Dynamics and Measurement of Smokes. II The Aerodynamic Diameter of Chain Aggregates in the Transition Regime

Abstract

Aerodynamic diameters of linear Fe₂ O₃ chain aggregates with up to 160 primary particles ranging in mean diameter from 0.035 to 0.12 μm were measured with a cylindrical aerosol centrifuge. Aerodynamic diameters closely follow the semiempirical expression

were ρ/ρ_o is the ratio of particle density to unit density; D_I and o_g are the mean diameter and geometric standard deviation of the primary particles, respectively; and n is the average number of primary particles in a chain found at the location corresponding to D _AE. The coefficient k is a function of D _I only. In the continuum limit (D_I $$ 0.02 μm) κ appears to reach a constant value that is consistent with earlier measurements by Stöber et al. (1970). The results suggest a predominant settling orientation normal to the chain axis for centrifuge measurements. Values for the slip coefficient as a function of chain thickness and length are derived, and imperfections in the chain configuration are explained in terms of a coefficient X.