167 research outputs found
Wide-gap Couette flows of dense emulsions: Local concentration measurements, and comparison between macroscopic and local constitutive law measurements through magnetic resonance imaging
Flows of dense emulsions show many complex features among which long range
nonlocal effects pose a problem for macroscopic characterization. In order to
get around this problem, we study the flows of several dense emulsions in a
wide-gap Couette geometry. We couple macroscopic rheometric experiments and
local velocity measurements through MRI techniques. As concentration
heterogeneities can be expected, we designed a method to measure the local
droplet concentration in emulsions with a MRI device. In contrast to dense
suspensions of rigid particles where very fast migration occurs under shear, we
show that no migration takes place in dense emulsions even for strains as large
as 100 000 in our systems. As a result of the absence of migration and of
finite size effect, we are able to determine very precisely the local
rheological behavior of several dense emulsions. As the materials are
homogeneous, this behavior can also be inferred from purely macroscopic
measurements. We thus suggest that properly analyzed purely macroscopic
measurements in a wide-gap Couette geometry can be used as a tool to study the
local constitutive laws of dense emulsions. All behaviors are basically
consistent with Herschel-Bulkley laws of index 0.5, but discrepancies exist at
the approach of the yield stress due to slow shear flows below the apparent
yield stress in the case of a strongly adhesive emulsion. The existence of a
constitutive law accounting for all flows contrasts with previous results
obtained within a microchannel by Goyon et al. (2008): the use of a wide-gap
Couette geometry is likely to prevent here from nonlocal finite size effects;
it also contrasts with the observations of B\'ecu et al. (2006)
Three-dimensional jamming and flows of soft glassy materials
Various disordered dense systems such as foams, gels, emulsions and colloidal
suspensions, exhibit a jamming transition from a liquid state (they flow) to a
solid state below a yield stress. Their structure, thoroughly studied with
powerful means of 3D characterization, exhibits some analogy with that of
glasses which led to call them soft glassy materials. However, despite its
importance for geophysical and industrial applications, their rheological
behavior, and its microscopic origin, is still poorly known, in particular
because of its nonlinear nature. Here we show from two original experiments
that a simple 3D continuum description of the behaviour of soft glassy
materials can be built. We first show that when a flow is imposed in some
direction there is no yield resistance to a secondary flow: these systems are
always unjammed simultaneously in all directions of space. The 3D jamming
criterion appears to be the plasticity criterion encountered in most solids. We
also find that they behave as simple liquids in the direction orthogonal to
that of the main flow; their viscosity is inversely proportional to the main
flow shear rate, as a signature of shear-induced structural relaxation, in
close similarity with the structural relaxations driven by temperature and
density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm
Akhet Khufu: Archaeo-astronomical Hints at a Common Project of the Two Main Pyramids of Giza, Egypt
Abstract. The architectural complexes composed by the two main pyramids of Giza together with their temples are investigated from an interdisciplinary point of view, taking into account their astronomical alignments as well as their relationships with the visible landscape. Combining already known facts together with new clues, the work strongly supports the idea that the two complexes were conceived as parts of a common project
High-intensity laser-accelerated ion beam produced from cryogenic micro-jet target
We report on the successful operation of a newly developed cryogenic jet target at high intensity
laser-irradiation. Using the frequency-doubled Titan short pulse laser system at Jupiter Laser Fa-
cility, Lawrence Livermore National Laboratory, we demonstrate the generation of a pure proton
beam a with maximum energy of 2 MeV. Furthermore, we record a quasi-monoenergetic peak at
1.1 MeV in the proton spectrum emitted in the laser forward direction suggesting an alternative
acceleration mechanism. Using a solid-density mixed hydrogen-deuterium target, we are also able
to produce pure proton-deuteron ion beams. With its high purity, limited size, near-critical density,
and high-repetition rate capability, this target is promising for future applications
Oval Domes: History, Geometry and Mechanics
An oval dome may be defined as a dome whose plan or profile (or both) has an oval form. The word Aoval@ comes from the latin Aovum@, egg. Then, an oval dome has an egg-shaped geometry. The first buildings with oval plans were built without a predetermined form, just trying to close an space in the most economical form. Eventually, the geometry was defined by using arcs of circle with common tangents in the points of change of curvature. Later the oval acquired a more regular form with two axis of symmetry. Therefore, an “oval” may be defined as an egg-shaped form, doubly symmetric, constructed with arcs of circle; an oval needs a minimum of four centres, but it is possible also to build polycentric ovals.
The above definition corresponds with the origin and the use of oval forms in building and may be applied without problem until, say, the XVIIIth century. Since then, the teaching of conics in the elementary courses of geometry made the cultivated people to define the oval as an approximation to the ellipse, an “imperfect ellipse”: an oval was, then, a curve formed with arcs of circles which tries to approximate to the ellipse of the same axes. As we shall see, the ellipse has very rarely been used in building.
Finally, in modern geometrical textbooks an oval is defined as a smooth closed convex curve, a more general definition which embraces the two previous, but which is of no particular use in the study of the employment of oval forms in building.
The present paper contains the following parts: 1) an outline the origin and application of the oval in historical architecture; 2) a discussion of the spatial geometry of oval domes, i. e., the different methods employed to trace them; 3) a brief exposition of the mechanics of oval arches and domes; and 4) a final discussion of the role of Geometry in oval arch and dome design
Crossed-beam energy transfer : Polarization effects and evidence of saturation
Recent results on crossed-beam energy transfer are presented. Wavelength tuning was used to vary the amount of energy transfer between two beams in a quasi-stationary plasma with carefully controlled conditions. The amount of transfer agreed well with calculations assuming linear ion acoustic waves (IAWs) with amplitudes up to . Increasing the initial probe intensity to access larger IAW amplitudes for otherwise fixed conditions yields evidence of saturation. The ability to manipulate a beam's polarization, which results from the anisotropic nature of the interaction, is revisited; an example is provided to demonstrate how polarization effects in a multibeam situation can dramatically enhance the expected amount of energy transfer
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
- …