Density Relaxation of Liquid–Vapor Critical Fluids in Earthʹs Gravity

Experimental results of the density relaxation of a liquid-vapor critical fluid in Earthʹs gravity over a temperature regime of severe density stratification are preseInted. A 10-mm-diameter x 1-mm-thick disk-shaped sample of SF6 was placed in a Twyman-Green phase shifting iInterferometer with a phase uncertainty of 1/65 of a wavelength over 60 h. Relaxations to equilibrium stratification were observed for a temperature range from 1.0 to 29.6 mK above Tc, where Tc. is the critical temperature. The iInterferometry provided a density distribution history over the full extent of the sample cell. Two types of initial density states were established before stepping to the final temperature (density) states for relaxation: (1) the two-phase state at Tc —50 mK and (2) the equilibrium state at Tc+ 100 mK. Upper and lower portions of the cell relaxed differently for these two initial states. For the Tc + 100 mK initial state, relaxation to T < Tc + 3 mK showed an early density overshoot followed by an additional long-time relaxation not seen in the other relaxation sequences. Otherwise, relaxations were faster and increasingly nondiffusive (without a unique exponential description) as the final state became closer to the critical temperature