Can you discover a new type of hydrodynamic instability? A UWM colleague from our department, Dr. Robert Balmer, did it almost 50 years ago. The phenomenon is the following: imagine that you have a horizontal pipe partially filled with a viscous liquid. The pipe rotates about its axis. Instead of staying at a constant level, the liquid will form cells. Balmer realized that the phenomenon is quite complex mathematically and that it is different from known textbook types of hydrodynamic instabilities, such as the Kelvin–Helmholtz, Plateau–Rayleigh, or Rayleigh–Taylor instability. Balmer called this phenomenon “the Hygrocyst” (“liquid cell”) and published about it in R. T. Balmer, 1970, The Hygrocyst–a Stability Phenomenon in Continuum Mechanics, Nature 227, 600 – 601 (08 August 1970); doi:10.1038/227600a0