SAXS from 3He-4He mixtures in aerogel

The behavior of binary mixtures in porous media can differ substantially from that of their bulk counterparts. Normally, one of the constituents of the mixture has a greater affinity for the pore surface, and some degree of preferential adsorption takes place. As a result, locally the concentration can differ significantly from its average value. Phase Separation on a global scale is often prevented by free energy barriers associated with the formation or motion of interfaces. Instead, the coexisting phases form domains spanning many pores or strands. In 3He-4He mixtures the preferential adsorption of the 4He on to the silica network leads to the appearance of superfluidity in parts of the mixture phase diagram where the bulk mixture is in the normal fluid state, and the coexistence curve is now contained within the superfluid part of the phase diagram. To completely understand the nature of superfluidity and coexistence in helium mixtures in porous media some information on the spatial distribution of preferentially adsorbed 4He as well as size distribution of coexisting domains is needed. We will present SAXS data that attempt to address these questions.