Density response and equilibration in a pure fluid near the liquid-vapor critical point:: 3He

We report measurements of the local density response inside a quasi 1-D horizontal He-3 fluid layer to a step-like change Delta T of the boundary temperature, where \Delta T\ approximate to 80 mu K and much smaller than \T -T-c\ where T-c is the critical temperature. These experiments used a new cell design, described in the text, and were carried out along the critical isochore <(rho)over bar> = rho(c) both above and below T-c. The observed temporal and spatial density response delta rho(t, z) and its equilibration time are described adequately by the relations developed from the thermodynamic theory of Onuki and Ferrell. We verify that over the temperature range of low stratification, where computer simulations and closed-form calculations can be compared they are in exact agreement. The systematic differences of experimental results fr on? predictions can be accounted for by the departure of the cell from the ideal 1-D geometry. The much larger disagreement between the experimental and predicted equilibration time scale in earlier experiments is also explained. Finally, deviations fi om linearity observed in the density response for steps \Delta T\ larger than approximate to 90 K mu are reported and the implications of such nonlinearity for the delta rho(t, z) profile and especially the effective relaxation time tau(eff) are analyzed. We also discuss the predicted onset of convection near T-c for the conditions in our experiment. In the Appendix, the likely sources for systematic deviations in the density response function for the experimental cell from calculations in the ideal I-D geometry ale presented and their effects calculated The so-obtained response function Z(F)(omega, z) is compared with previously published data.