OPTICAL INVESTIGATIONS OF GRANULAR AEROGEL FILLS

The light scattering behaviour of granular silica aerogels is measured at a fixed wavelength of 633 nm. Two different correlation lengths a(c,bulk) and a(c,sur), which characterize the bulk and surface scattering properties, are derived. A value of a(c,bulk) = (45 +/- 5) nm is typical for the bulk scattering, whereas surface imperfections, such as dust grains and scratches as well as bubbles and streaks, cause a strongly forward-peaked scattering distribution with a(c,sur) = (600 +/-50) nm. The spectral scattering coefficient in the visible region is also measured. By modelling the data, the contributions of large and small inhomogeneities to the scattering coefficient S can be separated. Typical values are S(bulk) = (65 +/- 10) m-1 at a wavelength of 633 nm and S(bulk) = (115 +/- 20) m-1 at 543 nm. For the large inhomogeneities a wavelength-independent scattering coefficient of S(sur), = (100 +/- 30) m-1 is obtained. We have also measured the directional-hemispherical transmission t(dh) of a loose granular fill as a function of granule size. The influence of the aerogel nanostructure on t(dh) is determined from radiation transport calculations. Because of the forward-peaked scattering distribution of the large inhomogeneities, t(dh) is predominantly affected by the bulk.