(15a) Particles in Contact: The Key Challenge in Solids Processing

One of the grand challenges in Particle Technology is how to characterize the contact mechanisms and contact forces between particles. Mechanical particle properties are the key parameters in discrete element models (DEM) which have been developed during the last decades and gradually reach “predictive power”. Here and in many other aspects of Particle Technology, it is helpful to distinguish between process and materials functions. The process functions describe the effects of external parameters such as energy input, rotor revolution, machine geometry on the motion and stresses acting on particles in the process. As a result, the particles in the process or in the machine (such as mills, mixers or granulators) are stressed by these external forces and react accordingly depending on their material properties. In order to make DEM truly predictive, intrinsic material properties such as Young’s modulus, hardness, plastic deformation or fracture toughness of the particles must be known quantitatively. For instance, we have shown how to model packing densities of glass beads as a function of particle size in dependency of intrinsic material properties. However, measured particle properties (and their distributions) are only known in a few rare cases.

The (nano-)mechanics of particles across all length scales down to the nanoscale are highly relevant for many applications including adhesion, friction, powder flow, comminution and tribology. Particular interesting new fields are additive manufacturing and 3D printing which “promise” a revolution in industrial manufacturing based on powder technologies. Nanomechanical elastic and inelastic particle properties have been measured systematically in a novel nanoindentation device installed within a scanning electron microscope to obtain simultaneously force-displacement information and optical images of deformed and broken particles. Quantitative data of intrinsic particle properties (and their respective distributions) are derived with high statistical relevance from force-displacement curves of single oxide, metal and polymer particles. The force-displacement of well-defined model particles can be used to obtain so far unknown distributions of stress energy and stress number distributions in mills and other solids processing devices. Finally, we show how novel 2D materials such as graphene, MoS₂ or BN can be produced by scalable mechanical delamination. Here, the challenge is also to characterize the delaminated nanosheets which is accomplished by advanced ultracentrifugation techniques. All these results are discussed in the context of unifying principles of product design and solids processing.