Drag on Gasborne Nanoparticles in the High Knudsen Number Regime

Abstract

At small Knudsen numbers (continuum regime) and sufficiently small Reynolds numbers, the friction resistance of particles with the carrier gas is described by means of the Stokes formula assuming no slip of the gas molecules at the particle surface. However, at high Knudsen numbers, this assumption does not hold anymore and the deviation from the continuum character is considered with the slip or Cunningham correction. In the present work, measurements of the separation curve of nanoparticles in a single-stage low-pressure impactor are presented to systematically investigate the disagreement of classical theory of friction resistance of nanoparticles in low-pressure flows. At the same time, the impaction process of nanoparticles is simulated with a CFD code based on the classic theory. An effective Cunningham correction is determined as a function of the Knudsen number from fitting the calculated separation curve to the measured one. Moreover, the so far unresolved finding of too low densities determined for metal nanoparticles by low-pressure impaction is explained for the first time.

Keywords

• nanoparticle
• cunningham correction
• high knudsen number
• drag