Diffusive mobility of fractal aggregates over the entire knudsen number range

Abstract

We determine the effective mobility radius for fractal aggregate particles. Our method is to use static light scattering to measure the radius of gyration R-g of the aggregates, and dynamic light scattering to measure the diffusion coefficient hence the mobility radius R-m. The range of our results can be specified by the Knudsen number Kn, which is the mean free path of the medium molecules divided by the radius of the aggregate. Our results apply to the entire range of Kn from the continuum limit (Kn=0) to the free molecular limit (Kn much greater than 1). In the continuum regime we find R-m/R-g=0.97+/-0.05 when the aggregate fractal dimension is D-f similar or equal to 2.15, and 0.70+/-0.05 when D(f)similar or equal to 1.75 . The latter result is independent of Kn for Kn less than or similar to 1.3. The free molecular mobility goes as R-m=aN(0.44+/-0.03) where a is the monomer radius and N is the number of monomers per aggregate. Since R(g)similar to aN(1/Df), R-m/R-g is not a constant when Kn is large. We find for all Kn that the functionality of R-m/R-g must always begin with the comet N-->1 limit, and this affects experimental observation. [S1063-651X(99)00409-2].