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 10µsec, 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. Thermal equilibration in one-component fluids takes place increasingly faster near the gas-liquid critical point at fixed volume [1,2,3,4,5,6,7,8,9,10], despite the fact that the thermal diffusion constant D tends to zero at the criticality. This is because the thermal diffusion layer at the boundary expands and sounds emitted cause adiabatic compression and heating in the whole cell after many traversals in the container. This heating mechanism is much intensified near the critical point due to the critical enhancement of thermal expansion of the layer. If the boundary temperature T w is fixed, the interior temperature approaches T w on the timescale of the piston time [2],where L is the cell length and γ = C p /C V is the specificheat ratio growing near the critical point. This time is much shorter than the isobaric equilibration time L 2 /4D by the very small factor (γ − 1) −2 [11]. The previous experiments have detected only slow temperature and density changes in the interior region on timescales of order 1 sec. The aim of this letter is to report ultra-sensitive, high-speed observation of sound propagation through a cell filled with CO 2 on the critical isochore close to the critical point T c = 304.12K. We can detect density changes of order 10

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Microgravity experiments have been conducted on the International Space Station in order to clarify the transition processes of the Marangoni convection in liquid bridges of high Prandtl number fluid. The use of microgravity allows us to generate large liquid bridges, 30 mm in diameter and up to 60 mm in length. Three-dimensional particle tracking velocimetry (3-D PTV) is used to reveal complex flow patterns that appear after the transition of the flow field to oscillatory states. It is found that a standingwave oscillation having an azimuthal mode number equal to one appears in the long liquid bridges. For the liquid bridge 45 mm in length, the oscillation of the flow field is observed in a meridional plane of the liquid bridge, and the flow field exhibits the presence of multiple vortical structures traveling from the heated disk toward the cooled disk. Such flow behaviors are shown to be associated with the propagation of surface temperature fluctuations visualized with an IR camera. These results indicate that the oscillation of the flow and temperature field is due to the propagation of the hydrothermal waves. Their characteristics are discussed in comparison with some previous results with long liquid bridges. It is shown that the axial wavelength of the hydrothermal wave observed presently is comparable to the length of the liquid bridge and that this result disagrees with the previous linear stability analysis for an infinitely long liquid bridge. List of symbolsAr Aspect ratio [-] D Disk diameter [m] f Frequency [Hz] H Length of the liquid bridge [m] m Azimuthal mode number [-] Ma Marangoni number [-] Ma c Critical Marangoni number [-] Pr Prandtl number [-] r Radial position [m] t Time [s] T Oscillation period [s] or temperature [K] T c , T h Cooled-disk temperature and heated-disk temperature [K] DT Temperature difference [K] DT c Critical temperature difference [K] V Liquid bridge volume [m 3 ] V 0 Gap volume [m 3 ] Vr Volume ratio (=V/V 0 ) [-] z Axial position [m] Greek symbols a Thermal diffusivity [m 2 /s] k z Axial wavelength [m] m Kinematic viscosity [m 2 /s] q Density [kg/m 3 ] T.