Granular Weissenberg Effect

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The aim of the proposed project is the experimental and numerical characterization of a "granular Weissenberg effect", a newly discovered and not yet fully understood phenomenon occurring when a system of non-spherical particles is sheared.Granular materials made of non-spherical particles are common in a wide range of applications, such as pharmaceutical and food industries, various technical fields and in geology. Shearing, pouring or shaking of such granulates can lead to the formation of local or global orientational orders, which significantly influence the macroscopic static and dynamic properties. This affects, for example, packing densities, repose angles, flow behaviour and friction forces.In previous investigations we found a phenomenon, which by now is not understood: when shearing granular material of non-spherical particles in a container of cylindrical geometry, we observe a significant elevation of the surface near the center of rotation, similar to the well known Weissenberg effect in non-Newtonian fluids. Albeit it is generally acknowledged that shear in such materials is restricted to localized shear bands, a large-scale rearrangement of the material, that is, a secondary flow, is observed. Details are described in the project proposal. Our hypothesis of the origin of this flow is the formation of a convection roll driven by normal stresses in or near the shear zone. The reported effect is only observed in granular matter of pronounced isotropic particles; for spherical particles or nearly spherical particles it is not detected. Despite obvious similarities, at present time it cannot be decided whether this phenomenon is related to the Weissenberg effect in other complex fluids (which is not fully understood as well). In this proposal, we use the term "granular Weissenberg effect" as a working hypothesis, based only on the phenomenological similarity. The proposed research project aims to identify the physical origin of the effect observed in granular materials.To this end, we combine experimental investigations with large scale numerical simulations. By means of tomographic methods (X-ray and NMR) we can perform three-dimensional characterization of the granulate bed. Single particle tracking is also possible. Invasive methods (such as excavation) can supplement as a cost-effective and time-saving alternative. The planned numerical simulations allow a targeted variation of material parameters as well as direct access to microscopic and macroscopic ensemble sizes for comparison with the experiment. We plan to characterize ensembles of prolate and oblate particles and compare them with the behaviour of spherical particles. We start from the hypothesis that effect is largely influenced by the aspect ratio and the shape of the particles (e.g. cylinder, ellipsoid), however, the research shall not be restricted to these quantities only.