4,553 research outputs found
Drop impact on superheated surfaces
At impact of a liquid droplet on a smooth surface heated above the liquid's
boiling point, the droplet either immediately boils when it contacts the
surfaces (``contact boiling''), or without any surface contact forms a
Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film
boiling''), or both forms the Leidenfrost layer and ejects tiny droplets upward
(``spraying film boiling''). We experimentally determine conditions under which
impact behaviors in each regime can be realized. We show that the dimensionless
maximum spreading of impacting droplets on the heated surfaces in both
gentle and spraying film boiling regimes shows a universal scaling with the
Weber number \We (\gamma\sim\We^{2/5}) -- regardless of surface temperature
and of liquid properties -- which is much steeper than for the impact on
non-heated (hydrophilic or hydrophobic) surfaces (\gamma\sim\We^{1/4}). We
also intereferometrically measure the vapor thickness under the droplet
Improvement of uncoupled Hartree-Fock expectation values for physical properties
Hartree-Fock calculation method as zero-order approximation for determining atomic and molecular second-order propertie
Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces
Droplets impacting on a superheated surface can either exhibit a contact
boiling regime, in which they make direct contact with the surface and boil
violently, or a film boiling regime, in which they remain separated from the
surface by their own vapor. The transition from the contact to the film boiling
regime depends not only on the temperature of the surface and kinetic energy of
the droplet, but also on the size of the structures fabricated on the surface.
Here we experimentally show that surfaces covered with carbon-nanofibers delay
the transition to film boiling to much higher temperature compared to smooth
surfaces. We present physical arguments showing that, because of the small
scale of the carbon fibers, they are cooled by the vapor flow just before the
liquid impact, thus permitting contact boiling up to much higher temperatures
than on smooth surfaces. We also show that, as long as the impact is in the
film boiling regime, the spreading factor of impacting droplets follows the
same \We^{3/10} scaling (with \We the Weber number) found for smooth
surfaces, which is caused by the vapor flow underneath the droplet.Comment: 10 pages, 6 figure
Evaluating the character and preservation of DNA within allophane clusters in buried soils on Holocene tephras, northern New Zealand
Clay minerals possess sorptive capacities for organic and inorganic matter, including DNA (Lorenz and Wackernagel, 1994), and hence reduce the utilization and degradation of organic matter or DNA by microorganisms. Buried allophane-rich soils on tephras (volcanic-ash beds) on the North Island, dated using tephrochronology, provide a valuable paleobiological ‘laboratory’ for studying the preservation of ancient DNA (aDNA) (Haile et al., 2007). Allophane comprises Al-rich nanocrystalline spherules ~3.5-5 nm in diameter (Fig. 1) with extremely large surface areas (up to 1000 m2 g-1). Moreover, allophanic soils are strongly associated with organic matter (Parfitt, 2009), and so we hypothesize that allophane also plays an important role for DNA protection within such soils
A Spitzer Study of the Mass Loss Histories of Three Bipolar Pre-Planetary Nebulae
We present the results of far-infrared imaging of extended regions around
three bipolar pre-planetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS
163423814, at 70 and 160 m with the MIPS instrument on the Spitzer
Space Telescope. After a careful subtraction of the point spread function of
the central star from these images, we place constraints on the existence of
extended shells and thus on the mass outflow rates as a function of radial
distance from these stars. We find no apparent extended emission in AFGL 2688
and OH 231.8+4.2 beyond 100 arcseconds from the central source. In the case of
AFGL 2688, this result is inconsistent with a previous report of two extended
dust shells made on the basis of ISO observations. We derive an upper limit of
M yr and M
yr for the dust mass loss rate of AFGL 2688 and OH 231.8, respectively,
at 200 arcseconds from each source. In contrast to these two sources, IRAS
163423814 does show extended emission at both wavelengths, which can be
interpreted as a very large dust shell with a radius of 400 arcseconds
and a thickness of 100 arcseconds, corresponding to 4 pc and 1 pc,
respectively, at a distance of 2 kpc. However, this enhanced emission may also
be galactic cirrus; better azimuthal coverage is necessary for confirmation of
a shell. If the extended emission is a shell, it can be modeled as enhanced
mass outflow at a dust mass outflow rate of M
yr superimposed on a steady outflow with a dust mass outflow rate of
M yr. It is likely that this shell has swept
up a substantial mass of interstellar gas during its expansion, so these
estimates are upper limits to the stellar mass loss rate.Comment: 31 pages, 12 figures, accepted to A
Forces behind China's Surging Trade: Competitiveness or Policy Driven?
This paper delves into China's differential growths in trade flows with high income countries by focusing on bilateral content of trade data over the time period 1962-2005. Unlike other studies, we account for end use of traded goods ranging from primary, intermediate, and finished goods because China's policies impact all segments China's trade flows. China's trade growth patterns with major high income countries clearly indicate that the adjacency-neighborhood partners alone is unlikely to explain its unprecedented growth in exports and imports. China's outstanding performance in trade growth can be traced back to the 1970s with changes in its policies and increased involvement in the international segmentation of production processes and preferential tariff treatment to assembling and processing activities.China, international trade, growth, policies, International Relations/Trade,
Developing Country Trade: Implications of China’s Changing Trade and Competitiveness in Intensive and Extensive Margin Goods
This paper delves into China’s differential growths in trade flows with high income and developing countries by focusing on bilateral content of trade data over the time period 1978-2005. Unlike other studies, we account for end use of traded goods ranging from primary, intermediate, and finished goods because China’s policies impact all segments China’s trade flows. In the last 28 years, China has specialized in deficits in the upstream production segments (parts and components) and rapid diversification in consumption goods (extensive margin). While in the late 1970s China’s export and import growth on all goods with major high income countries is outstanding in the most recent years China’s trade growth with developing countries has taken the lead while China is gaining in extensive margins goods trade. This general pattern evolving is in agreement with some of the new trade theory that gives a dominant role to an expansion of the number of export varieties (the extensive margin), which provides an additional channel for welfare gains from trade.China, international trade, growth, intensive, extensive margins, developing countries, International Relations/Trade,
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