MEGraMa: Magnetically excited granular particles in sounding rocket experiment. The dynamics under driving as well as without it can be characterized.

Granular matter is defined as a collection of particles that are large enough that thermal motion ceases to be important, and dissipation upon collisions of particles dominates the dynamics. Granular matter is both relevant in many applications and for the fundamental understanding of many-particle systems.

Intertwining theory, simulation, and experiments, the macroscopic behavior of granular systems is investigated on the basis of properties on the particle level such as dissipative interactions, particle shape and size, as well as charges, and system composition.

With increasing densities, granular gases, granular fluids, and granular packings are all discussed within the framework of statistical physics of disordered media, carried from equilibrium into out-of-equilibrium. For granular gases, magnetic forces can be utilized as a thermostat in microgravity, allowing for dynamics closest to but different from thermal motion. Subsequently, many aspects of granular gas dynamics can be investigated to support and cross-check the development of textbook knowledge in this field.

For the denser systems, light scattering and rheology are used to probe agitated granular fluids. Both conventional laser sources and Terahertz radiation is used to capture structure and dynamics in this regime. The scattering experiment Soft-Matter Dynamics was installed by Alexander Gerst on board the International Space Station ISS in 2018.

The densest state of granular packings is defined by individual particles forming permanent contacts: The contacts can be monitored quantitatively by stress-birefringence in both 2D and 3D. The behavior of the entire system is also measured by sound which becomes progressively slower the lower the pressure on the pile gets. In order to  avoid gradients, experiments are also performed in microgravity.

Experiments in the group are complemented and motivated by microscopic theory for granular fluids and granular rheology. Similarly, numerical simulation is used to investigate basic predictions and to expand on the parameter space not accessible otherwise.

Transmission of stresses in granular packings demonstrated by stress-birefringence. The method allows the determination of minute changes in particle contact forces very sensitively, which can be used to monitor critical behavior as well as qualitative features of heterogeneous stress distribution in granular media.

Research on granular matter at the institute currently focuses on:

• Understanding of macroscopic properties of granular matter from particle-particle interactions
• Microgravity and laboratory experiments for granular gases, fluids, and packings
• Theory and experiments for granular rheology