Slip Correction Measurements for Solid Spherical Particles by Modulated Dynamic Light Scattering

Abstract

The theoretical expression for the drag force on a spherical particle moving with low Reynolds number in a gas is customarily written by multiplying the Stokes' law expression by a slip correction factor of the form C x = 1 +Kn[γ1 + γ2 exp(-γ3/Kn)], where Kn is the particle Knudsen number and γ1, γ2, and γ3 are empirical constants. We have measured the drag forces on spherical polystyrene latex particles suspended in dry air using the modulated dynamic light scattering technique which is fundamentally different from the Millikan cell approach. Data are time autocorrelation functions of the intensity of light scattered by single test particles from the intersection volume of two coherent laser beams. The data provide detailed information about test particle Brownian motion including the value of the particle diffusion coefficient. Each test particle was held in air suspension by an electrostatic trap to permit measurements to be made on the same particle at air pressures ranging from 760 to 0.2 torr. Data ...