Disordered materials made of atoms, molecules or micro- or millimeter size particles can flow like a liquid, but can also be arrested like a solid while still maintaining the structure of a liquid. Examples include molecular glasses, quickclay, foams, or a pile of sand or sugar grains. We investigate the physics of this flow-no flow transition in dense colloidal suspensions. These particles exhibit Brownian motion, and therefore possess thermal energy;  yet they are large enough  to be resolved individually by optical microscopy. Three-dimensional tracking of the particles in real time is possible by using fast confocal microscopy to acquire three-dimensional images. This offers a unique opportunity to obtain insight into the mechanism of flow and arrest of amorphous materials.
We use the individual particle trajectories to visualize the strain distribution in slowly sheared suspensions (top image), and we investigate structural rearrangements on the single particle level (bottom image). We identify zones of high local shear strain (“shear transformation zones”), and analyze the spatial and temporal distribution of these zones to obtain insight into the onset of homogeneous flow, and shear banding, which is commonly observed in these materials at sufficiently high shear rates.

See also: P. Schall et al., Science 318, p.1895 (Dec 2007)