Adsorption and capillary condensation of 4He on Nuclepore: Third-sound and capacitance measurements.

Abstract

The adsorption of $^{4}\mathrm{He}$ films on Nuclepore filter materials of several pore sizes has been studied for temperatures in the range 1.4 K\ensuremath{\lesssim}T\ensuremath{\lesssim}1.8 K using simultaneous measurements of the time of flight of pulsed third sound and capacitance measurements of the mass of $^{4}\mathrm{He}$ in the pores. The third-sound and capacitance data for values of the film thickness 4<h(layers)<25, which span capillary condensation of the pores, show strongly hysteretic behavior. Correlations between the third-sound and capacitance data aid in understanding the film configuration(s) for a given value of the helium film thickness. Good agreement is found between the capacitance data and the theory which describes the behavior of helium films in cylindrical geometries due to Saam and Cole, if a distribution in pore radii is assumed. Good agreement is also found between the third-sound time-of-flight data and the theory of Cohen, Guyer, and Machta which describes the velocity of third sound on Nuclepore. The nature of the Kosterlitz-Thouless transition in the $^{4}\mathrm{He}$ film is briefly explored on both porous Nuclepore and a sample of the substrate material without holes.