788 research outputs found
Thermomechanical material modelling based on a hybrid free energy density depending on pressure, isochoric deformation and temperature
AbstractIn order to represent temperature-dependent mechanical material properties in a thermomechanical consistent manner it is common practice to start with the definition of a model for the specific Helmholtz free energy. Its canonical independent variables are the Green strain tensor and the temperature. But to represent calorimetric material properties under isobaric conditions, for example the exothermal behaviour of a curing process or the dependence of the specific heat on the temperature history, the temperature and the pressure should be taken as independent variables. Thus, in the field of calorimetry the Gibbs free energy is usually used as thermodynamic potential whereas in continuum mechanics the Helmholtz free energy is normally applied. In order to simplify the representation of calorimetric phenomena in continuum mechanics a hybrid free energy density is introduced. Its canonical independent variables are the isochoric Green strain tensor, the pressure and the temperature. It is related to the Helmholtz free energy density by a Legendre transformation. In combination with the additive split of the stress power into the sum of isochoric and volumetric terms this approach leads to thermomechanical consistent constitutive models for large deformations. The article closes with applications of this approach to finite thermoelasticity, curing adhesives and the glass transition
Sliding Blocks Revisited: A simulational Study
A computational study of sliding blocks on inclined surfaces is presented.
Assuming that the friction coefficient is a function of position, the
probability for the block to slide down over a length is
numerically calculated. Our results are consistent with recent experimental
data suggesting a power-law distribution of events over a wide range of
displacements when the chute angle is close to the critical one, and suggest
that the variation of along the surface is responsible for this.Comment: 6 pages, 4 figures. submitted to Int. J. Mod. Phys. (Proc. Brazilian
Wokshop on Simulational Physics
Stochastic Model for the Motion of a Particle on an Inclined Rough Plane and the Onset of Viscous Friction
Experiments on the motion of a particle on an inclined rough plane have
yielded some surprising results. For example, it was found that the frictional
force acting on the ball is viscous, {\it i.e.} proportional to the velocity
rather than the expected square of the velocity. It was also found that, for a
given inclination of the plane, the velocity of the ball scales as a power of
its radius. We present here a one dimensional stochastic model based on the
microscopic equations of motion of the ball, which exhibits the same behaviour
as the experiments. This model yields a mechanism for the origins of the
viscous friction force and the scaling of the velocity with the radius. It also
reproduces other aspects of the phase diagram of the motion which we will
discuss.Comment: 19 pages, latex, 11 postscript figures in separate uuencoded fil
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Development of bioluminescent sensors for interrogating cyclic di-nucleotide signaling
Bioluminescence is the spectacular natural phenomenon in which living organisms produce and emit light. Natural bioluminescent protein systems have been discovered and characterized in over 30 species, ranging from fireflies to fungi and bacteria. Significant effort has been put towards engineering and improving these natural systems so that they may be used as tools for interrogating biological processes. As opposed to fluorescence, because bioluminescence requires no excitation light to produce a signal, it has proven extremely useful for imaging in highly autofluorescent samples, such as within deep tissues of whole organisms. To date, however, most bioluminescent imaging systems have been developed solely for the study of eukaryotic systems, and few have focused on the study of bacteria. Bacteria naturally colonize highly diverse and complex environments, from gastrointestinal tracts to soil and plant surfaces, that have proven difficult to study with currently available fluorescent tools. To allow for the study of bacterial signaling within these complex environments, new bioluminescent sensors developed specifically for bacterial signaling are needed.Here, we describe the development and application of bioluminescent sensors for the bacterial cyclic di-nucleotide (CDN) signaling molecule cyclic di-GMP. Cyclic di-GMP is nearly ubiquitous in bacteria and plays a key role in the controlling motility and biofilm formation. As a first-generation bioluminescent sensor system, we developed intensity-based bioluminescent sensors for cyclic di-GMP. These sensors proved useful as in vitro tools for studying cyclic di-GMP, however were not amenable to live cell imaging. To move beyond purely in vitro systems, we developed next-generation ratiometric bioluminescent sensors for cyclic di-GMP. This next-generation system led to significantly improved sensor properties and allowed for the imaging of small numbers of live bacterial cells in an animal tissue-like model system. Finally, to expand these sensor systems to CDNs other than cyclic di-GMP, we applied a novel directed evolution approach to find sensors that respond to cyclic GMP-AMP. The first round of directed evolution was not successful, but work is ongoing on this front. Collectively, the work presented here lays the groundwork for using bioluminescent sensor systems to interrogate signaling in bacteria in their natural environments, which was previously not possible
Plug flow and the breakdown of Bagnold scaling in cohesive granular flows
Cohesive granular media flowing down an inclined plane are studied by
discrete element simulations. Previous work on cohesionless granular media
demonstrated that within the steady flow regime where gravitational energy is
balanced by dissipation arising from intergrain forces, the velocity profile in
the flow direction scales with depth in a manner consistent with the
predictions of Bagnold. Here we demonstrate that this Bagnold scaling does not
hold for the analogous steady-flows in cohesive granular media. We develop a
generalization of the Bagnold constitutive relation to account for our
observation and speculate as to the underlying physical mechanisms responsible
for the different constitutive laws for cohesive and noncohesive granular
media.Comment: 8 pages, 10 figure
Prediction of final infarct volume from native CT perfusion and treatment parameters using deep learning
CT Perfusion (CTP) imaging has gained importance in the diagnosis of acute
stroke. Conventional perfusion analysis performs a deconvolution of the
measurements and thresholds the perfusion parameters to determine the tissue
status. We pursue a data-driven and deconvolution-free approach, where a deep
neural network learns to predict the final infarct volume directly from the
native CTP images and metadata such as the time parameters and treatment. This
would allow clinicians to simulate various treatments and gain insight into
predicted tissue status over time. We demonstrate on a multicenter dataset that
our approach is able to predict the final infarct and effectively uses the
metadata. An ablation study shows that using the native CTP measurements
instead of the deconvolved measurements improves the prediction.Comment: Accepted for publication in Medical Image Analysi
Mesoscopic motion of atomic ions in magnetic fields
We introduce a semiclassical model for moving highly excited atomic ions in a
magnetic field which allows us to describe the mixing of the Landau orbitals of
the center of mass in terms of the electronic excitation and magnetic field.
The extent of quantum energy flow in the ion is investigated and a crossover
from localization to delocalization with increasing center of mass energy is
detected. It turns out that our model of the moving ion in a magnetic field is
closely connected to models for transport in disordered finite-size wires.Comment: 4 pages, 2 figures, subm. to Phys.Rev.A, Rap.Co
Solution of the Schr\"odinger Equation for Quantum Dot Lattices with Coulomb Interaction between the Dots
The Schr\"odinger equation for quantum dot lattices with non-cubic,
non-Bravais lattices built up from elliptical dots is investigated. The Coulomb
interaction between the dots is considered in dipole approximation. Then only
the center of mass (c.m.) coordinates of different dots couple with each other.
This c.m. subsystem can be solved exactly and provides magneto- phonon like
collective excitations. The inter-dot interaction is involved only through a
single interaction parameter. The relative coordinates of individual dots form
decoupled subsystems giving rise to intra-dot excitations. As an example, the
latter are calculated exactly for two-electron dots.
Emphasis is layed on qualitative effects like: i) Influence of the magnetic
field on the lattice instability due to inter-dot interaction, ii) Closing of
the gap between the lower and the upper c.m. mode at B=0 for elliptical dots
due to dot interaction, and iii) Kinks in the single dot excitation energies
(versus magnetic field) due to change of ground state angular momentum. It is
shown that for obtaining striking qualitative effects one should go beyond
simple cubic lattices with spherical dots. We also prove a more general version
of the Kohn Theorem for quantum dot lattices. It is shown that for observing
effects of electron- electron interaction between the dots in FIR spectra
(breaking Kohn's Theorem) one has to consider dot lattices with at least two
dot species with different confinement tensors.Comment: 11 figures included as ps-file
Determinants of the Presence and Size of Intracranial Aneurysms in the General Population The Rotterdam Study
BACKGROUND AND PURPOSE: The prevalence of unruptured intracranial aneurysms (UIAs) in the adult population is ≈3%.
Rupture of an intracranial aneurysm can have devastating consequences, which emphasizes the importance of identification
of potentially modifiable determinants for the presence and size of UIAs. Our aim was to study the association of a broad
spectrum of potential determinants with the presence and size of UIAs in a general adult population.
METHODS: Between 2005 and 2015, 5841 participants from the population-based Rotterdam Study (mean age, 64.4
years, 45.0% male) underwent brain magnetic resonance imaging (1.5T). These scans were evaluated for the presence of
incidental UIAs. We determined number and volume of the UIAs. Using logistic and linear regression models, we assessed
the association of cardiovascular, lifestyle and emerging inflammatory and hormonal determinants with the presence and
volume of UIAs.
RESULTS: In 134 (2.3%) participants, ≥1 UIAs were detected (149 UIAs in total), with a median volume of 61.1 mm3
(interquartile range, 33.2–134.0). In multivariable models, female sex (odds ratio, 1.92 [95% CI, 1.33–2.84]), hypertension
(odds ratio, 1.73 [95% CI, 1.13–2.68]), and current smoking (odds ratio, 3.75 [95% CI, 2.27–6.33]) were associated with
the presence of UIAs. We found no association of alcohol use, physical activity, or diet quality with UIA presence. Finally, we
found white blood cell count to relate to larger aneurysm volume (difference in volume of 33.6 mm3 per 109/L increase in
white blood cell [95% CI, 3.92–63.5]).
CONCLUSIONS: In this population-based study, female sex, hypertension, and smoking, but no other lifestyle determinants, were
associated with the presence of UIAs. White blood cell count is associated with size of UIAs. Preventive strategies should
focus on treating hypertension and promoting cessation of smoking
The chemistry of ZnWO<sub>4</sub> nanoparticle formation
The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals
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