Light scattering study of collective effects during evaporation and condensation in a disordered porous material

We combine high-resolution isotherms measurements and light scattering technics to study over a broad temperature range collective effects during the condensation and evaporation of helium from Vycor, a prototypic disordered porous material. For evaporation, our results provide the first direct evidence for a crossover from a percolation collective mechanism at low temperature to a local cavitation mechanism at high temperature. No long-range collective effects are detected during condensation. We compare these results to recent theoretical predictions emphasizing the specific role of disorder, and discuss their relevance for determining pores sizes distributions in disordered porous materials.