Steepening of density gradient in a free expansion

A one-dimensional free-expansion experiment for a mixture of two disparate gases is presented, in analogy with the mechanism of shock formation following the collapse of matter in a supernova. The gas pressure and density are measured simultaneously. In the free expansion of a pure gas, the pressure and density increase in such a manner that the temperature rises monotonically. A preexisting density inhomogeneity, on the other hand, steepens, and develops into a discontinuities upon interaction with the endwall, accompanied by an instability.