Dynamic shape factors of sphere aggregates in an electric field and their dependence on the Knudsen number

Abstract

Abstract Dynamic shape factors in an electric field for sphere aggregates of up to five monodisperse particles and their dependence on the Knudsen number in the range Kn = 0.2–1.2, were investigated using a commercial electrostatic classifier (TSI 3071). The results have been compared with data from the literature obtained with electrical and inertial spectrometers as well as with diffusion batteries and conclusions are drawn about the probable orientation of the aggregates during settling. For clusters of more than three primary particles no preferred orientation was observed over the Knudsen range investigated and the measured dynamic shape factors showed good agreement with those obtained with inertial spectrometers (aerosol centrifuges) indicating a random orientation during settling in both types of instruments. Chains of 3 and 4 primary particles were found to be oriented with their polar-axis parallel with the electric field during settling for all particle sizes studied. For doublets a significant increase of the dynamic shape factor with increasing Knudsen number was found. Extrapolations from a linear regression line gave dynamic shape factors of 1.036 and 1.073 for Kn = 0 and 1.28, respectively. The value of 1.036 is in good agreement with that expected for doublets settling with their polar-axis parallel with the electric field and the value of 1.073 is in agreement with recently published experimental data at Kn = 1.28 obtained with diffusion batteries in which doublets should move with random orientation. A similar behaviour was observed for triplet clusters. At Knudsen numbers below 0.5 these oblate shaped particles seemed to settle with their polar axis perpendicular with the electric field but at higher Knudsen numbers they seemed to move with random orientation. The observed change in orientation with increasing Knudsen number for these triplet particles was however not significant.