48 research outputs found
Equation of state of a granular gas homogeneously driven by particle rotations
We report an experimental study of a dilute "gas" of magnetic particles
subjected to a vertical alternating magnetic field in a 3D container. Due to
the torque exerted by the field on the magnetic moment of each particle, a
spatially homogeneous and chaotic forcing is reached where only rotational
motions are driven. This forcing differs significantly from boundary-driven
systems used in most previous experimental studies on non equilibrium
dissipative granular gases. Here, no cluster formation occurs, and the equation
of state displays strong analogy with the usual gas one apart from a geometric
factor. Collision statistics is also measured and shows an exponential tail for
the particle velocity distribution. Most of these observations are well
explained by a simple model which uncovers out-of-equilibrium systems
undergoing uniform "heating".Comment: Europhysics Letters (2013) in pres
Nonlinear waves on the surface of a fluid covered by an elastic sheet
We experimentally study linear and nonlinear waves on the surface of a fluid
covered by an elastic sheet where both tension and flexural waves take place.
An optical method is used to obtain the full space-time wave field, and the
dispersion relation of waves. When the forcing is increased, a significant
nonlinear shift of the dispersion relation is observed. We show that this shift
is due to an additional tension of the sheet induced by the transverse motion
of a fundamental mode of the sheet. When the system is subjected to a random
noise forcing at large scale, a regime of hydro-elastic wave turbulence is
observed with a power-law spectrum of the scale in disagreement with the wave
turbulence prediction. We show that the separation between relevant time scales
is well satisfied at each scale of the turbulent cascade as expected
theoretically. The wave field anisotropy, and finite size effects are also
quantified and are not at the origin of the discrepancy. Finally, the
dissipation is found to occur at all scales of the cascade contrary to the
theoretical hypothesis, and could thus explain this disagreement.Comment: Journal of Fluid Mechanics (2013
Acoustic coupling between two air bubbles in water
The acoustic coupling between two air bubbles immersed in water is clearly
demonstrated. The system is acoustically forced, and its response is detected.
The experimental results confirm that both theoretically predicted eigenmodes,
respectively symmetrical and antisymmetrical, do exist. Their frequencies,
measured as a function of the bubbles spacing, follow theoretical estimations
within a 10% accuracy.Comment: 14 pages, 6 figures, submitted to European Physical Journal E (2nd
version
Twisting and buckling: a new undulation mechanism for artificial swimmers
We present an artificial swimmer consisting in a long cylinder of ferrogel
which is polarized transversely and in opposite directions at each extremity.
When it is placed on a water film and submitted to a transverse oscillating
magnetic field, this artificial worm undulates and swims. Whereas symmetry
breaking is due to the field gradient, the undulations of the worm result from
a torsional buckling instability as the polarized ends tend to align with the
applied magnetic field. The critical magnetic field above which buckling and
subsequent swimming is observed may be predicted using elasticity equations
including the effect of a magnetic torque. As the length of the worm is varied,
several undulation modes are observed which are in good agreement with the
bending modes of an elastic rod with free ends
The adiabatic invariant of any harmonic oscillator : an unexpected application of Glauber's formalism
In this theoretical paper, we propose a general derivation of the adiabatic invariant of the n-degree-of-freedom harmonic oscillator, available whichever the physical nature of the oscillator and of the parametrical excitation it undergoes. This derivation is founded on the use of the classical Glauber variables and ends up with this simplest result: the oscillator's adiabatic invariant is just the sum of all the semiclassical quanta numbers associated with its different eigenmodes
New ferrofluid composites: Magnetism and optics experiments
International audienc
Oblate-Prolate Transition of Ellipsoidal Giant Magnetoliposomes: Experiments Showing an Anisotropic Spontaneous Curvature (Chapter 11)
International audienceThis chapter contains sections titled:IntroductionLiposome PreparationObservationsAnalysis of Deformations Under a Magnetic FieldInterpretatio
Nager grâce à un mécanisme de torsion/flexion
Nous présentons un nageur artificiel qui progresse grâce à un mécanisme original. Il consiste en un cylindre de ferrogel polarisé transversalement et en sens opposé à ses extrémités. Placé à la surface d'un film d'eau et soumis à un champ magnétique oscillant, le ver ondule et nage. Alors que la brisure de symétrie résulte de l'inhomogénéité du champ magnétique, les ondulations du ver sont dues à une instabilité de torsion/flexion liée à l'alignement des moments magnétiques des extrémités avec le champ appliqué. Les ondes de flexion ainsi excitées régissent alors les modes de nage du ver
The Huayruro Project: mapping the Calicanto Inca area buried by the A.D. 1600 Huaynaputina eruption, with geophysical imaging and remote sensing
We present geophysical and remote sensing observations near the Quinistaquillas town (southern Peru), in the framework of the HUAYRURO Project. This Inca zone was buried during the A.D. 1600 Huaynaputina eruption, the most important volcanic phenomenon of the last 400 years. The eruption had a global impact, due to the volume of emitted ash (2-3 times the one emitted by Vesuvius in A.D. 79). This lead to a 1.13°C cooling of the planet and caused a worlwide agricultural crisis. During the eruption, the Calicanto-Chimpapampa zone was covered by ashes and pyroclastic flows, with a thickness in the range [1 - 20] m. From 2015 to 2017, remote sensing and geophysical methods were deployed to map a ~ 1 km*2 km area, up to 3-m depth
Physical impacts of the CE 1600 Huaynaputina eruption on the local habitat: Geophysical insights
[ESP] El impacto climático global de la erupción del volcán Huaynaputina (IEV6) en 1600 está bien documentado pero las consecuencias regionales sobre las construcciones y los habitantes están poco conocidas. La localización de varios pueblos sepultados bajo los depósitos espesos del Huaynaputina no es claramente mencionada en las crónicas españolas. Investigaciones geofísicas realizadas durante el periodo 2015-2016 sobre diferentes sitios de ruinas a menos de 16 km del cráter constituyen la parte inicial de un proyecto denominado “Huayruro”, cuyo objetivo es entender mejor los impactos físicos y socio-económicos de esta erupción. Varios métodos e instrumentos geofísicos fueron utilizados: un drone y modelos numéricos de terreno de alta resolución, un geo-radar con imágenes 3D del subsuelo, el magnetismo, las imágenes infra-rojas y el electro-magnetismo. Esta investigación geofísica preliminar ha permitido identificar la futura estratégia y la mejor instrumentación para cartografiar el área del antiguo pueblo enterrado de Calicanto, localizando con precisión su extensión y los muros de las habitaciones. Este mapeo servirá para los futuros estudios tefro-estratigráficos y arqueológicos. El objetivo final del proyecto es diseminar los resultados del estudio multidisciplinar al público incluyendo la creación de un museo de sitio