  The mechanical properties of aggregated colloids depend on the mutual
interplay of inter-particle potentials, contact forces, aggregate structure and
material properties of the bare particles. Owing to this variety of influences
experimental results from macroscopic mechanical testings were mostly compared
to time-consuming, microscopic simulations rather than to analytical theories.
The aim of the present paper was to relate both macroscopic and microscopic
mechanical data with each other and simple analytical models. We investigated
dense amorphous aggregates made from monodisperse poly-methyl methacrylate
(PMMA) particles (diameter: 1.6\mu m) via nanoindentation in combination with
confocal microscopy. The resulting macroscopic information were complemented by
the three dimensional aggregate structure as well as the microscopic strain
field. The strain field was in reasonable agreement with the predictions from
analytical continuum theories. As a consequence the measured force-depth curves
could be analyzed within a theoretical framework that has frequently been used
for nanoindentation of atomic matter such as metals, ceramics and polymers. The
extracted values for hardness and effective elastic modulus represented average
values characteristic of the aggregate. On base of these parameters we discuss
the influence of the strength of particle bonds by introducing polystyrene (PS)
between the particles.
