Inverse-opal conducting polymer monoliths in microfluidic channels

Inverse opal monolithic flow-through structures of polyaniline (PANI) were achieved in microfluidic channels for lab-on-a-chip (LOC) applications. In order to achieve the uniformly porous monolith, polystyrene (PS) colloidal crystal (CC) templates were fabricated in channel. An inverse opal PANI structure was achieved on-chip, through a two-step process involving the electrochemical growth of PANI and subsequent removal of the template. The effect of electropolymerisation on these structures is discussed. It was found that growth time is critical in achieving an ordered structure with well-defined flow-through pores. This is significant in order to fabricate optimal porous PANI structures that maximise surface area of the monolith and also provide well-defined flow profiles through the micro-channel

Optimal transient growth in thin-interface internal solitary waves

The dynamics of perturbations to large-amplitude Internal Solitary Waves (ISW) in two-layered flows with thin interfaces is analyzed by means of linear optimal transient growth methods. Optimal perturbations are computed through direct-adjoint iterations of the Navier-Stokes equations linearized around inviscid, steady ISWs obtained from the Dubreil-Jacotin-Long (DJL) equation. Optimal perturbations are found as a function of the ISW phase velocity $c$ (alternatively amplitude) for one representative stratification. These disturbances are found to be localized wave-like packets that originate just upstream of the ISW self-induced zone (for large enough $c$) of potentially unstable Richardson number, $Ri < 0.25$. They propagate through the base wave as coherent packets whose total energy gain increases rapidly with $c$. The optimal disturbances are also shown to be relevant to DJL solitary waves that have been modified by viscosity representative of laboratory experiments. The optimal disturbances are compared to the local WKB approximation for spatially growing Kelvin-Helmholtz (K-H) waves through the $Ri < 0.25$ zone. The WKB approach is able to capture properties (e.g., carrier frequency, wavenumber and energy gain) of the optimal disturbances except for an initial phase of non-normal growth due to the Orr mechanism. The non-normal growth can be a substantial portion of the total gain, especially for ISWs that are weakly unstable to K-H waves. The linear evolution of Gaussian packets of linear free waves with the same carrier frequency as the optimal disturbances is shown to result in less energy gain than found for either the optimal perturbations or the WKB approximation due to non-normal effects that cause absorption of disturbance energy into the leading face of the wave.Comment: 33 pages, 22 figure

New Approaches for Evaluating Lidar-Derived Shoreline

This study presents and compares two new methods of assessing the uncertainty of lidar-derived National Shoreline mapped by NOAA’s National Geodetic Survey: an empirical (ground-based) approach and a stochastic (Monte Carlo) approach. OCIS codes: (280.3640) Lidar; (120.2830) Height measurements; (000.4430) Numerical approximation and analysi

A model for large-amplitude internal solitary waves with trapped cores

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Nonlinear Processes in Geophysics 17 (2010): 303-318, doi:10.5194/npg-17-303-2010.Large-amplitude internal solitary waves in continuously stratified systems can be found by solution of the Dubreil-Jacotin-Long (DJL) equation. For finite ambient density gradients at the surface (bottom) for waves of depression (elevation) these solutions may develop recirculating cores for wave speeds above a critical value. As typically modeled, these recirculating cores contain densities outside the ambient range, may be statically unstable, and thus are physically questionable. To address these issues the problem for trapped-core solitary waves is reformulated. A finite core of homogeneous density and velocity, but unknown shape, is assumed. The core density is arbitrary, but generally set equal to the ambient density on the streamline bounding the core. The flow outside the core satisfies the DJL equation. The flow in the core is given by a vorticity-streamfunction relation that may be arbitrarily specified. For simplicity, the simplest choice of a stagnant, zero vorticity core in the frame of the wave is assumed. A pressure matching condition is imposed along the core boundary. Simultaneous numerical solution of the DJL equation and the core condition gives the exterior flow and the core shape. Numerical solutions of time-dependent non-hydrostatic equations initiated with the new stagnant-core DJL solutions show that for the ambient stratification considered, the waves are stable up to a critical amplitude above which shear instability destroys the initial wave. Steadily propagating trapped-core waves formed by lock-release initial conditions also agree well with the theoretical wave properties despite the presence of a "leaky" core region that contains vorticity of opposite sign from the ambient flow.This work is supported as part of the Office of Naval Research NLIWI and IWISE program grants N00014-06-1- 0798 and N00014-09-1-0227

Magnetic field-induced soft mode in spin-gapped high-Tc superconductors

We present an explanation of the dynamical in-gap spin mode in LSCO induced by an applied magnetic field H as recently observed by J. Chang et al. Our model consists of a phenomenological spin-only Hamiltonian, and the softening of the spin mode is caused by vortex pinning of dynamical stripe fluctuations which we model by a local ordering of the exchange interactions. The spin gap vanishes experimentally around H=7T which in our scenario corresponds to the field required for overlapping vortex regions.Comment: 4 pages, 3 fig

Chandra Observation of the Radio Source / X-ray Gas Interaction in the Cooling Flow Cluster Abell 2052

We present a Chandra observation of Abell 2052, a cooling flow cluster with a central cD that hosts the complex radio source 3C 317. The data reveal ``holes'' in the X-ray emission that are coincident with the radio lobes. The holes are surrounded by bright ``shells'' of X-ray emission. The data are consistent with the radio source displacing and compressing, and at the same time being confined by, the X-ray gas. The compression of the X-ray shells appears to have been relatively gentle and, at most, slightly transonic. The pressure in the X-ray gas (the shells and surrounding cooler gas) is approximately an order of magnitude higher than the minimum pressure derived for the radio source, suggesting that an additional source of pressure is needed to support the radio plasma. The compression of the X-ray shells has speeded up the cooling of the shells, and optical emission line filaments are found coincident with the brightest regions of the shells.Comment: accepted for publication in ApJ Letters; for high-resolution color figures, see http://www.astro.virginia.edu/~elb6n/abell2052.htm

The spatio-temporal tuning of the mechanisms in the control of saccadic eye movements

AbstractWe compared the spatio-temporal tuning of perception to the mechanisms that drive saccadic eye movements. Detection thresholds were measured for Gabor-targets presented left or right of fixation (4 or 8deg eccentricity), at one of four spatial frequencies (1, 2, 4 or 8cpd) oscillating at one of three temporal frequencies (1, 8 or 16Hz). We then measured saccade latency to each target presented at various multiples of detection threshold. Consistent with previous research, latency decreased as a function of contrast. However, at equal detection performance, we found no systematic difference in saccadic latency and no difference in average oculometric performance (% correct saccade direction) across the different target spatio-temporal frequencies. Furthermore, position error remained fairly constant across all conditions. The results are consistent with the idea that the spatio-temporal signals used for perception are the same as those used by the mechanisms driving saccadic eye movements

Production of polystyrene spheres for use as a templating material for polyaniline monolith structures.

Polystyrene (PS) spheres are potentially useful as a reproducible, sacrificial templating material for monolith columns once they can be utilised to create a uniform microstructured packing which enables a higher monolith batch to batch reproducibility. To achieve PS spheres which can meet these requirements, their synthesis was optimised. Parameters investigated included variation of reactant concentrations, along with optimisation of reaction conditions temperature, agitation speed and nitrogen flow during aeration. Temperature and agitati on played vital roles in the size and homogeneity of the synthesised PS spheres. Temperature affected the equilibrium concentration of monomer in the aqueous phase. When reaction temperature was increased, sphere size reduced and as reaction temperature decreased sphere size increased. A similar trend was seen when agitation speed was varied. At higher agitation speed average PS sphere size decreased as the rate of polymerisation increased. At lower agitation speed the average PS sphere size increased as the rate of polymerisation decreased. Ensuring fluctuations in both temperature and agitation were kept to a minimum was key to maintaining reproducibility. Any fluctuation above ~10% in either temperature or agitation speed affected standard deviation irreversibly. The facile dissolution of the PS spheres was also investigated. If the spheres produced could not be dissolved, their use as a sacrificial templating material would not be possible. By decreasing the original concentration of cross-linker, dissolution increased dramaticall

Inverse-opal conducting polymer monoliths in micro-fluidic channels.

Inverse opal monolithic flow-through structures of polyaniline (PANI) were achieved in microfluidic channels for lab-on-a-chip (LOC) applications. In order to achieve the uniformly porous monolith, polystyrene (PS) colloidal crystal (CC) templates were fabricated in channel. An inverse opal PANI structure was achieved on-chip, through a two-step process involving the electrochemical growth of PANI and subsequent removal of the template. The effect of electropolymerisation on these structures is discussed. It was found that growth time is critical in achieving an ordered structure with well-defined flow-through pores. This is significant in order to fabricate optimal porous PANI structures that maximise surface area of the monolith and also provide well-defined flow profiles through the micro-channel