Plasmas, which are made of charged particles like ions or electrons, are useful in experiments for condensed matter physics. Plasmas are good models of gases, however, by adding dust to the system the plasma can mimic a liquid or a solid (where the particles remain almost stationary and interact only with their nearest neighbours).
Scientists in Iowa are using "dusty" plasmas - electrons loaded with micron-sized spheres - to study the structure and movements within the plasma at a microscopic level. By firing a supersonic particle into the plasma they can observe the pattern of shock waves produced. This pattern is known as a Mach cone.
The formation of Mach cones in solids has interesting applications in seismology. For example, sound waves travelling in a liquid-filled bore hole move faster than the speed of sound in the surrounding rock. This causes a Mach cone to propagate through the rock. AIP