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

Colloidal aggregates tested via nanoindentation and simultaneous 3D imaging