We have investigated the formation of helium droplets in two physical situations. In the first one, droplets are atomised from superfluid or normal liquid by a fast helium vapour flow. In the second, droplets of normal liquid are formed inside porous
glasses during the process of helium condensation. The context, aims, and results of these experiments are reviewed, with focus on the specificity of light scattering by helium. In particular, we discuss how, for different reasons, the closeness to unity of the index of refraction of helium allows in both cases to minimise the problem of multiple scattering and obtain results which it would not be possible to get using other fluids.
We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a firs
t application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media.
We report on thermodynamic and optical measurements of the condensation process of $^4$He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and
of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.
