Open Area Fraction Research Articles

Laminar forced convection heat transfer to ordered and disordered single rows of cylinders

We study laminar forced convection heat transfer to or from a single row of equidistantly and non-equidistantly spaced parallel cylindrical wires, perpendicular to the flow direction. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on the wire diameter and the free stream velocity) between 0.001 and 600. For equidistantly spaced rows of cylindrical wires we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant rows exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single cylinder in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant rows is lower than in equidistant rows for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.

Infrared Plasmonic Transmission Resonances of Gold Film with Hexagonally Ordered Hole Arrays on ZnSe Substrate

The high fractional open area of metal thin film coatings, with two dimensional, hexagonally ordered, close packed arrays of holes, makes them of interest for the incorporation of plasmonic effects into a variety of optical devices. Gold films with hexagonal patterns of circular holes have been created on ZnSe infrared windows. The films have 2.50 μm diameter holes and a hexagonal lattice parameter of 3.06 μm which places the primary transmission resonances of the ZnSe/gold interface at ~1,400 cm−1 (7.14 μm) and that of the air/gold interface at ~3,800 cm−1 (2.63 μm). This geometry produces useful transmission across the whole traditional mid-infrared range. The dispersion of these resonances has been measured by changing the angle of incident light. The data is modeled with explicit momentum matching equations in two different, high symmetry geometries, allowing the effective index of refraction to vary with wavelength. The response of these resonances to the addition of an acetaldehyde coating is described.

Temperature-Dependent Morphology Evolution and Surface Plasmon Absorption of Ultrathin Gold Island Films

Ultrathin gold island films on transparent substrates display a characteristic surface plasmon (SP) absorption band in which the peak position and full width at half-maximum (fwhm) are highly sensitive to the film morphology. In the present study, we investigated the temperature dependence of morphological evolution and the corresponding unique surface plasmon resonance (SPR) properties variation of the ultrathin gold island films (5 nm nominal thickness) upon rapid thermal annealing for 180 s at different temperatures ranging from 100 to 700 °C. The morphological evolution of the ultrathin gold film upon the thermal annealing-induced dewetting was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the optical properties variation was characterized by a transmission UV–vis-NIR spectroscopy. A strong temperature dependence of morphological evolution and optical properties variation as a function of thermal treatment conditions was identified. The blue shift and band narrowing of the SP absorption band can be correlated with various morphological characteristics, e.g., the increased open area fraction of island films, average separation between islands or nanoparticles (NPs) and the decreased aspect ratio (length divided by width) upon increasing thermal treatment temperatures. The temperature dependence of the transmission localized surface plasmon resonance (T-LSPR) may enable a science-based design of optical sensing and dynamic thermal sensors upon the morphological manipulation of ultrathin metallic surface nanostructures by thermal dewetting.

Metal mesh heating element size effect in resistance welding of thermoplastic composites

The objective of this work is to determine the effects of metal mesh heating element size on resistance welding of thermoplastic composites. The materials to be resistance-welded consisted of carbon fiber/poly-ether-ketone-ketone (CF/PEKK), carbon fiber/poly-ether-imide (CF/PEI) and glass fiber/PEI (GF/PEI). Four different metal mesh sizes were used as heating elements. The samples were welded in a lap shear joint configuration and mechanically tested. Maximum Lap Shear Strengths of 52, 47 and 33 MPa were obtained for the CF/PEKK, CF/PEI and GF/PEI specimens, respectively. The ratio of the heating element’s fraction of open area and wire diameter was shown to be the most important parameter to be considered when selecting an appropriate heating element size.

Experimental Studies on a Swirling Fluidized Bed with Annular Distributor

This paper reports the experimental findings on a swirling fluidized bed which operates with an annular-blade distributor. Spherical PVC particles (which fall under type D in Geldart’s classification) with two different sizes (5.75 mm and 9.84 mm) were used as bed material to study different bed configurations, taking into account the variation of fraction of open area (12.9% and 17.2%), blade overlapping angles (9° and 12°) and bed weight (500 gram to 2000 gram) for superficial velocities is from 1 m/s to 8 m/s. The performance of the swirling fluidized bed was assessed in terms of pressure drop values, minimum fluidization velocity and fluidization quality by physical observation on regimes of operation. The most significant finding is that the pressure drop of the bed increased with superficial velocity after minimum fluidization, in contrast with a conventional fluidized bed. It was also found that the blade geometry has less effect on bed performance, compared to fraction of open area and particle size. New regimes of operation were discovered, and designated as wave regime and two-layer bed regime. The best configuration obtained in the current work, which provides stable swirling at lower pressure drops, is the one with 60 distributor blades and 12° overlapping angle, operating with 9.84 mm particles.

Three-color electrowetting display device for electronic paper

Three-color electrowetting (EW) reflective display devices have been achieved using a vertical stack structure with colored oils. Fundamental properties (interface tension and geometry) of the EW process were investigated. Arrays of ∼1000–2000 pixels were constructed with pixel sizes of 200×600 and 300×900 μm2. Each color layer can be switched independently. Open (oil-free) area fraction of ∼80% in each cell is achieved at ∼10–12 V. Switch-on times range from 5 to 15 ms, suitable for video content display. These characteristics demonstrate that the EW vertical structure has the potential to produce attractive video-speed color reflective electronic paper devices.

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.

Aerosol Deposition on Electroformed Wire Screens

Insect screens, which are usually an integral component of an air sampling inlet, can cause inadvertent deposition of larger aerosol particles. Numerical and experimental studies were performed to characterize aerosol deposition on commercially available electroformed wire screens for aerodynamic particle diameters between 3 and 20 μ m, Stokes numbers between 0.49 and 20, wire widths between 35 and 160 μ m, and screen open area fractions of 0.56 to 0.90. With increasing values of Stokes numbers, the actual collection efficiency increases gradually to a maximum value that is asymptotic to the fraction of open area. Deposition is characterized in terms of a standardized screen efficiency, which is the actual efficiency divided by the areal solidarity (1–fraction of open area). A correlation equation has been developed for the electroformed mesh screens, which relates the standardized efficiency to the fraction of open area, the Stokes number, the interception parameter, and the Reynolds number based on wire...

Strong effect on dentin after the use of high concentrations of citric acid: An assessment with co-site optical microscopy and ESEM

Objectives To perform a longitudinal analysis of the effect of four substances (citric acid at 1%, 5% and 10% concentrations and 17% EDTA) on dentin. Methods Sixteen human molars had their crowns removed exposing cervical root dentin. A grinding procedure was used to produce a standardized smear layer and to create a smooth surface for analysis. Environmental scanning electron microscopy (ESEM) was used for qualitative observations while co-site optical microscopy (CSOM) and image analysis (IA) provided a quantitative comparison of the effect of the substances for several experimental times from 15 to 300 s. The methods allowed the observation of the same areas after each experimental time. The open tubule area fraction was measured for several CSOM image fields, at each experimental time, for the four substances. Thus, it was possible to follow the phenomenon and quantitatively analyze the effect of the substances. Kolmogorov–Smirnov, Kruskal–Wallis H and Wilcoxon signed-ranks (with Bonferroni correction) tests were used to analyze the data. The level of significance was set at p < 0.05. Results The CA solutions showed the greatest chelating effect, which was proportional to concentration (EDTA: 9.4–32.8% (15–300 s); 1% CA: 21.1–32.2% (15–300 s); 5% CA: 32.4–43.1% (15–300 s); 10% CA: 35.1–39.5% (15–30 s). Erosive effects were observed on inter and peritubular dentin for 5% CA and 10% CA after 60 s. Significance ESEM allowed a detailed qualitative comparison of the effect of the four substances. CSOM and IA provided a quantitative comparison method with good statistical significance. Future studies should focus on the consequences of dentin erosion caused by citric acid.

Dispersion Study of the Infrared Transmission Resonances of Freestanding Ni Microarrays

Nickel films (several-micrometer thickness, with 5.2-μm square holes in a square lattice array with 12.7-μm hole-to-hole spacing) exhibit Ebbesen's extraordinary transmission effect in the infrared; that is, they transmit a higher fraction of incident infrared light than the fractional open area of the holes. The role of surface plasmons (SPs) in this phenomenon is much debated, so we have obtained a data set whereby this idea and others can be tested against empirically determined dispersion curves. Unpolarized, zero-order transmission spectra have been recorded by rotating the mesh (from −2° to 60° in 1° steps) relative to the spectrometer's incident beam about an axis along the mesh's nearest hole-to-hole spacing in order to create the dispersion diagram. The data are numerically analyzed for peak centers that are then projected outside of the light line (by SP momentum matching equations) to two SP dispersion curves that are spaced in frequency by a splitting. With this caveat, all of the observed structure is accounted for by a simple SP model.

Measuring the continuity of diffusion barriers on porous films using γ-ray energy spectra of escaping positronium

Diffusion barriers for capping porous low dielectric constant films are important for preventing metal migration into a semiconductor circuit. Using the fact that positrons implanted into a porous dielectric form ortho-positronium (o-Ps) copiously, Gidley et al. [D. W. Gidley, W. F. Frieze, T. L. Dull, J. Sun, A. F. Yee, C. V. Nguyen, and D. Y. Yoon, Appl. Phys. Lett. 76, 1282 (2000)], have been able to measure open area fractions as low as 10−5 in porous dielectric film barrier layers from the increase in the ortho-positronium lifetime and intensity associated with positronium escape into vacuum. We demonstrate that it is possible to obtain comparable sensitivities by measuring the gamma-ray energy spectrum of the escaping positronium.

Use of non-invasive X-ray microtomography for characterizing microstructure of extruded biopolymer foams

Understanding foam microstructure formation is important for a priori design and engineering of new biopolymer-based products for both food and industrial applications. However, this has been hindered by unavailability of an imaging technology to characterize the cellular structure of foams accurately. This study investigated a non-invasive imaging technology, X-ray microtomography (XMT), for visualization and measurement of microstructural features of biopolymer foams. Brittle corn starch foams with two levels (5% and 15%) of whey protein concentrate (34% protein) factorialized with two moisture contents (26% or 34%) were produced using extrusion. XMT allowed non-invasive imaging of sample cross-sections at various depths, and facilitated accurate and hitherto impossible measurements of features like true cell size distribution (bi-modal), average diameter (0.58 to 2.27 mm), open wall area fraction (0.068 to 0.099), cell wall thickness (0.09 to 0.15 mm), and true void fraction (0.63 to 0.84). Results indicated XMT is superior to conventional imaging techniques for characterizing foam microstructure.

Flooding of Gas−Solids Countercurrent Flow in Fluidized Beds

Flooding experiments were carried out in a semicircular pilot-scale cold model of Syncrude's two fluid cokers. This unit circulates solids downward through a stripper zone and externally back to the top of the vessel, facilitating countercurrent gas/solid contacting. The onset of flooding for FCC particles was determined by analysis of differential pressure signals, supported by visual observations. The solids circulation flux at flooding increased with decreasing superficial gas velocity, with increasing fractional open area, with increasing slot width between adjacent baffles, and with decreasing baffle top included angle. A semiempirical model gives predictions of the onset of flooding that are in good agreement with both cold-flow model results and available commercial data.

Accessing Surface Plasmons with Ni Microarrays for Enhanced IR Absorption by Monolayers

Surface plasmons have been accessed with mid-IR light on Ni microarrays of subwavelength apertures using zeroth-order FTIR transmission spectra. A number of transmission resonances are observed throughout the mid-IR region, which is particularly interesting because nickel’s dielectric properties are unfavorable in the visible. On the strongest resonance, these microarrays transmit about 3 times the intensity of light that is directly incident upon the holes (i.e., they exhibit Ebbesen’s extraordinary transmission effect). A study is presented of the dispersion and line shape of resonances for a mesh of well-defined geometry (6.5-Im-wide square holes, square lattice with 12.7-Im hole-to-hole spacing, and 5-Im thickness). The dispersion diagram reveals large band gaps (as a fraction of the resonance energy) that are complicated by the lifting of nominally degenerate resonances. The line shape of the most isolated resonance was fit to a damped harmonic oscillator model revealing the complex dielectric parameters including the damping constant. Lifetimes obtained from the damping constants range from 80 to 510 fs and exhibit an exponential dependence on the resonance wavelength. A great enhancement of resonant transmission relative to the fractional open area has also been observed upon stacking two or more meshes on top of each other. The metallic microarray transmission geometry converts the photon energy to surface plasmon polaritons traveling along the metal surface and therefore through coatings, membranes, or monolayers on the mesh. This dramatically changes the path length in absorption experiments from about twice the thickness of the coating (as it is in reflection absorption experiments) to the thickness of the microchannel and more, which could amount in practice to a 1000-fold enhancement in the fraction of light absorbed. These observations suggest opportunities for using these resonances in sensitive detection schemes with vibrational spectroscopies to detect molecular surface species. We show one exciting outcome that involves some unusually large absorbances by 1-dodecanethiolate monolayers on these Ni meshes.

Preparation of SiC-based cellular substrate by pressure-pulsed chemical vapor infiltration into honeycomb-shaped paper preforms

Using a pressure-pulsed chemical vapor infiltration technique, SiC was infiltrated from a SiCl4 (4%)–CH4 (4%)–H2 gas phase into carbonized paper preforms at 1100°C. SiC-based cellular substrates with cell wall thicknesses of 25, 50 and 100 μm were obtained by using honeycomb-shaped paper preforms as the templates. The reduction of both wall thickness t and cell pitch d of SiC-based honeycomb substrate successfully led to an increase in geometric surface area per unit volume S, keeping pressure drop ΔP at constant, besides; ΔP decreased without lowering S by the reduction of t and the increase in d. The pressure drop in the prepared honeycomb substrates depended on S 2α−3 value, where α was open frontal area fraction. The compressive strength of the honeycomb substrates with t of 25 μm was about 7 MPa. The strength increased in proportion with 1 − α, which corresponded to the volume fraction of the cell wall in the honeycomb substrate.

Permeability of self-affine fractures

The permeability of self-affine fractures with various roughness exponents H is investigated by direct three-dimensional numerical simulations. A scaling behavior with an exponent H is exhibited in the self-affine scale range. Permeability can be related to the fractional open area and to the percolation probability by simple models.

Transmission of low-energy (< 10 eV) O + ions through Au films on TiO 2(110)

In an attempt to identify the fundamental processes that influence ion transport through metallic surface layers, we have studied the transmission of O + ions through discontinuous Au films adsorbed on TiO 2(110). A low energy (< 10 eV) O + ion beam is generated via electron stimulated desorption when an Au-dosed TiO 2(110) substrate is bombarded with a focused 250 eV electron beam. Low energy ion scattering data indicate that Au evaporated under ultrahigh vacuum conditions at ∼ 300 K forms three-dimensional clusters on TiO 2(110). As the Au coverage increases, the formation of Au clusters on TiO 2(110) blocks a fraction of the TiO 2 surface and the O + yield is attenuated. However, for high coverages (≥30% Au covered substrate) the O + signal decreases at a faster rate than the TiO 2 open area fraction. We attribute the attenuation of the O + yield for high Au coverages mainly to blocking of O + by Au clusters, to deflection of trajectories by the image force between ions and Au clusters, and to charge transfer between desorbing O + and neighboring Au clusters.

Hydraulic permeability of agarose gels

AbstractA new technique was developed to measure the hydraulic permeability of reinforced gel membranes and allows calculation of the Darcy permeability (κ) of the gel. It was applied to agarose with concentrations ranging from 2.0 to 7.3%. To create membranes that would be thin enough to yield easily measured filtration rates at modest applied pressures and yet withstand handling, gels were cast on woven polyester meshes. The resulting membranes had thicknesses of 70–100 μm and a fractional open area of 0.32. To correct for the presence of the mesh, finite‐element solutions were obtained for the pressure field in the 3‐D region occupied by the gel. For the meshes used here, the hydraulic permeability of the reinforced membrane was calculated to be 0.47–0.55 times that for a layer of pure gel, the exact value depending on the thickness of the composite membrane. The principal determinant of κ was the agarose concentration, but there was a secondary effect of applied pressure. The Darcy permeability extrapolated to zero applied pressure (κ0) varied from 616 nm2 for 2.0% agarose to 22 nm2 for 7.3% agarose. At a given gel concentration, the value for κ0 was as much as twice the value for κ measured at the maximum pressure difference of 20 kPa. This method can be adapted to a variety of other gel materials.

Extreme-ultraviolet polarization and filtering with gold transmission gratings

The polarization and transmission characteristics of freestanding gold transmission gratings, with 200-nm periods, for extreme-ultraviolet (EUV) radiation (1 < 200 nm) have been measured. We find that EUV transmission through the gratings is dominated by the waveguide characteristics of the gratings and that polarization efficiencies of 90% for wavelengths of 121.6 nm are achievable. Both the EUV polarization and transmission properties are in good agreement with a complete vector, numerical solution of Maxwell's equations. The fraction of open area to total area of the grating has been measured using a 10-keV proton beam and was found to be in good agreement with the microscopic slit and wire dimensions that were obtained by scanning electron microscopy. The use of these gratings for particle measurements in the presence of intense EUV radiation is briefly discussed.

Evaporation of Water With Single and Multiple Impinging Air Jets

An experimental investigation of impingement water evaporation under a single jet and arrays of circular jets was made. The parametric study included the effects of jet Reynolds number and standoff spacing for both single and multiple jets, as well as surface-to-nozzle diameter ratio and fractional nozzle open area for single and multiple jets, respectively. The nozzle exit temperature of the air jet, about the same as that of the laboratory, was 3–6° C higher than that of the evaporating water. Predictive equations are provided for mass transfer coefficient in terms of the flow and geometric conditions.