Impact on soft sand: void collapse and jet formation

Very fine sand is prepared in a well-defined and fully decompactified state by letting gas bubble through it. After turning off the gas stream, a steel ball is dropped on the sand. On impact of the ball, sand is blown away in all directions ("splash") and an impact crater forms. When this cavity collapses, a granular jet emerges and is driven straight into the air. A second jet goes downwards into the air bubble entrained during the process, thus pushing surface material deep into the ground. The air bubble rises slowly towards the surface, causing a granular eruption. In addition to the experiments and the discrete particle simulations we present a simple continuum theory to account for the void collapse leading to the formation of the upward and downward jets.     

Segment dynamics in thin polystyrene films probed by single-molecule optics

Single fluorescent molecules (represented by spheres with a volume equal to the actual van der Waals volume of the molecule) has been embedded in a polystyrene matrix (left). Such molecules act as probes for the study of polymer nanoscale (segmental scale) dynamics in thin films deposited on a glass cover slide (right).     

Real-space observation of ultraslow light in photonic crystal waveguides

We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least 3 ps, movement of this field is hardly discernible: its group velocity would be at most c/1000. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.