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. We will discuss the interplay between the various possible modes of motion with an experimental example of polydisperse silica particles (average diameter 7µm) were hydrophobized with long alkyl chains and dispersed in an index-matching liquid [1]. The particles show a weak attraction. Photobleaching the central plane of individual particles generates an optical anisotropy without changing particle interaction. In a series of 3D images, we follow trajectories and rotation of single particles [1]. In a combined experiments of mechanical nanoindentation and high speed confocal microscopy, we correlate the mechanical load with structural rearrangements in the sample.

Reference:
[1] Jennifer Wenzl, Ryohei Seto, Marcel Roth, Hans-Jürgen Butt, and Günter K. Auernhammer; “Measurement of rotation of individual spherical particles in cohesive granulates.” Granul. Matter, (2012), DOI 10.1007/s10035-012-0383-7, in press.