12,822 research outputs found
Torsion and bending of nucleic acids studied by subnanosecond time-resolved fluorescence depolarization of intercalated dyes
Subnanosecond time‐resolved fluorescence depolarization has been used to monitor the reorientation of ethidium bromide intercalated in native DNA, synthetic polynucleotide complexes, and in supercoiled plasmid DNA. The fluorescence polarization anisotropy was successfully analyzed with an elastic model of DNA dynamics, including both torsion and bending, which yielded an accurate value for the torsional rigidity of the different DNA samples. The dependence of the torsional rigidity on the base sequence, helical structure, and tertiary structure was experimentally observed. The magnitude of the polyelectrolyte contribution to the torsional rigidity of DNA was measured over a wide range of ionic strength, and compared with polyelectrolyte theories for the persistence length. We also observed a rapid initial reorientation of the intercalated ethidium which had a much smaller amplitude in RNA than in DNA
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LENS® and SFF: Enabling Technologies for Optimized Structures
Optimized, lightweight, high-strength structures are needed in many applications from aerospace
to automotive. In pursuit of such structures, there have been proposed analytical solutions and
some specialized FEA solutions for specific structures such as automobile frames. However,
generalized 3D optimization methods have been unavailable for use by most designers.
Moreover, in the cases where optimized structural solutions are available, they are often hollow,
curving, thin wall structures that cannot be fabricated by conventional manufacturing methods.
Researchers at Sandia National Laboratories and the University of Rhode Island teamed to solve
these problems. The team has been pursuing two methods of optimizing models for generalized
loading conditions, and also has been investigating the methods needed to fabricate these
structures using Laser Engineered Net Shaping™ (LENS®) and other rapid prototyping
methods. These solid freeform fabrication (SFF) methods offer the unique ability to make
hollow, high aspect ratio features out of many materials. The manufacturing development
required for LENS to make these complex structures has included the addition of rotational axes
to Sandia’s LENS machine bringing the total to 5 controlled axes. The additional axes have
required new efforts in process planning. Several of the unique structures that are only now
possible through the use of SFF technology are shown as part of the discussion of this exciting
new application for SFF.Mechanical Engineerin
Time-resolved spectroscopy of macromolecules: Effect of helical structure on the torsional dynamics of DNA and RNA
The torsional rigidity of DNA and RNA is measured via the fluorescence depolarization technique
String spectra near some null cosmological singularities
We construct cosmological spacetimes with null Kasner-like singularities as
purely gravitational solutions with no other background fields turned on. These
can be recast as anisotropic plane-wave spacetimes by coordinate
transformations. We analyse string quantization to find the spectrum of string
modes in these backgrounds. The classical string modes can be solved for
exactly in these time-dependent backgrounds, which enables a detailed study of
the near singularity string spectrum, (time-dependent) oscillator masses and
wavefunctions. We find that for low lying string modes(finite oscillation
number), the classical near-singularity string mode functions are non-divergent
for various families of singularities. Furthermore, for any infinitesimal
regularization of the vicinity of the singularity, we find a tower of string
modes of ultra-high oscillation number which propagate essentially freely in
the background. The resulting picture suggests that string interactions are
non-negligible near the singularity.Comment: Latex, 30pgs; v2. minor clarifications, references adde
Humpback and Fin Whaling in the Gulf of Maine from 1800 to 1918
The history of whaling in the Gulf of Maine was reviewed primarily to estimate removals of humpback whales, Megaptera novaeangliae, especially during the 19th century. In the decades from 1800 to 1860, whaling effort consisted of a few localized, small-scale, shore-based enterprises on the coast of Maine and Cape Cod, Mass. Provincetown and Nantucket schooners occasionally conducted short cruises for humpback whales in New England waters. With the development of bomb-lance technology at mid century, the ease of killing humpback whales and fin whales, Balaenoptera physalus, increased. As a result, by the 1870’s there was considerable local interest in hunting rorquals (baleen whales in the family Balaenopteridae, which include the humpback and fin whales) in the Gulf of Maine. A few schooners were specially outfitted to take rorquals in the late 1870’s and 1880’s although their combined annual take was probably no more than a few tens of whales. Also in about 1880, fishing steamers began to be used to hunt whales in the Gulf of Maine. This steamer fishery grew to include about five vessels regularly engaged in whaling by the mid 1880’s but dwindled to only one vessel by the end of the decade. Fin whales constituted at least half of the catch, which exceeded 100 animals in some years. In the late 1880’s and thereafter, few whales were taken by whaling vessels in the Gulf of Maine
Infinite products involving binary digit sums
Let denote the Thue-Morse sequence with values . The
Woods-Robbins identity below and several of its generalisations are well-known
in the literature
\begin{equation*}\label{WR}\prod_{n=0}^\infty\left(\frac{2n+1}{2n+2}\right)^{u_n}=\frac{1}{\sqrt
2}.\end{equation*} No other such product involving a rational function in
and the sequence seems to be known in closed form. To understand these
products in detail we study the function
\begin{equation*}f(b,c)=\prod_{n=1}^\infty\left(\frac{n+b}{n+c}\right)^{u_n}.\end{equation*}
We prove some analytical properties of . We also obtain some new identities
similar to the Woods-Robbins product.Comment: Accepted in Proc. AMMCS 2017, updated according to the referees'
comment
Impact Ionization in ZnS
The impact ionization rate and its orientation dependence in k space is
calculated for ZnS. The numerical results indicate a strong correlation to the
band structure. The use of a q-dependent screening function for the Coulomb
interaction between conduction and valence electrons is found to be essential.
A simple fit formula is presented for easy calculation of the energy dependent
transition rate.Comment: 9 pages LaTeX file, 3 EPS-figures (use psfig.sty), accepted for
publication in PRB as brief Report (LaTeX source replaces raw-postscript
file
Toward the End of Time
The null-brane space-time provides a simple model of a big crunch/big bang
singularity. A non-perturbative definition of M-theory on this space-time was
recently provided using matrix theory. We derive the fermion couplings for this
matrix model and study the leading quantum effects. These effects include
particle production and a time-dependent potential. Our results suggest that as
the null-brane develops a big crunch singularity, the usual notion of
space-time is replaced by an interacting gluon phase. This gluon phase appears
to constitute the end of our conventional picture of space and time.Comment: 31 pages, reference adde
Non-equilibrium steady state of sparse systems
A resistor-network picture of transitions is appropriate for the study of
energy absorption by weakly chaotic or weakly interacting driven systems. Such
"sparse" systems reach a novel non-equilibrium steady state (NESS) once coupled
to a bath. In the stochastic case there is an analogy to the physics of
percolating glassy systems, and an extension of the fluctuation-dissipation
phenomenology is proposed. In the mesoscopic case the quantum NESS might differ
enormously from the stochastic NESS, with saturation temperature determined by
the sparsity. A toy model where the sparsity of the system is modeled using a
log-normal random ensemble is analyzed.Comment: 6 pages, 6 figures, EPL accepted versio
Contact area of rough spheres: Large scale simulations and simple scaling laws
We use molecular simulations to study the nonadhesive and adhesive
atomic-scale contact of rough spheres with radii ranging from nanometers to
micrometers over more than ten orders of magnitude in applied normal load. At
the lowest loads, the interfacial mechanics is governed by the contact
mechanics of the first asperity that touches. The dependence of contact area on
normal force becomes linear at intermediate loads and crosses over to Hertzian
at the largest loads. By combining theories for the limiting cases of nominally
flat rough surfaces and smooth spheres, we provide parameter-free analytical
expressions for contact area over the whole range of loads. Our results
establish a range of validity for common approximations that neglect curvature
or roughness in modeling objects on scales from atomic force microscope tips to
ball bearings.Comment: 2 figures + Supporting Materia
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