Several types of fumed-silica aggregates with differing surface areas were studied over a wide range of spatial resolution by employing both light and neutron scattering. At intermediate length scales, between 100 and 1000 Å, the aggregates are mass fractals with $D_m$≊1.7–2.0, in basic agreement with simulations of aggregating clusters. At short length scales below 100 Å where the nature of the surfaces of the primary particles dominates the scattering, some of the samples appear to be fractally rough. In particular, a higher surface area seems to be correlated not with smaller primary particles in the aggregates, as previously assumed, but with fractally rough surfaces having $D_s$ as high as 2.5. These may be the first materials discovered to have both mass and surface fractal structure.

• Received 14 October 1986

DOI:https://doi.org/10.1103/PhysRevA.35.2361