4,214 research outputs found
Influence of shear stress applied during flow stoppage and rest period on the mechanical properties of thixotropic suspensions
We study the solid mechanical properties of several thixotropic suspensions
as a function of the shear stress history applied during their flow stoppage
and their aging in their solid state. We show that their elastic modulus and
yield stress depend strongly on the shear stress applied during their
solid-liquid transition (i.e., during flow stoppage) while applying the same
stress only before or only after this transition may induce only second-order
effects: there is negligible dependence of the mechanical properties on the
preshear history and on the shear stress applied at rest. We also found that
the suspensions age with a structuration rate that hardly depends on the stress
history. We propose a physical sketch based on the freezing of a microstructure
whose anisotropy depends on the stress applied during the liquid-solid
transition to explain why the mechanical properties depend strongly on this
stress. This sketch points out the role of the internal forces in the colloidal
suspensions' behavior. We finally discuss briefly the macroscopic consequences
of this phenomenon and show the importance of using a controlled-stress
rheometer
Macroscopic behavior of bidisperse suspensions of noncolloidal particles in yield stress fluids
We study both experimentally and theoretically the rheological behavior of
isotropic bidisperse suspensions of noncolloidal particles in yield stress
fluids. We focus on materials in which noncolloidal particles interact with the
suspending fluid only through hydrodynamical interactions. We observe that both
the elastic modulus and yield stress of bidisperse suspensions are lower than
those of monodisperse suspensions of same solid volume fraction. Moreover, we
show that the dimensionless yield stress of such suspensions is linked to their
dimensionless elastic modulus and to their solid volume fraction through the
simple equation of Chateau et al.[J. rheol. 52, 489-506 (2008)]. We also show
that the effect of the particle size heterogeneity can be described by means of
a packing model developed to estimate random loose packing of assemblies of dry
particles. All these observations finally allow us to propose simple closed
form estimates for both the elastic modulus and the yield stress of bidisperse
suspensions: while the elastic modulus is a function of the reduced volume
fraction only, where is the estimated random loose
packing, the yield stress is a function of both the volume fraction and
the reduced volume fraction
Boron nitride for excitonics, nano photonics, and quantum technologies
We review the recent progress regarding the physics and applications of boron nitride bulk crystals and its epitaxial layers in various fields. First, we highlight its importance from optoelectronics side, for simple devices operating in the deep ultraviolet, in view of sanitary applications. Emphasis will be directed towards the unusually strong efficiency of the exciton-phonon coupling in this indirect band gap semiconductor. Second, we shift towards nanophotonics, for the management of hyper-magnification and of medical imaging. Here, advantage is taken of the efficient coupling of the electromagnetic field with some of its phonons, those interacting with light at 12 and 6 yin in vacuum. Third, we present the different defects that are currently studied for their propensity to behave as single photon emitters, in the perspective to help them becoming challengers of the NV centres in diamond or of the double vacancy in silicon carbide in the field of modern and developing quantum technologies.This work was financially supported in France by the contract BONASPES (ANR-19-CE30-0007-02) under the umbrella of the publicly funded Investissements d'Avenir program managed by the French ANR agency. This work has been supported in Spain the Spanish MINECO/FEDER under Contracts No. MAT2015-71035-R and No. MAT2016-75586-C4-1-P.Peer reviewe
Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078
We present preliminary results from an ongoing program devoted to a study of
small scale structure in the spatial distribution of molecular gas. Our work is
based on multi-epoch FUSE and visible observations of HD34078. A detailed
comparison of H2, CH and CH+ absorption lines is performed. No short term
variations are seen (except for highly excited H2) but long-term changes in
N(CH) are clearly detected when comparing our data to spectra taken about 10
years ago.Comment: 4 pages, 2 figures, To appear in the Proceedings of the XVII IAP
Colloquium "Gaseous Matter in Galaxies and Intergalactic Space
Inverse Transport Theory of Photoacoustics
We consider the reconstruction of optical parameters in a domain of interest
from photoacoustic data. Photoacoustic tomography (PAT) radiates high frequency
electromagnetic waves into the domain and measures acoustic signals emitted by
the resulting thermal expansion. Acoustic signals are then used to construct
the deposited thermal energy map. The latter depends on the constitutive
optical parameters in a nontrivial manner. In this paper, we develop and use an
inverse transport theory with internal measurements to extract information on
the optical coefficients from knowledge of the deposited thermal energy map. We
consider the multi-measurement setting in which many electromagnetic radiation
patterns are used to probe the domain of interest. By developing an expansion
of the measurement operator into singular components, we show that the spatial
variations of the intrinsic attenuation and the scattering coefficients may be
reconstructed. We also reconstruct coefficients describing anisotropic
scattering of photons, such as the anisotropy coefficient in a
Henyey-Greenstein phase function model. Finally, we derive stability estimates
for the reconstructions
Inverse Diffusion Theory of Photoacoustics
This paper analyzes the reconstruction of diffusion and absorption parameters
in an elliptic equation from knowledge of internal data. In the application of
photo-acoustics, the internal data are the amount of thermal energy deposited
by high frequency radiation propagating inside a domain of interest. These data
are obtained by solving an inverse wave equation, which is well-studied in the
literature. We show that knowledge of two internal data based on well-chosen
boundary conditions uniquely determines two constitutive parameters in
diffusion and Schroedinger equations. Stability of the reconstruction is
guaranteed under additional geometric constraints of strict convexity. No
geometric constraints are necessary when internal data for well-chosen
boundary conditions are available, where is spatial dimension. The set of
well-chosen boundary conditions is characterized in terms of appropriate
complex geometrical optics (CGO) solutions.Comment: 24 page
A prescriptive approach to qualify and quantify customer value for value-based requirements engineering
Recently, customer-based product development is becoming a popular paradigm. Customer expectations and needs can be identified and transformed into requirements for product design with the help of various methods and tools. However, in many cases, these models fail to focus on the perceived value that is crucial when customers make the decision of purchasing a product. In this paper, a prescriptive approach to support value-based requirements engineering (RE) is proposed, describing the foundations, procedures and initial applications in the context of RE for commercial aircraft. An integrated set of techniques, such as means-ends analysis, part-whole analysis and multi-attribute utility theory is introduced in order to understand customer values in depth and width. Technically, this enables identifying the implicit value, structuring logically collected statements of customer expectations and performing value modelling and simulation. Additionally, it helps to put in place a system to measure customer satisfaction that is derived from the proposed approach. The approach offers significant potential to develop effective value creation strategies for the development of new product
Counterterms vs. Dualities
We investigate and clarify the mutual compatibility of the higher order
corrections arising in supergravity and string theory effective actions and the
non-linear duality symmetries of these theories. Starting from a conventional
tree level action leading to duality invariant equations of motion, we show how
to accommodate duality invariant counterterms given as functionals of both
electric and magnetic fields in a perturbative expansion, and to deduce from
them a non-polynomial bona fide action satisfying the Gaillard-Zumino
constraint. There exists a corresponding consistency constraint in the
non-covariant Henneaux-Teitelboim formalism which ensures that one can always
restore diffeomorphism invariance by perturbatively solving this functional
identity. We illustrate how this procedure works for the R^2 \nabla F \nabla F
and F^4 counterterms in Maxwell theory.Comment: 15 page
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