Measurement of Acoustic Speed in a Critical Fluid by Direct Observation of ``Piston Eｆｆect''

Abstract

Thermal energy applied to a pure material near its critical point immediately makes a strong expansion. This causes an adiabatic compression in the front of the expansion and a large density difference is formed there. The difference with adiabatic energy is transported as an acoustic wave. This rapid heat transportation is called ‘‘Piston Effect. ’’ In our study, an ultra-sensitive and high-speed density measurement system was developed to observe the elementary process of‘‘ Piston Effect. ’’ To demonstrate the performance of the system, the measurement of sound velocity in a critical fluid was conducted. Numerical simulations were also made to confirm the experimental results. Using the FFT method to both of　the experiment and the simulation, the sound velocity could be precisely evaluated. The velocity profile versus temperature from the experimental results shows a good agreement with the simulation results and theoretical prediction, that is,　the velocity rapidly decreases as the fluid approaches its critical point. This indicates that our experiments and simulations complementally enable us to quantitatively discuss the critical phenomena.