1,875 research outputs found
Breakdown of a conservation law in incommensurate systems
We show that invariance properties of the Lagrangian of an incommensurate
system, as described by the Frenkel Kontorova model, imply the existence of a
generalized angular momentum which is an integral of motion if the system
remains floating. The behavior of this quantity can therefore monitor the
character of the system as floating (when it is conserved) or locked (when it
is not). We find that, during the dynamics, the non-linear couplings of our
model cause parametric phonon excitations which lead to the appearance of
Umklapp terms and to a sudden deviation of the generalized momentum from a
constant value, signalling a dynamical transition from a floating to a pinned
state. We point out that this transition is related but does not coincide with
the onset of sliding friction which can take place when the system is still
floating.Comment: 7 pages, 6 figures, typed with RevTex, submitted to Phys. Rev. E
Replaced 27-03-2001: changes to text, minor revision of figure
Atomic Scale Sliding and Rolling of Carbon Nanotubes
A carbon nanotube is an ideal object for understanding the atomic scale
aspects of interface interaction and friction. Using molecular statics and
dynamics methods different types of motion of nanotubes on a graphite surface
are investigated. We found that each nanotube has unique equilibrium
orientations with sharp potential energy minima. This leads to atomic scale
locking of the nanotube.
The effective contact area and the total interaction energy scale with the
square root of the radius. Sliding and rolling of nanotubes have different
characters. The potential energy barriers for sliding nanotubes are higher than
that for perfect rolling. When the nanotube is pushed, we observe a combination
of atomic scale spinning and sliding motion. The result is rolling with the
friction force comparable to sliding.Comment: 4 pages (two column) 6 figures - one ep
Titan Science with the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the
successor to the Hubble Space Telescope (HST) but with a significantly larger
aperture (6.5 m) and advanced instrumentation focusing on infrared science
(0.6-28.0 m ). In this paper we examine the potential for scientific
investigation of Titan using JWST, primarily with three of the four
instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be
complementary. Five core scientific themes are identified: (i) surface (ii)
tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and
(v) stratospheric hazes. We discuss each theme in depth, including the
scientific purpose, capabilities and limitations of the instrument suite, and
suggested observing schemes. We pay particular attention to saturation, which
is a problem for all three instruments, but may be alleviated for NIRCam
through use of selecting small sub-arrays of the detectors - sufficient to
encompass Titan, but with significantly faster read-out times. We find that
JWST has very significant potential for advancing Titan science, with a
spectral resolution exceeding the Cassini instrument suite at near-infrared
wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In
particular, JWST will be valuable for time-domain monitoring of Titan, given a
five to ten year expected lifetime for the observatory, for example monitoring
the seasonal appearance of clouds. JWST observations in the post-Cassini period
will complement those of other large facilities such as HST, ALMA, SOFIA and
next-generation ground-based telescopes (TMT, GMT, EELT).Comment: 50 pages, including 22 figures and 2 table
Haptoglobin Polymorphism: A Novel Genetic Risk Factor for Celiac Disease Development and Its Clinical Manifestations
Background: Haptoglobin (Hp) α-chain alleles 1 and 2 account for 3 phenotypes that may influence the course of inflammatory diseases via biologically important differences in their antioxidant, scavenging, and immunomodulatory properties. Hp1-1 genotype results in the production of small dimeric, Hp2-1 linear, and Hp2-2 cyclic polymeric haptoglobin molecules. We investigated the haptoglobin polymorphism in patients with celiac disease and its possible association to the presenting symptoms.
Methods: We studied 712 unrelated, biopsy-proven Hungarian celiac patients (357 children, 355 adults; severe malabsorption 32.9%, minor gastrointestinal symptoms 22.8%, iron deficiency anemia 9.4%, dermatitis herpetiformis 15.6%, silent disease 7.2%, other 12.1%) and 384 healthy subjects. We determined haptoglobin phenotypes by gel electrophoresis and assigned corresponding genotypes.
Results: Hp2-1 was associated with a significant risk for celiac disease (P = 0.0006, odds ratio [OR] 1.54, 95% CI 1.20–1.98; prevalence 56.9% in patients vs 46.1% in controls). It was also overrepresented among patients with mild symptoms (69.2%) or silent disease (72.5%). Hp2-2 was less frequent in patients than in controls (P = 0.0023), but patients having this phenotype were at an increased risk for severe malabsorption (OR 2.21, 95% CI 1.60–3.07) and accounted for 45.3% of all malabsorption cases. Celiac and dermatitis herpetiformis patients showed similar haptoglobin phenotype distributions.
Conclusions: The haptoglobin polymorphism is associated with susceptibility to celiac disease and its clinical presentations. The predominant genotype in the celiac population was Hp2-1, but Hp2-2 predisposed to a more severe clinical course. The phenotype-dependent effect of haptoglobin may result from the molecule’s structural and functional properties
Importância de variáveis ambientais na soma térmica de cultivares de trigo.
Orientador: Genei Antonio Dalmago
Accelerating progress towards the SDGs: Collaborative policymaking in sanitation for integrated benefits in Sub-Saharan Africa
In 2020, 54% of the world had access to safe sanitation, but access further reduces to 21% in sub-Saharan Africa . With only eight years left to meet the Sustainable Development Goals (SDGs), there is an urgent need to accelerate action in the sanitation sector. Previous academic research highlights sanitation as vital for supporting cross-cutting outcomes in health, economics, climate, gender equality, and the environment. However, there is a gap in knowledge outputs for policymakers to highlights how action in sanitation can leverage investment from agencies and ministries to achieve multiple SDGs. Our transdisciplinary team (academics, policymakers, and practitioners) co-developed actionable outputs for diverse audiences to fill this gap, building on an existing global evidence review identifying 130 synergies between sanitation and the SDGs.
We identified priority focus areas influencing sanitation policies and financing in Sub-Saharan Africa to collaboratively develop visual aids, policy briefs, and this academic publication to engage cross-sectoral audiences. Our approach offers insights into the value of theoretical knowledge for policy outcomes in the sanitation sector. It is most likely that SDG6 Targets particularly related to sanitation will not be met by 2030 as the sector suffers from under investment. Co-creation between academia and the development sector is critical for consolidating knowledge/research and development practice to influence investment, progress and innovation
Thermal effects on atomic friction
We model friction acting on the tip of an atomic force microscope as it is
dragged across a surface at non-zero temperatures. We find that stick-slip
motion occurs and that the average frictional force follows ,
where is the tip velocity. This compares well to recent experimental work
(Gnecco et al, PRL 84, 1172), permitting the quantitative extraction of all
microscopic parameters. We calculate the scaled form of the average frictional
force's dependence on both temperature and tip speed as well as the form of the
friction-force distribution function.Comment: Accepted for publication, Physical Review Letter
Femtosecond laser microstructuring of zirconia dental implants
This study evaluated the suitability of femtosecond laser for microtexturizing cylindrical zirconia dental implants surface. Sixty-six cylindrical zirconia implants were used and divided into three groups: Control group (with no laser modification), Group A (microgropored texture), and Group B (microgrooved texture). Scanning electron microscopy observation of microgeometries revealed minimal collateral damage of the original surface surrounding the treated areas. Optical interferometric profilometry showed that ultrafast laser ablation increased surface roughness (Ra, Rq, Rz, and Rt) significantly for both textured patterns from 1.2× to 6×-fold when compared with the control group (p Group B 8.4% ± 0.42% > Group A 1.6% ± 0.35%) and aluminum (Control 4.3% ± 0.9% > Group B 2.3% ± 0.3% > Group A 1.16% ± 0.2%) in the laser-treated surfaces (p Group A 1.94% > Group B 1.72%) as the surfaces were processed with ultrashort laser pulses. We concluded that femtosecond laser microstructuring offers an interesting alternative to conventional surface treatments of zirconia implants as a result of its precision and minimal damage of the surrounding areas
The Static and Dynamic Lattice Changes Induced by Hydrogen Adsorption on NiAl(110)
Static and dynamic changes induced by adsorption of atomic hydrogen on the
NiAl(110) lattice at 130 K have been examined as a function of adsorbate
coverage. Adsorbed hydrogen exists in three distinct phases. At low coverages
the hydrogen is itinerant because of quantum tunneling between sites and
exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate
mediated interactions produce an ordered superstructure with c(2x2) symmetry,
and at higher coverages, hydrogen exists as a disordered lattice gas. This
picture of how hydrogen interacts with NiAl(110) is developed from our data and
compared to current theoretical predictions.Comment: 36 pages, including 12 figures, 2 tables and 58 reference
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