6,690 research outputs found
UK Strategy in the Gulf and Middle East After American Retrenchment
This is the final version. Available from SETA Foundation via the DOI in this recordThis paper considers the impact of the possible relative decline of the U.S. and its engagement in the Middle East and the Gulf in particular. US disengagement started under the Obama administration and seems to be continuing under the Trump administration. Applying theories of ‘rival hegemonic transition,’ possible post-Amercan successors are identified. In particular, the UK’s intents, capabilities, and strategies as it returns ‘East of Suez’ are examined, along with the geopolitical implications of such a return for the shifting balance of power in the Middle East
The Stochastic Dynamics of Rectangular and V-shaped Atomic Force Microscope Cantilevers in a Viscous Fluid and Near a Solid Boundary
Using a thermodynamic approach based upon the fluctuation-dissipation theorem
we quantify the stochastic dynamics of rectangular and V-shaped microscale
cantilevers immersed in a viscous fluid. We show that the stochastic cantilever
dynamics as measured by the displacement of the cantilever tip or by the angle
of the cantilever tip are different. We trace this difference to contributions
from the higher modes of the cantilever. We find that contributions from the
higher modes are significant in the dynamics of the cantilever tip-angle. For
the V-shaped cantilever the resulting flow field is three-dimensional and
complex in contrast to what is found for a long and slender rectangular
cantilever. Despite this complexity the stochastic dynamics can be predicted
using a two-dimensional model with an appropriately chosen length scale. We
also quantify the increased fluid dissipation that results as a V-shaped
cantilever is brought near a solid planar boundary.Comment: 10 pages, 15 images, corrected equation (8
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
Network formation of tissue cells via preferential attraction to elongated structures
Vascular and non-vascular cells often form an interconnected network in
vitro, similar to the early vascular bed of warm blooded embryos. Our
time-lapse recordings show that the network forms by extending sprouts, i.e.,
multicellular linear segments. To explain the emergence of such structures, we
propose a simple model of preferential attraction to stretched cells. Numerical
simulations reveal that the model evolves into a quasi-stationary pattern
containing linear segments, which interconnect above the critical volume
fraction of 0.2. In the quasi-stationary state the generation of new branches
offset the coarsening driven by surface tension. In agreement with empirical
data, the characteristic size of the resulting polygonal pattern is
density-independent within a wide range of volume fractions
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
Dynamics of a trapped Brownian particle in shear flows
The Brownian motion of a particle in a harmonic potential, which is
simultaneously exposed either to a linear shear flow or to a plane Poiseuille
flow is investigated. In the shear plane of both flows the probability
distribution of the particle becomes anisotropic and the dynamics is changed in
a characteristic manner compared to a trapped particle in a quiescent fluid.
The particle distribution takes either an elliptical or a parachute shape or a
superposition of both depending on the mean particle position in the shear
plane. Simultaneously, shear-induced cross-correlations between particle
fluctuations along orthogonal directions in the shear plane are found. They are
asymmetric in time. In Poiseuille flow thermal particle fluctuations
perpendicular to the flow direction in the shear plane induce a shift of the
particle's mean position away from the potential minimum. Two complementary
methods are suggested to measure shear-induced cross-correlations between
particle fluctuations along orthogonal directions.Comment: 14 pages, 7 figure
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
Calibration of optical tweezers with positional detection in the back-focal-plane
We explain and demonstrate a new method of force- and position-calibration
for optical tweezers with back-focal-plane photo detection. The method combines
power spectral measurements of thermal motion and the response to a sinusoidal
motion of a translation stage. It consequently does not use the drag
coefficient of the trapped ob ject as an input. Thus, neither the viscosity,
nor the size of the trapped ob ject, nor its distance to nearby surfaces need
to be known. The method requires only a low level of instrumentation and can be
applied in situ in all spatial dimensions. It is both accurate and precise:
true values are returned, with small error-bars. We tested this experimentally,
near and far from surfaces. Both position- and force-calibration were accurate
to within 3%. To calibrate, we moved the sample with a piezo-electric
translation stage, but the laser beam could be moved instead, e.g. by
acousto-optic deflectors. Near surfaces, this precision requires an improved
formula for the hydrodynamical interaction between an infinite plane and a
micro-sphere in non-constant motion parallel to it. We give such a formula.Comment: Submitted to: Review of Scientific Instruments. 13 pages, 5 figures.
Appendix added (hydrodynamically correct calibration
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