11,997 research outputs found
Elastic Instability Triggered Pattern Formation
Recent experiments have exploited elastic instabilities in membranes to
create complex patterns. However, the rational design of such structures poses
many challenges, as they are products of nonlinear elastic behavior. We pose a
simple model for determining the orientational order of such patterns using
only linear elasticity theory which correctly predicts the outcomes of several
experiments. Each element of the pattern is modeled by a "dislocation dipole"
located at a point on a lattice, which then interacts elastically with all
other dipoles in the system. We explicitly consider a membrane with a square
lattice of circular holes under uniform compression and examine the changes in
morphology as it is allowed to relax in a specified direction.Comment: 15 pages, 7 figures, the full catastroph
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Hydropyrolysis of high molecular weight organic matter in Murchison
Hydropyrolysis of the Murchison macromolecular material releases polyaromatic compounds including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation
Coherent laminar and turbulent motion of toroidal vortex bundles
Motivated by experiments performed in superfluid helium, we study numerically
the motion of toroidal bundles of vortex filaments in an inviscid fluid. We
find that the evolution of these large-scale vortex structures involves the
generalised leapfrogging of the constituent vortex rings. Despite three
dimensional perturbations in the form of Kelvin waves and vortex reconnections,
toroidal vortex bundles retain their coherence over a relatively large distance
(compared to their size), in agreement with experimental observations.Comment: 22 pages, 12 figure
Probing Correlated Ground States with Microscopic Optical Model for Nucleon Scattering off Doubly-Closed-Shell Nuclei
The RPA long range correlations are known to play a significant role in
understanding the depletion of single particle-hole states observed in (e, e')
and (e, e'p) measurements. Here the Random Phase Approximation (RPA) theory,
implemented using the D1S force is considered for the specific purpose of
building correlated ground states and related one-body density matrix elements.
These may be implemented and tested in a fully microscopic optical model for NA
scattering off doubly-closed-shell nuclei. A method is presented to correct for
the correlations overcounting inherent to the RPA formalism. One-body density
matrix elements in the uncorrelated (i.e. Hartree-Fock) and correlated (i.e.
RPA) ground states are then challenged in proton scattering studies based on
the Melbourne microscopic optical model to highlight the role played by the RPA
correlations. Effects of such correlations which deplete the nuclear matter at
small radial distance (r 2 fm) and enhance its surface region, are getting
more and more sizeable as the incident energy increases. Illustrations are
given for proton scattering observables measured up to 201 MeV for the
O, Ca, Ca and Pb target nuclei. Handling the RPA
correlations systematically improves the agreement between scattering
predictions and data for energies higher than 150 MeV.Comment: 20 pages, 7 figure
Stretching Instability of Helical Spring
We show that when a gradually increasing tensile force is applied to the ends
of a helical spring with sufficiently large ratios of radius to pitch and twist
to bending rigidity, the end-to-end distance undergoes a sequence of
discontinuous stretching transitions. Subsequent decrease of the force leads to
step-like contraction and hysteresis is observed. For finite helices, the
number of these transitions increases with the number of helical turns but only
one stretching and one contraction instability survive in the limit of an
infinite helix. We calculate the critical line that separates the region of
parameters in which the deformation is continuous from that in which stretching
instabilities occur, and propose experimental tests of our predictions.Comment: 5 pages, 4 figure
Space shuttle abort separation pressure investigation. Volume 2, Part B: Orbiter data at Mach 5
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Dynamics of (SUSY) AdS Space Isometry Breaking
Actions governing the dynamics of the Nambu-Goldstone modes resulting from
the spontaneous breaking of the SO(4,2) and isometries of five
dimensional anti-de Sitter space () and SUSY
spaces respectively due to a restriction of the motion to embedded four
dimensional space and four dimensional Minkowski space () probe
branes are presented. The dilatonic Nambu-Goldstone mode governing the motion
of the space probe brane into the covolume of the SUSY
space is found to be unstable. No such instablility appears in the other cases.
Gauging these symmetries leads to an Einstein-Hilbert action containing, in
addition to the gravitational vierbein, a massive Abelian vector field coupled
to gravity.Comment: 6 pages, 0 figures, to appear in Proceedings of IRGAC 2006: 2nd
International Conference on Quantum Theories and Renormalization Group in
Gravity and Cosmology, Barcelon
Creating correct aberrations: why blur isn’t always bad in the eye
In optics in general, a sharp aberration-free image is normally the desired goal, and the whole field of adaptive optics has developed with the aim of producing blur-free images. Likewise, in ophthalmic optics we normally aim for a sharp image on the retina. But even with an emmetropic, or well-corrected eye, chromatic and high order aberrations affect the image. We describe two different areas where it is important to take these effects into account and why creating blur correctly via rendering can be advantageous. Firstly we show how rendering chromatic aberration correctly can drive accommodation in the eye and secondly report on matching defocus-l generated using rendering with conventional optical defocus
Ocular accommodation and wavelength: The effect of longitudinal chromatic aberration on the stimulus-response curve.
The longitudinal chromatic aberration (LCA) of the eye creates a chromatic blur on the retina that is an important cue for accommodation. Although this mechanism can work optimally in broadband illuminants such as daylight, it is not clear how the system responds to the narrowband illuminants used by many modern displays. Here, we measured pupil and accommodative responses as well as visual acuity under narrowband light-emitting diode (LED) illuminants of different peak wavelengths. Observers were able to accommodate under narrowband light and compensate for the LCA of the eye, with no difference in the variability of the steady-state accommodation response between narrowband and broadband illuminants. Intriguingly, our subjects compensated more fully for LCA at nearer distances. That is, the difference in accommodation to different wavelengths became larger when the object was placed nearer the observer, causing the slope of the accommodation response curve to become shallower for shorter wavelengths and steeper for longer ones. Within the accommodative range of observers, accommodative errors were small and visual acuity normal. When comparing between illuminants, when accommodation was accurate, visual acuity was worst for blue narrowband light. This cannot be due to the sparser spacing for S-cones, as our stimuli had equal luminance and thus activated LM-cones roughly equally. It is likely because ocular LCA changes more rapidly at shorter wavelength and so the finite spectral bandwidth of LEDs corresponds to a greater dioptric range at shorter wavelengths. This effect disappears for larger accommodative errors, due to the increased depth of focus of the eye
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