1,409 research outputs found
Frictional dissipation of polymeric solids vs interfacial glass transition
We present single contact friction experiments between a glassy polymer and
smooth silica substrates grafted with alkylsilane layers of different coverage
densities and morphologies. This allows us to adjust the polymer/substrate
interaction strength. We find that, when going from weak to strong interaction,
the response of the interfacial junction where shear localizes evolves from
that of a highly viscous threshold fluid to that of a plastically deformed
glassy solid. This we analyse as resulting from an interaction-induced
``interfacial glass transition'' helped by pressure
Stability of hexagonal solidification patterns
We investigate the dynamics of cellular solidification patterns using
three-dimensional phase-field simulations. The cells can organize into stable
hexagonal patterns or exhibit unsteady evolutions. We identify the relevant
secondary instabilities of regular hexagonal arrays and find that the stability
boundaries depend significantly on the strength of crystalline anisotropy. We
also find multiplet states that can be reached by applying well-defined
perturbations to a pre-existing hexagonal array.Comment: Minor changes, mainly in introduction and conclusion, one reference
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Expected performances of a Laue lens made with bent crystals
In the context of the LAUE project devoted to build a Laue lens prototype for
focusing celestial hard X-/soft gamma-rays, a Laue lens made of bent crystal
tiles, with 20 m focal length, is simulated. The focusing energy passband is
assumed to be 90--600 keV. The distortion of the image produced by the lens on
the focal plane, due to effects of crystal tile misalignment and radial
distortion of the crystal curvature, is investigated. The corresponding
effective area of the lens, its point spread function and sensitivity are
calculated and compared with those exhibited by a nominal Laue lens with no
misalignment and/or distortion. Such analysis is crucial to estimate the
optical properties of a real lens, in which the investigated shortcomings could
be present.Comment: 20 pages, 14 figure
Perturbation of Tunneling Processes by Mechanical Degrees of Freedom in Mesoscopic Junctions
We investigate the perturbation in the tunneling current caused by
non-adiabatic mechanical motion in a mesoscopic tunnel junction. A theory
introduced by Caroli et al. \cite{bi1,bi2,bi3} is used to evaluate second order
self-energy corrections for this non-equilibrium situation lacking
translational invariance. Inelastic signatures of the mechanical degrees of
freedom are found in the current-voltage characteristics. These give
rise to sharp features in the derivative spectrum, .Comment: 22 pages LaTeX + 3 uuencoded PS picture
Driven activation versus thermal activation
Activated dynamics in a glassy system undergoing steady shear deformation is
studied by numerical simulations. Our results show that the external driving
force has a strong influence on the barrier crossing rate, even though the
reaction coordinate is only weakly coupled to the nonequilibrium system. This
"driven activation" can be quantified by introducing in the Arrhenius
expression an effective temperature, which is close to the one determined from
the fluctuation-dissipation relation. This conclusion is supported by
analytical results for a simplified model system.Comment: 5 pages, 3 figure
Dry Friction due to Adsorbed Molecules
Using an adiabatic approximation method, which searches for Tomlinson
model-like instabilities for a simple but still realistic model for two
crystalline surfaces in the extremely light contact limit, with mobile
molecules present at the interface, sliding relative to each other, we are able
to account for the virtually universal occurrence of "dry friction." The model
makes important predictions for the dependence of friction on the strength of
the interaction of each surface with the mobile molecules.Comment: four pages of latex, figure provide
Compton telescope with coded aperture mask: Imaging with the INTEGRAL/IBIS Compton mode
Compton telescopes provide a good sensitivity over a wide field of view in
the difficult energy range running from a few hundred keV to several MeV. Their
angular resolution is, however, poor and strongly energy dependent. We present
a novel experimental design associating a coded mask and a Compton detection
unit to overcome these pitfalls. It maintains the Compton performance while
improving the angular resolution by at least an order of magnitude in the field
of view subtended by the mask. This improvement is obtained only at the expense
of the efficiency that is reduced by a factor of two. In addition, the
background corrections benefit from the coded mask technique, i.e. a
simultaneous measurement of the source and background. This design is
implemented and tested using the IBIS telescope on board the INTEGRAL satellite
to construct images with a 12' resolution over a 29 degrees x 29 degrees field
of view in the energy range from 200 keV to a few MeV. The details of the
analysis method and the resulting telescope performance, particularly in terms
of sensitivity, are presented
Computational Methods for the Integrative Analysis of Genomics and Pharmacological Data
Since the pioneering NCI-60 panel of the late'80's, several major screenings of genetic profiling and drug testing in cancer cell lines have been conducted to investigate how genetic backgrounds and transcriptional patterns shape cancer's response to therapy and to identify disease-specific genes associated with drug response. Historically, pharmacogenomics screenings have been largely heterogeneous in terms of investigated cell lines, assay technologies, number of compounds, type and quality of genomic data, and methods for their computational analysis. The analysis of this enormous and heterogeneous amount of data required the development of computational methods for the integration of genomic profiles with drug responses across multiple screenings. Here, we will review the computational tools that have been developed to integrate cancer cell lines' genomic profiles and sensitivity to small molecule perturbations obtained from different screenings
Static Versus Dynamic Friction: The Role of Coherence
A simple model for solid friction is analyzed. It is based on tangential
springs representing interlocked asperities of the surfaces in contact. Each
spring is given a maximal strain according to a probability distribution. At
their maximal strain the springs break irreversibly. Initially all springs are
assumed to have zero strain, because at static contact local elastic stresses
are expected to relax. Relative tangential motion of the two solids leads to a
loss of coherence of the initial state: The springs get out of phase due to
differences in their sizes. This mechanism alone is shown to lead to a
difference between static and dynamic friction forces already. We find that in
this case the ratio of the static and dynamic coefficients decreases with
increasing relative width of the probability distribution, and has a lower
bound of 1 and an upper bound of 2.Comment: 10 pages, 2 figures, revtex
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