Evidence for a disorder induced phase transition in the condensation of 4He in aerogels

We report on thermodynamic and optical measurements of the condensation process of ${}^4$He in two silica aerogels of same porosity 95%, but different microstructures resulting from different pH during synthesis. For a base-catalyzed aerogel, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder induced transition,in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalyzed aerogel, which we interpret as due to a larger disorder in this case.