1,806 research outputs found
A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy
We find that the jump-into-contact of the cantilever in the atomic force
microscope (AFM) is caused by an inherent instability in the motion of the AFM
cantilever. The analysis is based on a simple model of the cantilever moving in
a nonlinear force field. We show that the jump-into-contact distance can be
used to find the interaction of the cantilever tip with the surface. In the
specific context of the attractive van der Waals interaction, this method can
be realized as a new method of measuring the Hamaker constant for materials.
The Hamaker constant is determined from the deflection of the cantilever at the
jump-into-contact using the force constant of the cantilever and the tip radius
of curvature, all of which can be obtained by measurements. The results have
been verified experimentally on a sample of cleaved mica, a sample of Si wafer
with natural oxide and a silver film, using a number of cantilevers with
different spring constants. We emphasize that the method described here is
applicable only to surfaces that have van der Waals interaction as the
tip-sample interaction. We also find that the tip to sample separation at the
jump-into-contact is simply related to the cantilever deflection at this point,
and this provides a method to exactly locate the surface.Comment: 11 pages, 4 figures, 1 tabl
Fetal tracheolaryngeal airway obstruction: prenatal evaluation by sonography and MRI
We reviewed the sonographic and MRI findings of tracheolaryngeal obstruction in the fetus. Conditions that can cause tracheolaryngeal obstruction include extrinsic causes such as lymphatic malformation, cervical teratoma and vascular rings and intrinsic causes such as congenital high airway obstruction syndrome (CHAOS). Accurate distinction of these conditions by sonography or MRI can help facilitate parental counseling and management, including the decision to utilize the ex utero intrapartum treatment (EXIT) procedure
An exonic splicing enhancer in human IGF-I pre-mRNA mediates recognition of alternative exon 5 by the serine-arginine protein splicing factor-2/alternative splicing factor
The human IGF-I gene has six exons, four of which are alternatively spliced. Variations in splicing involving exon 5 may occur, depending on the tissue type and hormonal environment. To study the regulation of splicing to IGF-I exon 5, we established an in vitro splicing assay, using a model pre-mRNA containing IGF-I exons 4 and 5 and part of the intervening intron. Using a series of deletion mutants, we identified an 18-nucleotide purine-rich splicing enhancer in exon 5 that increases the splicing efficiency of the upstream intron from 6 to 35%. We show that the serine-arginine protein splicing factor-2/alternative splicing factor specifically promotes splicing in cultured cells and in vitro and is recruited to the spliceosome in an enhancer-specific manner. Our findings are consistent with a role for splicing factor-2/alternative splicing factor in the regulation of splicing of IGF-I alternative exon 5 via a purine-rich exonic splicing enhancer
Turbulence compressibility corrections
The basic objective of this research was to identify, develop and recommend turbulence models which could be incorporated into CFD codes used in the design of the National AeroSpace Plane vehicles. To accomplish this goal, a combined effort consisting of experimental and theoretical phases was undertaken. The experimental phase consisted of a literature survey to collect and assess a database of well documented experimental flows, with emphasis on high speed or hypersonic flows, which could be used to validate turbulence models. Since it was anticipated that this database would be incomplete and would need supplementing, additional experiments in the NASA Ames 3.5-Foot Hypersonic Wind Tunnel (HWT) were also undertaken. The theoretical phase consisted of identifying promising turbulence models through applications to simple flows, and then investigating more promising models in applications to complex flows. The complex flows were selected from the database developed in the first phase of the study. For these flows it was anticipated that model performance would not be entirely satisfactory, so that model improvements or corrections would be required. The primary goals of the investigation were essentially achieved. A large database of flows was collected and assessed, a number of additional hypersonic experiments were conducted in the Ames HWT, and two turbulence models (kappa-epsilon and kappa-omega models with corrections) were determined which gave superior performance for most of the flows studied and are now recommended for NASP applications
Design, analysis and test verification of advanced encapsulation systems, phase 2 program results
Optical, electrical isolation, thermal structural, structural deflection, and thermal tests are reported. The utility of the optical, series capacitance, and structural deflection models was verified
Cultivation of Heligmosomoides polygyrus:An immunomodulatory nematode parasite and its secreted products
Heligmosomoides polygyrus (formerly known as Nematospiroides dubius, and also referred to by some as H. bakeri) is a gastrointestinal
helminth that employs multiple immunomodulatory mechanisms to establish chronic infection in mice and closely resembles prevalent human
helminth infections. H. polygyrus has been studied extensively in the field of helminth-derived immune regulation and has been found to potently
suppress experimental models of allergy and autoimmunity (both with active infection and isolated secreted products). The protocol described in
this paper outlines management of the H. polygyrus life cycle for consistent production of L3 larvae, recovery of adult parasites, and collection of
their excretory-secretory products (HES)
The equivocal appendix at CT: prevalence in a control population
The purpose of the study was to determine the prevalence of appendices with an equivocal appearance at computed tomography (CT) in a control population. We retrospectively identified a control population of 150 patients who underwent CT of the abdomen and pelvis for evaluation of hematuria (without abdominal pain, fever, or colonic disease). One reader measured the diameter of the appendix and noted if the appendix was either isodense in appearance or airless and fluid filled. Sixty-seven of 150 cases (44.6%) demonstrated appendiceal diameter greater than 6Â mm. The appendix was collapsed or isodense in 34/150 cases (22.7%). Only ten of 150 or 6.6% of cases were isodense in combination with diameter greater than 6Â mm, and none had diameter greater than 10Â mm. Only one of 150 cases (0.67%) demonstrated airless fluid within the lumen, and the appendix measured less than 6Â mm. While the diameter of the normal appendix is frequently greater than 6Â mm, none measured greater than 10Â mm in combination with ambiguous morphology. Furthermore, in the normal appendix, airless fluid filling the lumen is a rare appearance with a prevalence of less than 1%. While appendicitis could undoubtedly occur in an isodense appendix between 6 and 10Â mm in diameter, such an appearance can occur in up to 6.6% of the normal populatio
Precision Measurement of the Radiative Decay of the Free Neutron
The standard model predicts that, in addition to a proton, an electron, and
an antineutrino, a continuous spectrum of photons is emitted in the
decay of the free neutron. We report on the RDK II experiment which measured
the photon spectrum using two different detector arrays. An annular array of
bismuth germanium oxide scintillators detected photons from 14 to 782~keV. The
spectral shape was consistent with theory, and we determined a branching ratio
of 0.00335 0.00005 [stat] 0.00015 [syst]. A second detector array
of large area avalanche photodiodes directly detected photons from 0.4 to
14~keV. For this array, the spectral shape was consistent with theory, and the
branching ratio was determined to be 0.00582 0.00023 [stat] 0.00062
[syst]. We report the first precision test of the shape of the photon energy
spectrum from neutron radiative decay and a substantially improved
determination of the branching ratio over a broad range of photon energies
Casimir-Polder forces, boundary conditions and fluctuations
We review different aspects of the atom-atom and atom-wall Casimir-Polder
forces. We first discuss the role of a boundary condition on the interatomic
Casimir-Polder potential between two ground-state atoms, and give a physically
transparent interpretation of the results in terms of vacuum fluctuations and
image atomic dipoles. We then discuss the known atom-wall Casimir-Polder force
for ground- and excited-state atoms, using a different method which is also
suited for extension to time-dependent situations. Finally, we consider the
fluctuation of the Casimir-Polder force between a ground-state atom and a
conducting wall, and discuss possible observation of this force fluctuation.Comment: 5 page
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