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X-ray tomography of granular and colloidal materials

X-ray tomography of granular and colloidal materials
place:ANKA seminar room, 1st floor, building 348
Affiliation:European Synchrotron Radiation Facility, France
Inviting person:Tilo Baumbach, KIT-ANKA, IPS, LAS
Speaker:Mario Scheel
Time:10:00 a.m.


When a small amount of liquid is added to a granulate, its stiffness changes dramatically. This is due to capillary bridges forming between mutually adjacent grains in the pile, which exert an attractive force by virtue of the surface tension of the liquid. The resulting mechanical properties of a wet granulate are surprisingly independent within a wide range of liquid content, as studied by measuring the critical fluidization acceleration for vertical agitation, the tensile strength, and the yield stress of the granulate for various liquid contents. This peculiar behaviour is explored experimentally and theoretically by means of the liquid distribution within the granulate. A model granulate consisting of glass beads is analyzed experimentally by X-ray tomography as a function of wettability. The constant mechanical properties of wet granulates for a large range of liquid contents is a result of ramified liquid structures, which dominate over all other possible morphologies in case of a wetting liquid. As the particle diameter gets much smaller the colloidal regime is reached. There, Brownian motion becomes important and crystals are formed from mondisperse 1.5 μm sized silica spheres in a suspension. Sediments of the colloids are characterized by X-ray methods. First high resolution SAXS was applied on a colloidal sediment containing crystalline and glassy regions. This allowed measuring the colloid diameter and polydispersity. Then micro tomography was used to separate glassy and crystalline domains, and the crystal size distribution was analyzed. Nano-tomography accesses the colloidal structure in real space, and local parameters could be extracted like the individual sphere position, the structures of the crystals, or localising defects relative to the crystal border. These parameters are hard to investigate by other measurement methods.