recherche –

La thixotropie (TIPE, stage de recherche et explications)

Tristan FERROIR — Sat, 14 Jun 2008 20:39:48 +0000

Qu’est-ce donc que ce mot barbare? Sous ce terme scientifique se cache un phénomène assez connu, globalement celui des sables mouvants. On dit d’un matériau qu’il est thixotrope si, en réponse à une contrainte (agitation mécanique, ondes sismiques, cisaillment) sa viscosité diminue fortement. Autrement dit, il passe de l’état solide à l’état liquide. Mais, là ou les fluides thixotropes sont très « forts » c’est que selon si on les laisse au repos 1 minute, 10 minutes, 1 heure etc… leur viscosité ne sera pas la même et donc si on les agite à nouveau, on obtiendra pas les mêmes résultats. Certains matériaux naturels sont thixotropes comme le ketchup, la mayonnaise mais leur temps de retour à l’équilibre est très faible si bien qu’on ne s’en rend quasimeznt pas compte. Des matériaux plus exotiques tels que l’argile bentonite offre une vision du phénomène beaucoup plus complète.

En géologie, ce comportement thixotrope observé par exemple dans des argiles peut avoir des conséquences dramatiques dans le cas de séismes comme ce fut le cas à Niigata (photo ci dessous). Mais ce comportement est aussi utilisé dans les boues de forage des pétroliers pour faciliter le travail du trépan.

J’ai eu l’occasion de travailler pas mal là dessus et je propose donc quelques ressources sur ce phénomène. Tout d’abord un TIPE fait en Spé BCPST sur le sujet, plus particulièrement sur la liquéfaction des sols à partir de l’exemple d’un sol en foret de Rambouillet (Note : 18 à l’agro, à G2E et aux ENS). Ensuite, un stage de recherche au cours de ma licence (L3) à l’ENS Lyon sur l’influence des vibrations dans la liquéfaction des matériaux thixotropes. Enfinn, une petite synthèse sur les grands types de fluides (fluide newtonien, fluide à seuil, fluide rhéo-épaissisant, fluide rhéo-fluidifiant et fluide thixotrope).

Et vous que pensez-vous de la thixotropie?

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

Tristan FERROIR — Thu, 21 Dec 2006 11:41:30 +0000

NEW PUBLICATION IN SCIENCE

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Comet 81P/Wild 2 Under a Microscope

Tristan FERROIR — Thu, 21 Dec 2006 11:40:06 +0000

NEW PUBLICATION IN SCIENCE

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

Tristan FERROIR — Tue, 21 Mar 2006 11:38:06 +0000

NEW PUBLICATION IN AMERICAN MINERALOGIST

The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high-pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The » angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state Þ t to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K’0 = 4.0.

A new high-pressure form of KAlSi3O8 under lower mantle conditions

Tristan FERROIR — Tue, 14 Dec 2004 11:30:39 +0000

NEW PUBLICATION

In situ X-ray diffraction measurements have been made on KAlSi3O8 hollandite using diamond anvil cell and multianvil apparatus combined with synchrotron radiation.
Both of the measurements with different techniques demonstrated that K-hollandite transforms to a new highpressure phase (hollandite II) at 22 GPa upon increasing pressure at room temperature. The X-ray diffraction peaks of the new phase were reasonably indexed on the basis of a monoclinic cell with I2/m space group. Hollandite II was also confirmed to be formed at high temperatures to 1200°C and pressures to 35 GPa, which was quenched to room temperature under pressure but converted back to hollandite at about 20 GPa on release of pressure. The present result is contradictory to earlier studies based mainly on quench method, which concluded that hollandite is stable up to 95 GPa at both room temperature and high temperatures up to 2300°C.

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Motion of a solid object through a pasty (thixotropic) fluid

Tristan FERROIR — Mon, 26 Jan 2004 11:35:37 +0000

NEW PUBLICATION IN PHYSCIS OF FLUIDS

For materials assumed to be simple yield stress fluids the velocity of an object should continuously increase from zero as the applied force increases from the critical value for incipient motion. We carried out experiments of fall of a sphere in a typical, thixotropic, pasty material ~a laponite suspension!. We either left a sphere falling in the fluid in different initial states of structure or vibrated the fluid in a given state of structure at different frequencies. In each case three analogous regimes appear either for increasing restructuring states of the fluid or decreasing frequencies: A rapid fall at an almost constant rate; a slower fall at a progressively decreasing velocity; a slow fall at a rapidly decreasing rate finally leading to apparent stoppage. These results show that the motion of an object, due to gravity in a pasty material, is a more complex dynamical process than generally assumed for simple yield stress fluids. A simple model using the basic features of the ~thixotropic! rheological behavior of these pasty materials makes it possible to explain these experimental trends. The fall of an object in such a fluid thus appears to basically follow a bifurcation process: For a sufficiently large force applied onto the object its rapid motion tends to sufficiently liquify the fluid around it so that its subsequent motion is more rapid and so on until reaching a constant velocity; on the contrary if the force applied onto the object is not sufficiently large the fluid around has enough time to restructure, which slows down the motion and so on until the complete stoppage of the object.