Colloidal aggregates tested via nano indentation and simultaneous 3D imaging: In this work we investigate the mechanical properties of aggregates made from mono-disperse poly-methyl methacrylate (PMMA) particles (diameter: 1.6 µm) via nano indentation in combination with confocal microscopy. In doing so the bare mechanical data are complemented by 3D coordinates of all particles within the aggregate as well as their individual displacements during the indentation. This information is used to identify reorganization processes and calculate the average strain field. We show that in case of dense amorphous aggregates the strain field is in reasonable agreement with the predictions from continuum theories. In semicrystalline assemblies heterogeneities and generation of dislocation defects are observed which is in accordance with the behavior of atomic crystal. As a consequence the measured force-depth curves can be quantitatively analyzed with the theory of Oliver and Pharr that is usually used for indentation of atomic matter such as metals, ceramics and polymers. The extracted values for hardness and effective elastic modulus represent average values characteristic of the aggregate. Beside to the variation in aggregate structure the influence of particle bonds is tested by introducing polystyrene (PS) between the particles.

The figure shows the indentation set-up (left), a computer reconstruction of the colloidal structure before indentation (middle) and the measured displacement field after indentation (right).

Viscoelastic rheology of colloid-liquid crystal composites: In suspensions of polymeric colloids in liquid crystals (LCs), solvent-particle interactions dominate and can be changed when the LC undergoes phase transition from isotropic to nematic. In case of poly-methyl methacrylate colloids and 4-pentyl-4-cyanobiphenyl (5CB), cooling through the isotropic-nematic phase transition results in a cellular network. The network formation is accompanied by a strong evolution of the mechanical properties. Until now, the mechanical response of the gel was attributed to the elastic interactions of the LC with the colloids. We used a homemade piezorheometer to measure the complex shear modulus of the sample in parallel plate geometry. Since the applied strains are very small, only the linear viscoelastic regime was tested. This limit guarantees a high degree of reproducibility. We gained insight into the underlying processes by measuring the frequency response for the whole cooling process. Temperature and frequency showed a strong correlation allowing for a superposition of the frequency spectra to form a single master curve similar to time-temperature-superposition. We propose that this superposition behavior is connected to the thermodynamics of the isotropic-nematic phase transition of 5CB located in the network walls. Additional experimental observations, such as hysteresis effects, support this assumption Similar effects have been observed by dielectric spectroscopy.

A mixture of PMMA colloids and the liquid crystalline compound 5CB the aggregation is induced by the nematic-to-isotropic phase transition. It turned out that the macroscopic mechanical behavior is dominated by the interaction between the PMMA and the 5CB molecules. Similar effects have been observed by dielectric spectroscopy.