lopez, A., Benbelkacem, H., Pic, J. ‐S., & Debellefontaine, H. (2004). Oxidation pathways for ozonation of azo dyes in a semi‐batch reactor: A kinetic parameters approach. Environmental Technology, 25(3), 311-321. doi:10.1080/09593330409355465Meldon, J. H., Olawoyin, O. O., & Bonanno, D. (2007). Analysis of Mass Transfer with Reversible Chemical Reaction†. Industrial & Engineering Chemistry Research, 46(19), 6140-6146. doi:10.1021/ie0705397Navarro-Laboulais, J., Cardona, S. C., Torregrosa, J. I., Abad, A., & López, F. (2006). Structural identifiability analysis of the dynamic gas–liquid film model. AIChE Journal, 52(8), 2851-2863. doi:10.1002/aic.10901Navarro-Laboulais, J., Cardona, S. C., Torregrosa, J. I., Abad, A., & López, F. (2008). 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Quantification of the performance of chaotic micromixers on the basis of finite time Lyapunov exponents

Chaotic micromixers such as the staggered herringbone mixer developed by Stroock et al. allow efficient mixing of fluids even at low Reynolds number by repeated stretching and folding of the fluid interfaces. The ability of the fluid to mix well depends on the rate at which "chaotic advection" occurs in the mixer. An optimization of mixer geometries is a non trivial task which is often performed by time consuming and expensive trial and error experiments. In this paper an algorithm is presented that applies the concept of finite-time Lyapunov exponents to obtain a quantitative measure of the chaotic advection of the flow and hence the performance of micromixers. By performing lattice Boltzmann simulations of the flow inside a mixer geometry, introducing massless and non-interacting tracer particles and following their trajectories the finite time Lyapunov exponents can be calculated. The applicability of the method is demonstrated by a comparison of the improved geometrical structure of the staggered herringbone mixer with available literature data.Comment: 9 pages, 8 figure

Anyons in a weakly interacting system

We describe a theoretical proposal for a system whose excitations are anyons with the exchange phase pi/4 and charge -e/2, but, remarkably, can be built by filling a set of single-particle states of essentially noninteracting electrons. The system consists of an artificially structured type-II superconducting film adjacent to a 2D electron gas in the integer quantum Hall regime with unit filling fraction. The proposal rests on the observation that a vacancy in an otherwise periodic vortex lattice in the superconductor creates a bound state in the 2DEG with total charge -e/2. A composite of this fractionally charged hole and the missing flux due to the vacancy behaves as an anyon. The proposed setup allows for manipulation of these anyons and could prove useful in various schemes for fault-tolerant topological quantum computation.Comment: 7 pages with 3 figures. For related work and info visit http://www.physics.ubc.ca/~fran

Atomic-scale confinement of optical fields

In the presence of matter there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically-defined gaps reaching below 0.5 nm. The existence of atomically-confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically-resolved spectroscopic imaging, deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the realization of novel quantum-optical devices