Astrong Dependence Research Articles

Increased range and contrast in fog with circularly polarized imaging.

Fogs, low lying clouds, and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on optical sensors for navigation whose performance is degraded by highly scattering environments. In our previous simulation work, we have shown that polarized light can penetrate through a scattering environment such as fog. We have demonstrated that circularly polarized light maintains its initial polarization state better than linearly polarized light, even through large numbers of scattering events and thus ranges. This has recently been experimentally verified by other researchers. In this work, we present the design, construction, and testing of active polarization imagers at short-wave infrared and visible wavelengths. We explore multiple polarimetric configurations for the imagers, focusing on linear and circular polarization states. The polarized imagers were tested at the Sandia National Laboratories Fog Chamber under realistic fog conditions. We show that active circular polarization imagers can increase range and contrast in fog better than linear polarization imagers. We show that when imaging typical road sign and safety retro-reflective films, circularly polarized imaging has enhanced contrast throughout most fog densities/ranges compared to linearly polarized imaging and can penetrate over 15 to 25m into the fog beyond the range limit of linearly polarized imaging, with a strong dependence on the interaction of the polarization state with the target materials.

Polycyclic aromatic hydrocarbons (PAHs) and oxy- and nitro-PAHs in ambient air of the Arctic town Longyearbyen, Svalbard

Abstract. Polycyclic aromatic hydrocarbons (PAHs) are not declining in Arctic air despite reductions in their global emissions. In Svalbard, the Longyearbyen coal-fired power plant is considered to be one of the major local sources of PAHs. Power plant stack emissions and ambient air samples, collected simultaneously at 1 km (UNIS) and 6 km (Adventdalen) transect distance, were analysed (gaseous and particulate phases separately) for 22 nitro-PAHs, 8 oxy-PAHs, and 16 parent PAHs by gas chromatography in combination with single quadrupole electron capture negative ionization mass spectrometry (GC-ECNI-MS) and gas chromatography in combination with triple quadrupole electron ionization mass spectrometry (GC-EI-MS/MS). Results confirm low levels of PAH emissions (∑16 PAHs =1.5 µg kg−1 coal) from the power plant. Phenanthrene, 9,10-anthraquinone, 9-fluorenone, fluorene, fluoranthene, and pyrene accounted for 85 % of the plant emission (not including naphthalene). A dilution effect was observed for the transect ambient air samples: 1.26±0.16 and 0.63±0.14 ng m−3 were the sum of all 47 PAH derivatives for UNIS and Adventdalen, respectively. The PAH profile was homogeneous for these recipient stations with phenanthrene and 9-fluorenone being most abundant. Multivariate statistical analysis confirmed coal combustion and vehicle and marine traffic as the predominant sources of PAHs. Secondary atmospheric formation of 9-nitroanthracene and 2+3-nitrofluoranthene was evaluated and concluded. PAHs partitioning between gaseous and particulate phases showed a strong dependence on ambient temperatures and humidity. The present study contributes important data which can be utilized to eliminate uncertainties in model predictions that aim to assess the extent and impacts of Arctic atmospheric contaminants.

Phase stability of a reversible supramolecular polymer solution mixed with nanospheres

Theory is presented for the phase stability of mixtures containing nanospheres andnon-adsorbing reversible supramolecular polymers. This was made possible byincorporating the depletion thickness and osmotic pressure of reversible supramolecularpolymer chains into generalized free-volume theory, recently developed for investigating thephase behaviour of colloidal spheres mixed with interacting polymers (Fleer and Tuinier2008 Adv. Colloid Interface Sci. 143 1–47). It follows that the fluid–fluid phasestability region where reversible supramolecular polymer chains can be mixed withnanospheres is sensitive to the energy of scission between the monomers and to thenanoparticle radius. One can then expect the fluid–fluid coexistence curves to have astrong dependence on temperature and that shifting of phase boundaries withina single experimental system should be possible by varying the temperature.The calculations reveal the width of the stability region to be rather small. Thisimplies that phase homogeneity of product formulations containing reversiblesupramolecular polymers is only possible at low nanoparticle concentrations.

Relaxation to thermal equilibrium in the self-gravitating sheet model

We revisit the issue of relaxation to thermal equilibrium in the so-called ‘sheet model’,i.e. particles in one dimension interacting by attractive forces independent of theirseparation. We show that this relaxation may be very clearly detected and characterized byfollowing the evolution of order parameters defined by appropriately normalized momentsof the phase space distribution which probe its entanglement in space and velocitycoordinates. For a class of quasi-stationary states which result from the violentrelaxation of rectangular waterbag initial conditions, characterized by their virial ratioR0, weshow that relaxation occurs on a timescale which (i) scales approximately linearly in the particle numberN and (ii) also shows astrong dependence on R0, with quasi-stationary states from colder initial conditions relaxing much more rapidly.The temporal evolution of the order parameter may be well described by a stretchedexponential function. We study finally the correlation of the relaxation times with theamplitude of fluctuations in the relaxing quasi-stationary states, as well as the relationbetween temporal and ensemble averages.

Isoscaling in Statistical Sequential Decay Model

A sequential decay model is used to study isoscaling, i.e. thefactorization of the isotope ratios from sources of different isospinsand sizes over a broad range of excitation energies, into fugacity termsof proton and neutron number,R21(N,Z) = Y2(N,Z)/Y1(N,Z) = Ce(αN+βZ). It isfound that the isoscaling parameters α and β have astrong dependence on the isospin difference of equilibrated sourceand excitation energy, no significant influence of the source sizeon α and β has been observed. It is found that α andβ decrease with the excitation energy and are linearfunctions of 1/T and Δ(Z/A)2 or Δ(N/A)2 of thesources. Symmetry energy coefficient Csym is constrained fromthe relationship of α and source Δ(Z/A)2, β andsource Δ(N/A)2.

Morphology-controllable fabrication of ordered platinum nanoshells with reducedsymmetry

This paper presents a novel method for fabricating an ordered array of metallic nanoshellswith a controllable shape by a combination of a porous polymer template and ananocrystal-seeded electroless plating technique. The morphology of hollow particles has astrong dependence on the seed-deposition time onto the surfaces of the original colloidaltemplate. Using this method, ordered Pt nanobowls (bowl-shaped shells) and, alternatively,nanocups (cup-shaped shells) are prepared. These materials show some intriguingproperties: (i) reduced symmetry of the building blocks; (ii) a well-ordered structure; and(iii) a high ratio of surface area to volume, all of which are useful in many areas such ascatalysts, sensors, and photonic crystals.

Dependence of bridging oxygen 17Oquadrupolar coupling parameters on Si–O distance and Si–O–Si angle

Ab initio quantum chemistry calculations and comparisons withexperimental 17O solid-state nuclear magneticresonance (NMR) investigations were used to determine the dependenceof the 17O quadrupolar coupling constantand asymmetry parameter on the first-coordination-sphere structurearound bridging oxygen. The quadrupolar asymmetry parameter was foundto be dependent on the Si–O–Si angle, in agreement withprevious studies, and independent of the Si–O distance. Incontrast, the quadrupolar coupling constant was found to have astrong dependence on Si–O distance as well asSi–O–Si angle. Analytical expressions describing thesedependences were proposed and used to develop an approach forrelating measured 17O quadrupolar coupling constantand asymmetry parameter values for bridging oxygen to theirSi–O–Si angle and average Si–O distance. Examplesof this approach were given using 17O NMR results from thecrystalline silica polymorphs, coesite, α-quartz, cristobalite, andferrierite.

Flow regimes in a wide ‘sliced-cylinder’ model of homogeneous beta-plane circulation

We report new experiments with the ‘sliced-cylinder’ β-plane model of Pedlosky & Greenspan (1967) and Beardsley (1969), but with a much wider basin such that the western boundary current and its eddies occupy a small fraction of the basin width. These experiments provide new insights into nonlinear aspects of the flow: the critical conditions for boundary current separation and the transition from stable to unstable flow are redefined, and a further transition from periodic to chaotic eddy shedding under strong anticyclonic forcing is also found. In the nonlinear regimes the western boundary current separates from the western wall and shoots into the interior as a narrow jet that undergoes a rapid adjustment to join with the broad slow interior flow. In the unstable regimes this adjustment involves eddy shedding. Each transition occurs at a fixed critical value of a Reynolds number Reγ based on the velocity and width scales for a purely viscous boundary current: the flow is unstable for Reγ > 123±4 and aperiodic for Reγ > 231±5. The results provide evidence that the mechanism causing instability is shear in the separated jet rather than the breaking of a large-amplitude Rossby wave. A quasi-geostrophic numerical model applied to the laboratory conditions yields a stability boundary and detailed characteristics of the flow largely consistent with those determined from the experiments. It also reveals a strong dependence of the circulation pattern on basin aspect ratio, and shows that an adverse higher-order pressure gradient is responsible for western boundary current separation in this model. Eddy–eddy interactions and feedback of fluctuations from the eddy formation region to upstream parts of the boundary current contribute to aperiodic behaviour. As a result of eddy shedding, passive tracer from each streamline in the boundary current can be stirred across much of the width of the basin.

Numerical and asymptotic results on the linear stability of a thin film spreading down a slope of small inclination

We model a thin liquid film moving down a slope using the lubrication approximation with a slip condition. The travelling-wave solution is derived for small inclination angle α, using singular perturbation methods, and compared to the numerical solution. For the linear stability analysis we combine numerical methods with the long-wave approximation and find a small but finite critical α* below which the flow remains linearly stable to spanwise perturbations. This is contrasted with the vanishing of the hump of the travelling-wave solution. Finally, the prevailing linear stability of the travelling-wave at small inclination angles is compared with recent related results using a precursor model. Here, though, a strong dependence on the magnitude of the contact angle is found, which we think has not been observed before.

Texture and Structure Evolution During Indirect Extrusion of an AlSiMgMn Aluminium Alloy

Texture evolution during indirect extrusion of cylindrical profiles of an AlSiMgMn aluminium alloy was investigated in the present work. The effect of extrusion temperature on texture development, through‐thickness texture variations and texture variations along the length of the extruded profiles were investigated. In all cases a combined 〈100〉 and 〈111〉 fibre texture developed. Generally, the 〈111〉 fibre was sharper than the 〈100〉 fibre. The texture evolution was rather similar along the length of the profile, while the variations through the thickness were considerable. In a region midways between the centre and the surface of the profiles, the 〈100〉 fibre was weakened, and in the surface regions the two fibres were significantly rotated. The texture measurements showed a strong dependence on extrusion temperature, in the sense that the higher the temperature, the sharper the 〈100〉 fibre texture. The influence of texture variations on formability were also investigated.

Anomalous magnetism of iron(II) complexes of methyl-substituted pyridylthiazoles

Bis-ligand iron(11) and nickel(11) complexes of the potentially tridentate chelating agents 2-(3-methyl-2-pyridylamino)-4-(2-pyridyl)thiazole and 2-(4-methyl- 2-pyridylamino)-4-(2-pyridyl)thiazole are described. The electronic spectra of the nickel(11) complexes show that the field strengths of the ligands are in the neighbourhood of the Crossover region for iron(11). The magnetic properties of the cationic iron(11) complexes are anomalous and consistent with a spin-equilibrium involving the nearly equi-energetic 5T2 and 1A1 states of the metal atom. The magnetism shows a strong dependence on the associated anion. 2-(6-Methyl-2-pyridylamino)-4- (2-pyridyl)thiazole is also described. In its bis-ligand iron(11) complexes it functions as a bidentate chelating agent and the complexes are high-spin. All three ligands may be deprotonated to yield inner complexes with iron(11) which in some instances have anomalous magnetic properties associated with the presence of a spin-equilibrium. 2-(6-Methyl-2-pyridyl)-4-(2-pyridyl)thiaole and its bis-ligand iron(11) and nickel(11) complexes are also described. The iron(11) complexes are high-spin and show normal temperature dependence of their magnetism.