AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Drag Measurements of Cylindrical Aerosol Particles in the Transition Regime
RANGANATHAN GOPALAKRISHNAN, Peter McMurry, Christopher Hogan Jr., University of Minnesota
Abstract Number: 637 Working Group: Aerosol Physics
Abstract Measurements of electrical mobility, and hence drag force, are commonly used to characterize submicrometer and nanometer sized aerosol particles. For spherical particles, Stokes law modified with the Knudsen number dependent slip correction factor can be used with confidence to link the particle diameter/radius to the observed drag force/electrical mobility. For non-spheres, however, questions remain regarding the correction approach to relate appropriate length scales describing the particle to the resulting drag force the particle experiences in a fluid flow. To address this issue we make measurements of the electrical mobility of well characterized high aspect ratio (cylindrical) aerosol particles in the momentum transfer transition regime, i.e. where the mean free path of the surrounding gas is on the order of the length scale of the particles. Two types of cylindrical aerosol particles are used for measurements. First, single walled carbon nanotubes are synthesized in an inverse diffusion flame, with diameters in the sub 10 nm range and lengths in the submicrometer to micrometer size range. Second, gold nanorods, synthesized in the liquid phase with both diameters and lengths in the sub 100 nm range, are aerosolized using an electrospray aerosol generator. A differential mobility analyzer (DMA) is used to isolate particles of a prescribed electrical mobility, and subsequently TEM analysis of mobility classified particles is used to precisely determine the particle length and diameter. From TEM measurements, theoretical prediction of the Knudsen number for such particles and subsequently their electrical mobilities are made using equations derived from dimensional analysis and Direct Simulation Monte Carlo (DSMC). The predicted electrical mobility for each analyzed particle is compared directly to its known electrical mobility under the measurement conditions, with the DMA transfer function and other experimental uncertainties accounted for.