Dynamics of density equilibration near the liquid-vapor critical point of³He

Abstract

The density change σρ(t) was measured as a function of time at two locations in the upper and the lower half of a flat cell, after a temperature step ΔT≪(T−T _c), whereT _c is the critical temperature. The series of measurements were made along the critical isochore. Direct observation of the sharp response from adiabatic energy transfer into the bulk fluid (“piston effect”), and the effect of slow diffusive relaxation to equilibrium are reported. The relaxation time is found to diverge asT _c is approached. The onset of convection for a step ΔT beyond a critical value could be observed, with a dramatic change of the temporal profile of σρ(t). Computer simulations of this profile in the absence of convection have been carried out and are compared with the experiment.