Aerodynamic behavior of fiber- and disc-like particles in a Millikan cell apparatus

Abstract

Silicon dioxide particles of fiber- or disc-like shapes (aspect ratio = 0.2–8.5) were used to determine the drag force on nonspherical particles at a low Knudsen number. The experiment was designed to study the effect of particle orientation on the dynamic shape factor and the slip correction factor in a Millikan cell apparatus. Results showed that the particles moved randomly when under the influence of gravity but were aligned in a definite direction in the presence of an electrical field. Using a nonlinear regression method, both the dynamic shape factor and slip correction factor of the particles could be simultaneously obtained from the data. Results showed that the shape factor of nonspherical particles varied with particle shape, aspect ratio and particle orientation. The shape factors for fiber-like particles ranged from 1.06 (for particles whose aspect ratio was 4.29 and whose major axis moved parallel to the flow) to 1.59 (for particles whose aspect ratio was 8.18 and whose major axis moved perpendicular to the flow). The shape factors for disc-like particles ranged from 1.08 (for particles whose aspect ratio was 0.44 and whose major axis moved perpendicular to the flow) to 1.27 (for particles whose aspect ratio was 0.29 and whose major axis moved parallel to the flow). In addition, the results showed that the slip correction factors of nonspherical particles were functions of particle shape, aspect ratio and orientation. For fiber-like particles, their slip correction factors were in agreement with those of the cylinders predicted by Dahneke (1973, J. Aerosol Sci. 4, 147). For disc-like particles with an aspect ratio of 0.29, the slip correction factors were in agreement with those of oblate spheroids.