High-speed observation of the piston effect near the gas-liquid critical   point
  We measured high-speed sound propagation in a near-critical fluid using a ultra-sensitive interferometer to investigate adiabatic changes of fluids on acoustic timescales. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10^{-7}g/cm^3 and a temperature change of order 10^{-5}deg. Very small heat inputs at a heater produced short acoustic pulses with width of order 10micro sec, which were broadened as they moved through the cell and encountered with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions well agree with our data. 
Yuichi Miura, Mitsuru Ohnishi, Shoichi Yoshihara, Katsuya Honda,   Masahiro Matsumoto, Jun Kawai, Masamichi Ishikawa, Hiroto...