Small-angle x-ray scattering measurements of the microstructure of liquid helium mixtures adsorbed in aerogel

Small-angle x-ray scattering (SAXS) was used to measure the microstructure of isotopic mixtures of 3He and 4He adsorbed into silica aerogels as a function of temperature and 3He concentration. The SAXS measurements could be well described by the formation of a nearly pure film of 4He which separates from the bulk mixture onto the aerogel strands and which thickens with decreasing temperature. Previous observations of a superfluid 3He -rich phase are consistent with superfluidity existing within this film phase. Observed differences between different density aerogels are explained in terms of the depletion of 4He from the bulk mixture due to film formation.