900 research outputs found
Strengthening of foamed composite materials
We investigate the shear elastic modulus of soft polymer foams loaded with
hard spherical particles and we show that, for constant bubble size and gas
volume fraction, strengthening is strongly dependent on the size of those
inclusions. Through an accurate control of the ratio that compares
the particle size to the thickness of the struts in the foam structure, we
evidence a transition in the mechanical behavior at . For
, every particle loading leads to a strengthening effect whose
magnitude depends only on the particle volume fraction. On the contrary, for
, the strengthening effect weakens abruptly as a function of
and a softening effect is even observed for .
This transition in the mechanical behavior is reminiscent of the so-called
"particle exclusion transition" that has been recently reported within the
framework of drainage of foamy granular suspensions [Haffner B, Khidas Y,
Pitois O. The drainage of foamy granular suspensions. J Colloid Interface Sci
2015. In Press.]. It involves the evolution for the geometrical configuration
of the particles with respect to the foam network, and it appears to control
the mechanics of such foamy systems
Flow and Jamming of Granular Suspensions in Foams
The drainage of particulate foams is studied under conditions where the
particles are not trapped individually by constrictions of the interstitial
pore space. The drainage velocity decreases continuously as the particle volume
fraction increases. The suspensions jam - and therefore drainage
stops - for values which reveal a strong effect of the particle
size. In accounting for the particular geometry of the foam, we show that
accounts for unusual confinement effects when the particles pack
into the foam network. We model quantitatively the overall behavior of the
suspension - from flow to jamming - by taking into account explicitly the
divergence of its effective viscosity at . Beyond the scope of
drainage, the reported jamming transition is expected to have a deep
significance for all aspects related to particulate foams, from aging to
mechanical properties
Shear induced drainage in foamy yield-stress fluids
Shear induced drainage of a foamy yield stress fluid is investigated using
MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes
the system at rest, a fast drainage is observed when a horizontal shear is
imposed. It is shown that the sheared interstitial material behaves as a
viscous fluid in the direction of gravity, the effective viscosity of which is
controlled by shear in transient foam films between bubbles. Results provided
for several bubble sizes are not captured by the R^2 scaling classically
observed for liquid flow in particulate systems, such as foams and thus
constitute a remarkable demonstration of the strong coupling of drainage flow
and shear induced interstitial flow. Furthermore, foam films are found to be
responsible for the unexpected arrest of drainage, thus trapping irreversibly a
significant amount of interstitial liquid.Comment: Published in Physical Review Letters.
http://prl.aps.org/abstract/PRL/v104/i12/e12830
Coupling of elasticity to capillarity in soft aerated materials
We study the elastic properties of soft solids containing air bubbles.
Contrary to standard porous materials, the softness of the matrix allows for a
coupling of the matrix elasticity to surface tension forces brought in by the
bubbles. Thanks to appropriate experiments on model systems, we show how the
elastic response of the dispersions is governed by two dimensionless
parameters: the gas volume fraction and a capillary number comparing the
elasticity of the matrix to the stiffness of the bubbles. We also show that our
experimental results are in good agreement with computations of the shear
modulus through a micro-mechanical approach.Comment: submitted to Soft Matte
Rheological behaviour of suspensions of bubbles in yield stress fluids
The rheological properties of suspensions of bubbles in yield stress fluids
are investigated through experiments on model systems made of monodisperse
bubbles dispersed in concentrated emulsions. Thanks to this highly tunable
system, the bubble size and the rheological properties of the suspending yield
stress fluid are varied over a wide range. We show that the macroscopic
response under shear of the suspensions depends on the gas volume fraction and
the bubble stiffness in the suspending fluid. This relative stiffness can be
quantified through capillary numbers comparing the capillary pressure to stress
scales associated with the rheological properties of the suspending fluid. We
demonstrate that those capillary numbers govern the decrease of the elastic and
loss moduli, the absence of variation of the yield stress and the increase of
the consistency with the gas volume fraction, for the investigated range of
capillary numbers. Micro-mechanical estimates are consistent with the
experimental data and provide insight on the experimental results.Comment: submitted to Journal of non Newtonian Fluid Mechanic
Theory of Polarization Attraction in Parametric Amplifiers Based on Telecommunication Fibers
We develop from first principles the coupled wave equations that describe
polarization-sensitive parametric amplification based on four-wave mixing in
standard (randomly birefringent) optical fibers. We show that in the
small-signal case these equations can be solved analytically, and permit us to
predict the gain experienced by the signal beam as well as its state of
polarization (SOP) at the fiber output. We find that, independently of its
initial value, the output SOP of a signal within the parametric gain bandwidth
is solely determined by the pump SOP. We call this effect of pulling the
polarization of the signal towards a reference SOP as polarization attraction,
and such parametric amplifier as the FWM-polarizer. Our theory is valid beyond
the zero polarization mode dispersion (PMD) limit, and it takes into account
moderate deviations of the PMD from zero. In particular, our theory is capable
of analytically predicting the rate of degradation of the efficiency of the
parametric amplifier which is caused by the detrimental PMD effect
Trapping polarization of light in nonlinear optical fibers: An ideal Raman polarizer
The main subject of this contribution is the all-optical control over the
state of polarization (SOP) of light, understood as the control over the SOP of
a signal beam by the SOP of a pump beam. We will show how the possibility of
such control arises naturally from a vectorial study of pump-probe Raman
interactions in optical fibers. Most studies on the Raman effect in optical
fibers assume a scalar model, which is only valid for high-PMD fibers (here,
PMD stands for the polarization-mode dispersion). Modern technology enables
manufacturing of low-PMD fibers, the description of which requires a full
vectorial model. Within this model we gain full control over the SOP of the
signal beam. In particular we show how the signal SOP is pulled towards and
trapped by the pump SOP. The isotropic symmetry of the fiber is broken by the
presence of the polarized pump. This trapping effect is used in experiments for
the design of new nonlinear optical devices named Raman polarizers. Along with
the property of improved signal amplification, these devices transform an
arbitrary input SOP of the signal beam into one and the same SOP towards the
output end. This output SOP is fully controlled by the SOP of the pump beam. We
overview the sate-of-the-art of the subject and introduce the notion of an
"ideal Raman polarizer"
Full vectorial analysis of polarization effects in optical nanowires
We develop a full theoretical analysis of the nonlinear interactions of the
two polarizations of a waveguide by means of a vectorial model of pulse
propagation which applies to high index subwavelength waveguides. In such
waveguides there is an anisotropy in the nonlinear behavior of the two
polarizations that originates entirely from the waveguide structure, and leads
to switching properties. We determine the stability properties of the steady
state solutions by means of a Lagrangian formulation. We find all static
solutions of the nonlinear system, including those that are periodic with
respect to the optical fiber length as well as nonperiodic soliton solutions,
and analyze these solutions by means of a Hamiltonian formulation. We discuss
in particular the switching solutions which lie near the unstable steady
states, since they lead to self-polarization flipping which can in principle be
employed to construct fast optical switches and optical logic gates
Long-term retrospective analysis of mackerel spawning in the North Sea: a new time series and modeling approach to CPR data
We present a unique view of mackerel (Scomber scombrus) in the North Sea based on a new time series of larvae caught by the Continuous Plankton Recorder (CPR) survey from 1948-2005, covering the period both before and after the collapse of the North Sea stock. Hydrographic backtrack modelling suggested that the effect of advection is very limited between spawning and larvae capture in the CPR survey. Using a statistical technique not previously applied to CPR data, we then generated a larval index that accounts for both catchability as well as spatial and temporal autocorrelation. The resulting time series documents the significant decrease of spawning from before 1970 to recent depleted levels. Spatial distributions of the larvae, and thus the spawning area, showed a shift from early to recent decades, suggesting that the central North Sea is no longer as important as the areas further west and south. These results provide a consistent and unique perspective on the dynamics of mackerel in this region and can potentially resolve many of the unresolved questions about this stock.lved questions about this stoc
Capture-induced transition in foamy suspensions
International audienceWe investigate the drainage behaviour of foamy granular suspensions. Results reveal large fluctuations in the drainage velocity as bubble size, particle size and gas volume fraction are varied for a given particle volume fraction. Particle capture is proved to control the overall drainage behaviour through the parameter lambda, which compares the particle size to the size of passage through constrictions within the foam pore space. lambda highlights a sharp transition: for lambda 1 particles are trapped and the resulting drainage velocity is strongly reduced. A phenomenological model is proposed to describe this behaviour
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