Thixotropy in macroscopic suspensions of spheres

An experimental study of the viscosity of a macroscopic suspension, i.e. a suspension for which Brownian motion can be neglected, under steady shear is presented. The suspension is prepared with a high packing fraction and is density-matched in a Newtonian carrier fluid. The viscosity of the suspension depends on the shear rate and the time of shearing. It is shown for the first time that a macroscopic suspension shows thixotropic viscosity, i.e. shear-thinning with a long relaxation time as a unique function of shear. The relaxation times show a systematic decrease with increasing shear rate. These relaxation times are larger when decreasing the shear rates, compared to those observed after increasing the shear. The time scales involved are about 10000 times larger than the viscous time scale and about 1000 times smaller than the thermodynamic time scale. The structure of the suspension at the outer cylinder of a viscometer is monitored with a camera, showing the formation of a hexagonal structure. The temporal decrease of the viscosity under shear coincides with the formation of this hexagonal pattern

AEROGEL CATALYST MODIFICATION TO IMPROVE CATALYTIC STABILITY DURING THE NITROXIDATION OF PROPYLENE

L'introduction de MgO ou de Fe3O4 dans l'aérogel binaire NiO/Al2O3 augmente très notablement la stabilité du catalyseur dans la nitroxydation du propylène.By introducing MgO or Fe3O4 into a binary NiO/Al2O3 aerogel the catalytic stability in the nitroxidation of propylene may be dramatically improved

Characterization and reactivity of Mg xFe 2-2xO 3-2x and Mg yZn 1-yFe 2O 4 solid solution spinels prepared through the supercritical drying method

Mixed oxides with the formula Mg xFe 2-2xO 3-2x (0{precedes above single-line equals sign}x<1) and Mg yZn 1-yFe 2O 4 (0{precedes above single-line equals sign}y{precedes above single-line equals sign}1) have been prepared as aerogels (i.e. gels obtained by hydrolysis of metal acetates or alkoxides in methanolic solution followed by drying under supercritical conditions). Mg xFe 2-2xO 3-2x samples with 0<x<0.5 are Mg-deficient spinels, giving rise to MgFe 2O 4+\u3b1-Fe 2O 3 mixtures when calcined at 773 K. With x=0.5 the stable phase MgFe 2O 4 is obtained. With 0.5<x{precedes above single-line equals sign}0.66 a Mg-excess spinel phase is obtained that is substantially stable event at 1073 K. Surface areas of the spinel phases range around 100 m 2/g. Mg yZn 1-yFe 2O 4 are stable solid solution spinels in the entire range, and convert progressively from normal to partially inverted spinels by increasing y. The bulk and surface properties of these materials have been, investigated by XRD, TG-DTA, FT-IR and FT-Raman spectroscopies, BET surface areas and SEM microscopy. Also FT-IR spectra of the surface OH groups and catalytic activity in the reduction of NO by NH 3 were investigated. \ua9 1995 Elsevier B.V. All rights reserved

The release of dispersed asbestos fibres from soils

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