1,277 research outputs found
Strain-rate sensitivity of glasses
AbstractWe report on the loading-rate dependence of localized plastic deformation in inorganic covalent, metallic, ionic and superionic glasses. For this, the strain-rate sensitivity is determined through instrumented nanoindentation in a load-controlled strain-rate jump test. Through relating the strain-rate sensitivity to the reduced temperature, the packing density, the network dimensionality and the average single bond strength of the system, a qualitative mechanistic description of the strain-mediating process is possible. A strong variability of strain-rate sensitivity is obtained only at intermediate values of packing density, network connectivity or bond strength, when other parameters such as chemical composition and specific structural arrangement are dominating the deformation process. On the other side, for high bond strength and connectivity or for high packing density, the strain-rate sensitivity of the considered glasses is always low, which is also confirmed through the dependence of strain-rate sensitivity on Poisson ratio. Here, only for glasses with a Poisson ratio of ~0.3–0.4 we observe a wide variability of the loading-rate dependence of local deformation. For higher or lower Poisson ratio, the observed dependence is always low: when the limiting factor in deformation is primarily network connectivity and bond strength or packing density, respectively, once an activation barrier is overcome, deformation is only weakly loading-rate-dependent. This is regardless of the height of the activation barrier. When approaching the glass transition temperature, high strain-rate sensitivity is observed only in glasses where non-Newtonian flow is expected also in the corresponding liquid
Polyelectrolyte Bundles
Using extensive Molecular Dynamics simulations we study the behavior of
polyelectrolytes with hydrophobic side chains, which are known to form
cylindrical micelles in aqueous solution. We investigate the stability of such
bundles with respect to hydrophobicity, the strength of the electrostatic
interaction, and the bundle size. We show that for the parameter range relevant
for sulfonated poly-para-phenylenes (PPP) one finds a stable finite bundle
size. In a more generic model we also show the influence of the length of the
precursor oligomer on the stability of the bundles. We also point out that our
model has close similarities to DNA solutions with added condensing agents,
hinting to the possibility that the size of DNA aggregates is under certain
circumstances thermodynamically limited.Comment: 10 pages, 8 figure
Conductance of the single-electron transistor: A comparison of experimental data with Monte Carlo calculations
We report on experimental results for the conductance of metallic
single-electron transistors as a function of temperature, gate voltage and
dimensionless conductance. In contrast to previous experiments our transistor
layout allows for a direct measurement of the parallel conductance and no ad
hoc assumptions on the symmetry of the transistors are necessary. Thus we can
make a comparison between our data and theoretical predictions without any
adjustable parameter. Even for rather weakly conducting transistors significant
deviations from the perturbative results are noted. On the other hand, path
integral Monte Carlo calculations show remarkable agreement with experiments
for the whole range of temperatures and conductances.Comment: 8 pages, 7 figures, revtex4, corrected typos, submitted to PR
Data Reduction Pipeline for the CHARIS Integral-Field Spectrograph I: Detector Readout Calibration and Data Cube Extraction
We present the data reduction pipeline for CHARIS, a high-contrast
integral-field spectrograph for the Subaru Telescope. The pipeline constructs a
ramp from the raw reads using the measured nonlinear pixel response, and
reconstructs the data cube using one of three extraction algorithms: aperture
photometry, optimal extraction, or fitting. We measure and apply both
a detector flatfield and a lenslet flatfield and reconstruct the wavelength-
and position-dependent lenslet point-spread function (PSF) from images taken
with a tunable laser. We use these measured PSFs to implement a -based
extraction of the data cube, with typical residuals of ~5% due to imperfect
models of the undersampled lenslet PSFs. The full two-dimensional residual of
the extraction allows us to model and remove correlated read noise,
dramatically improving CHARIS' performance. The extraction produces a
data cube that has been deconvolved with the line-spread function, and never
performs any interpolations of either the data or the individual lenslet
spectra. The extracted data cube also includes uncertainties for each spatial
and spectral measurement. CHARIS' software is parallelized, written in Python
and Cython, and freely available on github with a separate documentation page.
Astrometric and spectrophotometric calibrations of the data cubes and PSF
subtraction will be treated in a forthcoming paper.Comment: 18 pages, 15 figures, 3 tables, replaced with JATIS accepted version
(emulateapj formatted here). Software at
https://github.com/PrincetonUniversity/charis-dep and documentation at
http://princetonuniversity.github.io/charis-de
Simulational study of anomalous tracer diffusion in hydrogels
In this article, we analyze different factors that affect the diffusion
behavior of small tracer particles (as they are used e.g.in fluorescence
correlation spectroscopy (FCS)) in the polymer network of a hydrogel and
perform simulations of various simplified models. We observe, that under
certain circumstances the attraction of a tracer particle to the polymer
network strands might cause subdiffusive behavior on intermediate time scales.
In theory, this behavior could be employed to examine the network structure and
swelling behavior of weakly crosslinked hydrogels with the help of FCS.Comment: 11 pages, 11 figure
Molecular Dynamics Simulation of Semiflexible Polyampholyte Brushes - The Effect of Charged Monomers Sequence
Planar brushes formed by end-grafted semiflexible polyampholyte chains, each
chain containing equal number of positively and negatively charged monomers is
studied using molecular dynamics simulations. Keeping the length of the chains
fixed, dependence of the average brush thickness and equilibrium statistics of
the brush conformations on the grafting density and the salt concentration are
obtained with various sequences of charged monomers. When similarly charged
monomers of the chains are arranged in longer blocks, the average brush
thickness is smaller and dependence of brush properties on the grafting density
and the salt concentration is stronger. With such long blocks of similarly
charged monomers, the anchored chains bond to each other in the vicinity of the
grafting surface at low grafting densities and buckle toward the grafting
surface at high grafting densities.Comment: 8 pages,7 figure
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