6,010 research outputs found
Impedance of a sphere oscillating in an elastic medium with and without slip
The dynamic impedance of a sphere oscillating in an elastic medium is
considered. Oestreicher's formula for the impedance of a sphere bonded to the
surrounding medium can be expressed simply in terms of three lumped impedances
associated with the displaced mass and the longitudinal and transverse waves.
If the surface of the sphere slips while the normal velocity remains
continuous, the impedance formula is modified by adjusting the definition of
the transverse impedance to include the interfacial impedance.Comment: 10 pages, 2 figure
Consequences of asteroid fragmentation during impact hazard mitigation
The consequences of the fragmentation of an Earth-threatening asteroid due to an attempted deflection are examined in this paper. The minimum required energy for a successful impulsive deflection of a threatening object is computed and compared to the energy required to break up a small size asteroid. The results show that the fragmentation of an asteroid that underwent an impulsive deflection, such as a kinetic impact or a nuclear explosion, is a very plausible event.Astatistical model is used to approximate the number and size of the fragments as well as the distribution of velocities at the instant after the deflection attempt takes place. This distribution of velocities is a function of the energy provided by the deflection attempt, whereas the number and size of the asteroidal fragments is a function of the size of the largest fragment. The model also takes into account the gravity forces that could lead to a reaggregation of the asteroid after fragmentation. The probability distribution of the pieces after the deflection is then propagated forward in time until the encounter with Earth. A probability damage factor (i.e., expected damage caused by a given size fragment multiplied by its impact probability) is then computed and analyzed for different plausible scenarios, characterized by different levels of deflection energies and lead times
Giant viscosity enhancement in a spin-polarized Fermi liquid
The viscosity is measured for a Fermi liquid, a dilute He-He mixture,
under extremely high magnetic field/temperature conditions ( T, mK). The spin splitting energy is substantially greater than
the Fermi energy ; as a consequence the polarization tends to unity
and s-wave quasiparticle scattering is suppressed for . Using a
novel composite vibrating-wire viscometer an enhancement of the viscosity is
observed by a factor of more than 500 over its low-field value. Good agreement
is found between the measured viscosity and theoretical predictions based upon
a -matrix formalism.Comment: 4 pages, 4 figure
Order parameters in the Verwey phase transition
The Verwey phase transition in magnetite is analyzed on the basis of the
Landau theory. The free energy functional is expanded in a series of components
belonging to the primary and secondary order parameters. A low-temperature
phase with the monoclinic P2/c symmetry is a result of condensation of two
order parameters X_3 and \Delta_5 . The temperature dependence of the shear
elastic constant C_44 is derived and the mechanism of its softening is
discussed.Comment: 4 pages, 1 figur
Tribology of particle suspensions in rolling-sliding soft contacts
We investigate the lubrication of microsphere suspensions between compliant substrates, and probe the influence of matrix viscosity, particle phase volume, surface roughness and wetting, and slide-to-roll ratio (SRR). In general, the suspensions behave as a continuum in the elastohydrodynamic regime provided the film thickness, which is predicted from the product of speed and viscosity, is greater than the particle diameter. Below this, the frictional response is characteristic of the mixed and boundary regimes. In the boundary regime, friction is independent of phase volume above 5% and it is governed by the rolling friction associated with particles being entrained into the contact that is independent of SRR, which is made possible by substrate deformation. This study provides a benchmark for soft-tribology and biotribology studies involving more complex particle suspensions and particle-containing soft materials
Decreased glutathione biosynthesis contributes to EGFR T790M-driven erlotinib resistance in non-small cell lung cancer
Epidermal growth factor receptor (EGFR) inhibitors such as erlotinib are novel effective agents in the treatment of EGFR-driven lung cancer, but their clinical impact is often impaired by acquired drug resistance through the secondary T790M EGFR mutation. To overcome this problem, we analysed the metabonomic differences between two independent pairs of erlotinib-sensitive/resistant cells and discovered that glutathione (GSH) levels were significantly reduced in T790M EGFR cells. We also found that increasing GSH levels in erlotinib-resistant cells re-sensitised them, whereas reducing GSH levels in erlotinib-sensitive cells made them resistant. Decreased transcription of the GSH-synthesising enzymes (GCLC and GSS) due to the inhibition of NRF2 was responsible for low GSH levels in resistant cells that was directly linked to the T790M mutation. T790M EGFR clinical samples also showed decreased expression of these key enzymes; increasing intra-tumoural GSH levels with a small-molecule GST inhibitor re-sensitised resistant tumours to erlotinib in mice. Thus, we identified a new resistance pathway controlled by EGFR T790M and a therapeutic strategy to tackle this problem in the clinic
Isospin splittings of meson and baryon masses from three-flavor lattice QCD + QED
Lattice QCD simulations are now reaching a precision where isospin breaking
effects become important. Previously, we have developed a program to
systematically investigate the pattern of flavor symmetry beaking within QCD
and successfully applied it to meson and baryon masses involving up, down and
strange quarks. In this Letter we extend the calculations to QCD + QED and
present our first results on isospin splittings in the pseudoscalar meson and
baryon octets. In particular, we obtain the nucleon mass difference of
M_n-M_p=1.35(18)(8)\,\mbox{MeV} and the electromagnetic contribution to the
pion splitting M_{\pi^+}-M_{\pi^0}=4.60(20)\,\mbox{MeV}. Further we report
first determination of the separation between strong and electromagnetic
contributions in the scheme.Comment: 14 pages, 10 figures, text and figures added, Journal versio
The declining representativeness of the British party system, and why it matters
In a recent article, Michael Laver has explained âWhy Vote-Seeking Parties May Make Voters Miserableâ. His model shows that, while ideological convergence may boost congruence between governments and the median voter, it can reduce congruence between the party system and the electorate as a whole. Specifically, convergence can increase the mean distance between voters and their nearest party. In this article we show that this captures the reality of todayâs British party system. Policy scale placements in British Election Studies from 1987 to 2010 confirm that the pronounced convergence during the past decade has left the Conservatives and Labour closer together than would be optimal in terms of minimising the policy distance between the average voter and the nearest major party. We go on to demonstrate that this comes at a cost. Respondents who perceive themselves as further away from one of the major parties in the system tend to score lower on satisfaction with democracy. In short, vote-seeking parties have left the British party system less representative of the ideological diversity in the electorate, and thus made at least some British voters miserable
High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations
Structural evolution of a prospective hydrogen storage material, ammonia
borane NH3BH3, has been studied at high pressures up to 12 GPa and at low
temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a
disordered I4mm structure reversibly transforms into a new ordered phase. Its
Cmc21 structure was solved from the diffraction data, the positions of N and B
atoms and the orientation of NH3 and BH3 groups were finally assigned with the
help of density functional theory calculations. Group-theoretical analysis
identifies a single two-component order parameter, combining ordering and
atomic displacement mechanisms, which link an orientationally disordered parent
phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a
generic phase diagram for NH3BH3, mapping three experimentally found and one
predicted (P21) phases as a function of temperature and pressure, along with
the evolution of the corresponding structural distortions. Ammonia borane
belongs to the class of improper ferroelastics and we show that both
temperature- and pressure-induced phase transitions can be driven to be of the
second order. The role of N-H...H-B dihydrogen bonds and other intermolecular
interactions in the stability of NH3BH3 polymorphs is examined.Comment: 23 pages, 7 figure
- âŠ