Prof. Bob Behringer's group recently has a paper accepted in PR titled "Particle scale dynamics in granular impact." The image at left is a typical image from one of their experiments, where the bright particles are experiencing force. They are exploring the questions of: How does granular material respond to a high-speed impact by a foreign object, such as a meteor striking a planetary surface? How do the grains collectively push back against the intruder, bringing it to a stop? They answer these questions with novel impact experiments on granular materials made from a photoelastic substance. This approach has the special property of allowing them to see how individual grains bear forces, and where the energy of the object goes. High-speed videos reveal rich acoustic activity: as the intruder moves, lightning bolts of force emerge, traveling along networks of grains. These intermittent acoustic pulses along granular 'chains' are responsible for decelerating the intruder and carrying away energy. This study is particularly novel because it captures the granular behavior in a way that has not been done previously. It offers a new perspective for a process of considerable importance in natural phenomena and in industry, which has frequently been studied in the past, but without access to what the grains are doing.
