MOMENTUM SLIP CORRECTION FACTOR FOR SMALL PARTICLES IN 9 COMMON GASES

Experimental data for the slip correction to Stokes drag law, required for particles with sizes of the order of the gas mean free path, are reviewed and re-evaluated for a variety of common gases. An accepted correlation for the slip factor, C(Kn)=1 + Kn[α + β·exp (-γ/Kn)], is assumed and the best values for α, β and γ are sought for the various gases (Kn is the Knudsen number equal to the ratio of the gas mean free path to the particle radius). The review focuses on the oil-drop work of Ishida [Ishida, Y. (1923) Phys. Rev. 21, 550-563], whose experiments in the continuum-slip regime (Kn ≲ 0.4) provide accurate values for the parameter α for nine gases: air, argon, helium, hydrogen, methane, ethane, isobutane, nitrous oxide and carbon dioxide. Ishida's data are re-evaluated using the latest published values for the electric charge and gas properties. The resulting values for α range from 1.141 for hydrogen to 1.277 for helium; α for air fell mid-range at 1.207. Good agreement is found with other investigators. The oil-drop work of Eglin [Eglin, J. M. (1923) Phys. Rev. 22, 161-170] and Millikan [Millikan, R. A. (1923) Phys. Rev. 22, 1] provides slip correction data over a wider range of Kn for air, oxygen, carbon dioxide and helium, and shows that, within experimental uncertainty, (α + β) = 1.647 for all four gases in the free-molecular limit. The interpolation parameter γ was found to be 0.78, 0.92 and 2.0 for air, carbon dioxide and helium, respectively. © 1990.