Pores in fractal aerogels and their incidence on scaling

The nature of scanning electron-microscopy images of silica aerogels is shown to depend enormously on the conductive coating used. The analysis of such images, and of a large amount of other data, confirms that the samples become homogeneous at the point where their fractal regime terminates. There is no disturbing macroporosity that would prevent scaling in function of the density. Simulations of a very large percolation model confirm that the scaling law is obeyed by the pore-size distribution and emphasize that a few occurrences of large pores are to be expected in disordered fractals. The results also clarify the distinction between the acoustic and density correlation lengths.