Dynamical processes in granular matter

Measurement of rotation of individual spherical particles in cohesive granulates:
Single particle dynamics in colloidal systems include (in the simplest case) translations and rotations of the colloids. To explore these dynamical processes, we use a combination of 3D imaging and mechanical testing. We analyze structural changes using confocal microscopy while applying a mechanical load simultaneously. Fluorescently labeled polydisperse silica particles were hydrophobized with long alkyl chains and dispersed in an index-matching liquid. 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. We focus on particle translation and rotation in dependency of the local volume fraction. During compression, restructuring happens predominantly in regions of low packing density. We show that rotation plays an important role and is hence a key parameter for explaining dynamical processes in granular systems.

Schematic setup: The home-built laser scanning confocal microscope (LSCM).

Visualization of rotation and translation of particle one. Resting particles are shown by opaque gray, the others translucent. The red and blue arrows indicate the focal and normal vectors, the green arrow its vector product.

• 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 15 (4), 391, (2013), DOI 10.1007/s10035-012-0383-7.

See: http://link.springer.com/content/pdf/10.1007%2Fs10035-012-0383-7.pdf