Tristan FERROIR

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.

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.

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.