High-speed observation of the piston effect near the gas-liquid critical point

TL;DR

This study used high-speed interferometry to observe acoustic phenomena near the gas-liquid critical point, revealing minute adiabatic changes and pulse broadening effects that intensify close to criticality, supported by theoretical modeling.

Contribution

It provides detailed experimental measurements of sound propagation and adiabatic changes near the critical point, along with a theoretical framework explaining sound emission and heat-to-mechanical energy conversion.

Findings

Detected minute density and temperature changes during sound propagation near critical point.

Observed pulse broadening effects that increase as the critical point is approached.

Theoretical predictions align well with experimental data.

Abstract

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.