Thermal instability in the radiative cooling region behind a shock will cause upstream density fluctuations to collapse into thin sheets aligned parallel to the shock front. A linearized calculation demonstrates the development of this instability. Thermal conduction suppresses the development of small-scale perturbations. Estimates of the scale sizes for the fully developed condensations agree roughly with the scale sizes of fine structure observed in supernova shells such as the Cygnus Loop.
McCray, R., Stein, R.F., Kafatos, M. (1975) Thermal Instability in Supernova Shells,Astrophysical Journal, 196: 565-570. doi: 10.1086/153436