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Sand, coffee beans and mud all belong to a class of materials that we call granular media. Despite their relevance in industry and agriculture, the flow behaviour of these materials remains poorly understood. In particular, it is unclear how specific properties of the particles, governing the interactions at the microscopic level, influence the macroscopic flow response. In practice, it is often difficult to vary particle properties, such as stiffness or friction coefficient, in a controlled way. In this thesis, we investigate flows of granular materials with well-defined particle properties, by synthesizing the particles using novel methods. In Part I of the thesis, we investigate the role of friction in shear flows of granular suspensions. We present a method to produce millimetre-sized hydrogel particles, and investigate how the chemistry of the hydrogels affects the material friction coefficient, and subsequently determine how this relates to macroscopic flow behaviour. In Part II of the thesis, we study granular materials in systems where they are not driven by the walls, but rather from within the material. We study how passive particles driven by a single magnet can aid mixing in a microfluidic mixing chip. We also take care that the pressure drop, which limits the simple use of microfluidic chips, is greatly reduced compared to commercially available solutions. Finally, we investigate the role of geometric friction in a granular material in which each particle is individually driven to rotate. The activity of these 3D-printed particles, combined with frictional coupling of rotational and translational degrees of freedom, leads to the emergence of a granular material that displays collective behaviour. The thesis is concluded with a general discussion.

Driven granular media : mixing, friction & activity