INTERNAL WAVES IN XENON NEAR THE CRITICAL-POINT

Just above the liquid-vapor critical point, a fluid's large compressib ility causes a stable stratification in which the density varies by as much as 10% in 1 cm. This stratification supports internal gravity wa ves which we observed with an oscillator immersed in a near-critical x enon sample. We found the number and frequencies of the observable mod es depended on the sample cell's orientation, with only two modes seen in the horizontal cell. The frequencies of the two modes had differen t temperature dependences: with decreasing temperature, the higher fre quency increased monotonically from 0.7 to 2.8 Hz, but the lower frequ ency varied nonmonotonically, with a maximum of 1.0 Hz at 20 mK above the critical temperature. These temperature dependences continued to 2 0 mK below the critical temperature, where the xenon was separated int o liquid and vapor phases. We calculated these two frequencies by solv ing the eigenvalue problem of internal waves in a box containing a str atified fluid. The fluid's density profile was obtained from xenon's e quation of state. The calculated and measured frequencies agree to wit hin 15%. Analytical calculations based on simple approximations of the density profile provide insight into the observed temperature depende nces. (C) 1996 American Institute of Physics.