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Upward flame spread over corrugated cardboard
As part of a study of the combustion of boxes of commodities, rates of upward flame spread during early-stage burning were observed during experiments on wide samples of corrugated cardboard. The rate of spread of the flame front, defined by the burning pyrolysis region, was determined by visually averaging the pyrolysis front position across the fuel surface. The resulting best fit produced a power-law progression of the pyrolysis front, xp=Atn, where xp is the average height of the pyrolysis front at time t, n=3/2, and A is a constant. This result corresponds to a slower acceleration than was obtained in previous measurements and theories (e.g. n=2), an observation which suggests that development of an alternative description of the upward flame spread rate over wide, inhomogeneous materials may be worth studying for applications such as warehouse fires. Based upon the experimental results and overall conservation principles it is hypothesized that the non-homogeneity of the cardboard helped to reduce the acceleration of the upward spread rates by physically disrupting flow in the boundary layer close to the vertical surface and thereby modifying heating rates of the solid fuel above the pyrolysis region. As a result of this phenomena, a distinct difference was observed between scalings of peak flame heights, or maximum " flame tip" measurements and the average location of the flame. The results yield alternative scalings that may be better applicable to some situations encountered in practice in warehouse fires. © 2010 The Combustion Institute
Disrupting the wall accumulation of human sperm cells by artificial corrugation
Many self-propelled microorganisms are attracted to surfaces. This makes
their dynamics in restricted geometries very different from that observed in
the bulk. Swimming along walls is beneficial for directing and sorting cells,
but may be detrimental if homogeneous populations are desired, such as in
counting microchambers. In this work, we characterize the motion of human sperm
cells long, strongly confined to shallow chambers. We
investigate the nature of the cell trajectories between the confining surfaces
and their accumulation near the borders. Observed cell trajectories are
composed of a succession of quasi-circular and quasi-linear segments. This
suggests that the cells follow a path of intermittent trappings near the top
and bottom surfaces separated by stretches of quasi-free motion in between the
two surfaces, as confirmed by depth resolved confocal microscopy studies. We
show that the introduction of artificial petal-shaped corrugation in the
lateral boundaries removes the tendency of cells to accumulate near the
borders, an effect which we hypothesize may be valuable for microfluidic
applications in biomedicine.Comment: 9 pages, latex. In accepted version on April 14, v2: abstract
modified, information added to Sec. II.A and experiments added to Sec. III.A
and Fig.3. Sec. III.C was deleted. Requested references adde
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Large-area epitaxial growth of curvature-stabilized ABC trilayer graphene.
The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials
Corrugated Silicon Platelet Feed Horn Array for CMB Polarimetry at 150 GHz
Next generation cosmic microwave background (CMB) polarization anisotropy
measurements will feature focal plane arrays with more than 600 millimeter-wave
detectors. We make use of high-resolution photolithography and wafer-scale etch
tools to build planar arrays of corrugated platelet feeds in silicon with
highly symmetric beams, low cross-polarization and low side lobes. A compact
Au-plated corrugated Si feed designed for 150 GHz operation exhibited
performance equivalent to that of electroformed feeds: ~-0.2 dB insertion loss,
<-20 dB return loss from 120 GHz to 170 GHz, <-25 dB side lobes and <-23 dB
cross-polarization. We are currently fabricating a 50 mm diameter array with 84
horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol
telescopes. Our fabrication facilities permit arrays up to 150 mm in diameter.Comment: 12 pages; SPIE proceedings for Millimeter, Submillimeter, and
Far-Infrared Detectors and Instrumentation for Astronomy V (Conference 7741,
June 2010, San Diego, CA, USA
Diffusion of Colloidal Rods in Corrugated Channels
In many natural and artificial devices diffusive transport takes place in
confined geometries with corrugated boundaries. Such boundaries cause both
entropic and hydrodynamic effects, which have been studied only for the case of
spherical particles. Here we experimentally investigate diffusion of particles
of elongated shape confined into a corrugated quasi-two-dimensional channel.
Elongated shape causes complex excluded-volume interactions between particle
and channel walls which reduce the accessible configuration space and lead to
novel entropic free energy effects. The extra rotational degree of freedom also
gives rise to a complex diffusivity matrix that depends on both the particle
location and its orientation. We further show how to extend the standard
Fick-Jacobs theory to incorporate combined hydrodynamic and entropic effects,
so as, for instance, to accurately predict experimentally measured mean first
passage times along the channel. Our approach can be used as a generic method
to describe translational diffusion of anisotropic particles in corrugated
channels.Comment: 12 pages and 4 figure
Sedimentation rates test models of oceanic detachment faulting
This is the accepted manuscript version.The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/2014GL061555/full.Long-lived detachment faults play an important role in the construction of new oceanic
crust at slow-spreading mid-oceanic ridges. Although the corrugated surfaces of exposed
low-angle faults demonstrate past slip, it is difficult to determine whether a given fault is
currently active. If inactive, it is unclear when slip ceased. This judgment is crucial for
tectonic reconstructions where detachment faults are present, and for models of plate
spreading. We quantify variation in sediment thickness over two corrugated surfaces
near 16.5°N at the Mid-Atlantic Ridge using near-bottom CHIRP data. We show that the
distribution of sediment and tectonic features at one detachment fault is consistent with
slip occurring today. In contrast, another corrugated surface 20 km to the south shows a
sediment distribution suggesting that slip ceased ~150,000 years ago. Data presented here
provide new evidence for active detachment faulting, and suggest along-axis variations in
fault activity occur over tens of kilometers.This work was supported by the National Science Foundation grant number OCE-1155650
An analytical and experimental investigation of a 1.8 by 3.7 meter Fresnel lens solar concentrator
Line-focusing acrylic Fresnel lenses with application potential in the 200 to 370 C range are being analytically and experimentally evaluated. Investigations previously conducted with a 56 cm wide lens have been extended by the present study to experimentation/analyses with a 1.8 by 3.7 m lens. A measured peak concentration ratio of 64 with 90 percent of the transmitted energy focused into a 5.0 cm width was achieved. A peak concentration of 61 and a 90 percent target width of 4.5 cm were analytically computed. The experimental and analytical lens transmittance was 81 percent and 86 percent, respectively. The lens also was interfaced with a receiver assembly and operated in the collection mode. The collection efficiency ranged from 42 percent at 100 C to 26 percent at 300 C
Ferromagnetic Domain Structure of La0.78Ca0.22MnO3 Single Crystals
The magneto-optical technique has been employed to observe spontaneous
ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The
magnetic domain topology was found to be correlated with the intrinsic twin
structure of the investigated crystals. With decreasing temperature the regular
network of ferromagnetic domains undergoes significant changes resulting in
apparent rotation of the domain walls in the temperature range of 70-150 K. The
apparent rotation of the domain walls can be understood in terms of the
Jahn-Teller deformation of the orthorhombic unit cell, accompanied by
additional twinning.Comment: 7 pages, 5 figures, to be published in PR
Advanced microwave radiometer antenna system study
The practicability of a multi-frequency antenna for spaceborne microwave radiometers was considered in detail. The program consisted of a comparative study of various antenna systems, both mechanically and electronically scanned, in relation to specified design goals and desired system performance. The study involved several distinct tasks: definition of candidate antennas that are lightweight and that, at the specified frequencies of 5, 10, 18, 22, and 36 GHz, can provide conical scanning, dual linear polarization, and simultaneous multiple frequency operation; examination of various feed systems and phase-shifting techniques; detailed analysis of several key performance parameters such as beam efficiency, sidelobe level, and antenna beam footprint size; and conception of an antenna/feed system that could meet the design goals. Candidate antennas examined include phased arrays, lenses, and optical reflector systems. Mechanical, electrical, and performance characteristics of the various systems were tabulated for ease of comparison
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