Granular materials feature very intriguing mechanical properties, which differ vastly from their solid bulk materials. An everyday example of granular mechanics can be inferred from ground coffee powder. Being packed into vacuumized bags the particles strongly interdigitate resulting in a very hard and solid pack of coffee.
We therefore investigated the mechanical properties of ordered colloidal superstructures. In particular, with the aim of light-weight materials, we have been investigating colloidal mono- and multilayers comprising hollow silica spheres. Hollow capsules exhibit a buckling deformation under compressive load. The stiffness of the capsule itself depends on the ratio between the radius of the entire sphere and the square of the shell thickness. Using a nanoindentor, we investigated the rich parameter space of the cooperative mechanical properties of mono-, bi-, and trilayer films of monodisperse hollow silica nanoparticles.
The strain rate can also have a significant influence on the mechanical response of a given material. Particularly, high strain rates present during shock or blast impact events are not well understood on a nano- and mesoscale. A method to overcome this gap, has therefore been developed: LIPIT - laser induced projectile impact testing. This allows to impact nanostructured samples with few micrometer large "bullets" or particles with up to Mach10.