On the Effect of Particle Alignment in the DMA

Abstract

The Differential Mobility Analyzer (DMA) is designed to measure particle mobility diameter, which for spherical particles is equal to particle volume equivalent diameter. In contrast, the mobility diameter of aspherical particles is a function of the particle shape and orientation. The magnitude of the DMA electric fields is such that it can cause aspherical particles to align preferentially in a specific orientation. The same electric field and the sheath flow rate (q sh ) define the particle mobility diameter. But, the fact that particle orientation depends on the electric field makes the dynamic shape factor and hence the mobility diameter depend on q sh . Here, we describe an operating procedure that relies on a tandem DMA system, in which the second DMA is operated at a number of q sh , to obtain information about particle shape by measuring the effect of particle alignment on the particle mobility diameter. We show how the relationship between the mobility diameter and q sh can even be used to physically separate particles according to their shapes. In addition we explore the use of simultaneous measurements of particle alignment and particle vacuum aerodynamic diameters to gain further information on particle shape and account for particle alignment in the calculations of dynamic shape factor. We first test this approach on doublets and compact triplets of PSL spheres, for which the orientation dependent dynamic shape factors are known. We then investigate applications on a number of polydisperse particle systems of various shapes. INTRODUCTIONFor spherical particles the mobility diameter is well defined and is equal to particle volume equivalent diameter. However, Address correspondence to Alla Zelenyuk, Pacific Northwest National Laboratory, PO Box 999, MSIN K8-88, Richland, WA 99354, USA. E-mail: alla.zelenyuk@pnl.gov because particle mobility diameter is a measure of particle behavior and not of particle intrinsic property, for aspherical particles whose behavior in the DMA depends on the particle shape and the electric field, the relationship between the mobility diameter and volume equivalent diameter can be complex. Very little attention has been paid to the effects of asphericity on the particle mobility diameter and to how to use the relationship between the two to obtain information about particle shape.With the advent of aerosol instrumentation it is becoming more common to use Differential Mobility Analyzers (DMAs) to classify or select particles with a narrow distribution of mobility diameters for interrogation by a second instrument to determine, for example, particle density (Katrib et al. 2004;McMurry et al. 2002;Slowik et al. 2004;Zelenyuk et al. 2006), or hygroscopicity and composition (Buzorius et al. 2002). Combining a number of instruments in a series with a DMA upfront makes the need to take into account particle asphericity all the more important because the rigorous interpretation of these detailed measurements, must take particle shape into account. For particles of unknown shapes...

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