Microcrystal Morphology

Framboids are constituted by microcrystals with approximately log-normal size-frequency distributions, and 95% of framboidal microcrystals are between 0.1 and 3.1 μ‎m. Nanocrystals are not generally observed in framboids. Packing efficiencies vary between close-packings in which the microcrystals occupy up to 74% of the framboid volume and random packings with a 56% volume of microcrystals. The ratios between framboid diameter and microcrystal size show a clear bimodal distribution which reflects the populations of close-packed ordered framboids and randomly organized framboids. Framboids may contain up to 500,000 microcrystals. The average numbers of microcrystals in both disordered and ordered framboids are similar, which suggests that the organization of microcrystals is the result of an additional process. Minerals that do not commonly produce equant crystals forms are unlikely to display the framboidal texture. Framboid microcrystals are essentially limited to isometric minerals like pyrite which produce equant crystals. Pyrite displays the greatest variety of crystal shapes among the common minerals. This means that pyrite is able to approximate forbidden fivefold symmetries such as the pentagonal dodecahedron, but with asymmetric pentagonal faces, and the icosahedron, again with different-sized triangular faces, as a combination of the octahedron and pyritohedron.

Seismic waves in the asteroid environment

Through numerical simulations, we investigate impact generated seismic wave transmission in granular media under extremely low pressure. This mimics the conditions in the interior of asteroids and other small planetary bodies. We find a dependency not only on the overburden pressure on the medium, but also on the velocity of the impact that generates the wave. This is, at extremely low values of overburden pressure, the wave speed depends no only on the imposed pressure, but also on the increment in pressure created by the passing of the wave. We study crystalline and random packings and find very similar behaviour though with different wave speeds as expected. We then relate our results to different mission-related events on asteroids.

Analytical and DEM studies of thermal stress in granular materials

The ability to predict thermal-induced stresses in granular materials is of practical importance across a range of disciplines ranging from process engineering to geotechnical engineering. This study presents an analytical formula to predict thermal-induced stress increments in mono-disperse granular materials subject to an initial isotropic stress state. A complementary series of DEM simulations were carried out to explore the applicability of the proposed analytical formula. The comparative analysis showed that the proposed expression can accurately predict stress changes in packings where there are negligible particle displacements as a consequence of the thermal loading (e.g. regular packings and medium/dense random packings); however large errors were observed in loose samples with a random packing.

X-ray nanotomography of dry colloidal packings

Random packings are crucial in understanding arrangement and geometry of particles. Random packings of dry small particles may be subject to adhesion or friction, as expected theoretically and numerically. We explore experimentally random packings of dry colloids with X-ray nanotomography that directly provides three-dimensional structural and geometric information of dry colloidal packings. We find that dry colloidal packings, as characterized by contact number and packing density, are quite consistent with adhesive loose packings that significantly deviate from random loose packings for hard spheres. This study may offer direct evidence for adhesive loose packings comprising dry small particles, as proven by X-ray nanotomography.

Computational analysis of transport in three-dimensional heterogeneous materials

Porous and heterogeneous materials are found in many applications from composites, membranes, chemical reactors, and other engineered materials to biological matter and natural subsurface structures. In this work we propose an integrated approach to generate, study and upscale transport equations in random and periodic porous structures. The geometry generation is based on random algorithms or ballistic deposition. In particular, a new algorithm is proposed to generate random packings of ellipsoids with random orientation and tunable porosity and connectivity. The porous structure is then meshed using locally refined Cartesian-based or unstructured strategies. Transport equations are thus solved in a finite-volume formulation with quasi-periodic boundary conditions to simplify the upscaling problem by solving simple closure problems consistent with the classical theory of homogenisation for linear advection–diffusion–reaction operators. Existing simulation codes are extended with novel developments and integrated to produce a fully open-source simulation pipeline. A showcase of a few interesting three-dimensional applications of these computational approaches is then presented. Firstly, convergence properties and the transport and dispersion properties of a periodic arrangement of spheres are studied. Then, heat transfer problems are considered in a pipe with layers of deposited particles of different heights, and in heterogeneous anisotropic materials.