First LDA measurements of nanoparticle velocities in a low-pressure impacting jet

Abstract

The velocity of nanoparticle agglomerates and 1 μ m latex spheres approaching an impaction surface from a free gas jet was measured by LDA at pressures of 7.4 and 1.8 kPa, respectively. The "aerodynamically transparent" carbon nano-agglomerates gave sufficient LDA signal while the drag force was dominated by the size of the individual primary particles (diameter about 11 nm). The particle velocity as a function of distance from the impaction plate was also modelled on the basis of a CFD simulation of the flow field and subsequent numerical integration of the equation of motion with the slip correction factor as an adjustable parameter. For a pressure of 7.4 kPa the measured and calculated velocities agreed very well, when using the Stokes formula with classical parameters for the slip correction. Surprisingly however, at 1.8 kPa the effective drag on the nanoparticles was lower than predicted by a factor of 17 and on the 1 μ m latex sphere by a factor of 6, respectively. The physical reason for this reduction of the drag is not known so far. The predicted particle impact velocity at 1.8 kPa was 54% of the maximum gas velocity. This factor is much lower than the often used factor of 0.85 derived by Marple. Copyright © American Association for Aerosol Research.