352 research outputs found
Unsteady Crack Motion and Branching in a Phase-Field Model of Brittle Fracture
Crack propagation is studied numerically using a continuum phase-field
approach to mode III brittle fracture. The results shed light on the physics
that controls the speed of accelerating cracks and the characteristic branching
instability at a fraction of the wave speed.Comment: 11 pages, 4 figure
Experimental analysis of lateral impact on planar brittle material: spatial properties of the cracks
The breakup of glass and alumina plates due to planar impacts on one of their
lateral sides is studied. Particular attention is given to investigating the
spatial location of the cracks within the plates. Analysis based on a
phenomenological model suggests that bifurcations along the cracks' paths are
more likely to take place closer to the impact region than far away from it, i.
e., the bifurcation probability seems to lower as the perpendicular distance
from the impacted lateral in- creases. It is also found that many observables
are not sensitive to the plate material used in this work, as long as the
fragment multiplicities corresponding to the fragmentation of the plates are
similar. This gives support to the universal properties of the fragmentation
process reported in for- mer experiments. However, even under the just
mentioned circumstances, some spatial observables are capable of distinguishing
the material of which the plates are made and, therefore, it suggests that this
universality should be carefully investigated
Viral RNAs are unusually compact.
A majority of viruses are composed of long single-stranded genomic RNA molecules encapsulated by protein shells with diameters of just a few tens of nanometers. We examine the extent to which these viral RNAs have evolved to be physically compact molecules to facilitate encapsulation. Measurements of equal-length viral, non-viral, coding and non-coding RNAs show viral RNAs to have among the smallest sizes in solution, i.e., the highest gel-electrophoretic mobilities and the smallest hydrodynamic radii. Using graph-theoretical analyses we demonstrate that their sizes correlate with the compactness of branching patterns in predicted secondary structure ensembles. The density of branching is determined by the number and relative positions of 3-helix junctions, and is highly sensitive to the presence of rare higher-order junctions with 4 or more helices. Compact branching arises from a preponderance of base pairing between nucleotides close to each other in the primary sequence. The density of branching represents a degree of freedom optimized by viral RNA genomes in response to the evolutionary pressure to be packaged reliably. Several families of viruses are analyzed to delineate the effects of capsid geometry, size and charge stabilization on the selective pressure for RNA compactness. Compact branching has important implications for RNA folding and viral assembly
Experimental analysis of lateral impact on planar brittle material
The fragmentation of alumina and glass plates due to lateral impact is
studied. A few hundred plates have been fragmented at different impact
velocities and the produced fragments are analyzed. The method employed in this
work allows one to investigate some geometrical properties of the fragments,
besides the traditional size distribution usually studied in former
experiments. We found that, although both materials exhibit qualitative similar
fragment size distribution function, their geometrical properties appear to be
quite different. A schematic model for two-dimensional fragmentation is also
presented and its predictions are compared to our experimental results. The
comparison suggests that the analysis of the fragments' geometrical properties
constitutes a more stringent test of the theoretical models' assumptions than
the size distribution
Calculations on the Size Effects of Raman Intensities of Silicon Quantum Dots
Raman intensities of Si quantum dots (QDs) with up to 11,489 atoms (about 7.6
nm in diameter) for different scattering configurations are calculated. First,
phonon modes in these QDs, including all vibration frequencies and vibration
amplitudes, are calculated directly from the lattice dynamic matrix by using a
microscopic valence force field model combined with the group theory. Then the
Raman intensities of these quantum dots are calculated by using a
bond-polarizability approximation. The size effects of the Raman intensity in
these QDs are discussed in detail based on these calculations. The calculations
are compared with the available experimental observation. We are expecting that
our calculations can further stimulate more experimental measurements.Comment: 21 pages, 7 figure
Oscillating Fracture in Rubber
We have found an oscillating instability of fast-running cracks in thin
rubber sheets. A well-defined transition from straight to oscillating cracks
occurs as the amount of biaxial strain increases. Measurements of the amplitude
and wavelength of the oscillation near the onset of this instability indicate
that the instability is a Hopf bifurcation
An accurate description of quantum size effects in InP nanocrystallites over a wide range of sizes
We obtain an effective parametrization of the bulk electronic structure of
InP within the Tight Binding scheme. Using these parameters, we calculate the
electronic structure of InP clusters with the size ranging upto 7.5 nm. The
calculated variations in the electronic structure as a function of the cluster
size is found to be in excellent agreement with experimental results over the
entire range of sizes, establishing the effectiveness and transferability of
the obtained parameter strengths.Comment: 9 pages, 3 figures, pdf file available at
http://sscu.iisc.ernet.in/~sampan/publications.htm
Towards attosecond high-energy electron bunches : controlling self-injection in laser wakefield accelerators through plasma density modulation
Self-injection in a laser-plasma wakefield accelerator (LWFA) is usually achieved by increasing the laser intensity until the threshold for injection is exceeded. Alternatively, the velocity of the bubble accelerating structure can be controlled using plasma density ramps, reducing the electron velocity required for injection. We present a model describing self-injection in the short bunch regime for arbitrary changes in the plasma density. We derive the threshold condition for injection due to a plasma density gradient, which is confirmed using particle-in-cell (PIC) simulations that demonstrate injection of sub-femtosecond bunches. It is shown that the bunch charge, bunch length and separation of bunches in a bunch train can be controlled by tailoring the plasma density profile
Optical pumped ultra-short pulse CO2 lasers as drivers of laser-plasma accelerators and other applications
Optically pumped CO 2 lasers can operate with high efficiency, high repetition rate and large bandwidths, suitable for producing ultra-short pulses at terawatts to petawatts, in contrast to conventional discharge-pumped CO2 lasers, which are restricted by the requirements of discharge dynamics in high-pressure gas. We show how an optically pumped CO2 laser can be realised and we consider its application in laser-driven acceleration. There is potential to replace conventional transversely excited atmospheric CO2 lasers with diode-pumped solid-state lasers as a pump laser for a high-pressure CO2 gain medium, making it suitable for amplifying ultra-short pulses. We show that by driving a laser plasma wakefield accelerator with an ultra-short pulse CO 2 laser, a very high charge, high average current, high energy accelerator can be constructed. This could have a major impact on the application of these novel accelerators and radiation sources based on them
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