Thermal Relaxation near the Critical Point

We have measured thermal-relaxation times (τ) near the critical point of He³. τ remains small in one-phase states throughout the critical region. In two-phase states, τ increases as the critical temperature (Tc) is approached in proportion to (Tc-T)^-1.0 at all densities. An extra increase in τ occurs in two-phase states very near the phase boundary. These observations suggest to us that the diffusion of matter across the liquid-vapor interface is a bottle-neck for equilibration. Our data are of value in interpreting puzzling features of several other experiments near critical points.