Dynamic shape factors of sphere aggregates in an electric field and their dependence on the Knudsen number

Hansson, Hans‐Christen; Ahlberg, Mats

doi:10.1016/0021-8502(85)90021-7

Cited by 20 publications

(16 citation statements)

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“…Specifically, Kasper and colleagues showed that during aerodynamic classification (with aerosol centrifuges and inertial impactors) such straight chain aggregates are oriented parallel to the flow field (Kasper, 1982), while with electrical mobility classification techniques, aggregates are parallel to the electric field (perpendicular to the flow) . It has also been found that doublets of submicrometer spheres orient parallel to sufficiently strong electric fields (Hansson & Ahlberg, 1985;Horvath, 1979), though the influence of electric field on particle orientation is less pronounced for smaller particles. The later measurements of Kousaka et al (1996) and Zelenyuk and Imre (2007) confirm that doublets orient themselves parallel to the field at high-field strengths (average intensity 45 Â 10 5 V/m for 100 nm primary particle doublets), that little-to-no preferential orientation occurs below a critical electric field strength, and that electrical alignment is considerably diminished for smaller particles.…”

Section: Resultsmentioning

confidence: 97%

The electrical mobilities and scalar friction factors of modest-to-high aspect ratio particles in the transition regime

Gopalakrishnan

McMurry

Hogan

2015

Journal of Aerosol Science

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Section: Resultsmentioning

confidence: 97%

The electrical mobilities and scalar friction factors of modest-to-high aspect ratio particles in the transition regime

Gopalakrishnan

McMurry

Hogan

2015

Journal of Aerosol Science

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“…Soot particles affect climate directly by absorbing and scattering sunlight and indirectly by serving as a cloud condenstaion nuclei or ice nuclei (IN) (Bond et al 2013). Soot deposited on snow and ice sheets can decrease the surface albedo, thereby accelerating melting (Hansson andAhlberg 1985, Ramanathan andCarmichael 2008). Furthermore, soot particles can increase absorption of solar radiation by decreasing cloud cover in the lower troposphere (Lohmann and Feichter 2005).…”

Section: Introductionmentioning

confidence: 99%

Morphology of diesel soot residuals from supercooled water droplets and ice crystals: implications for optical properties

China

Kulkarni

Scarnato

et al. 2015

Environ. Res. Lett.

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Freshly emitted soot particles are fractal-like aggregates, but atmospheric processes often transform their morphology. Morphology of soot particles plays an important role in determining their optical properties, life cycle and hence their effect on Earth's radiative balance. However, little is known about the morphology of soot particles that participated in cold cloud processes. Here we report results from laboratory experiments that simulate cold cloud processing of diesel soot particles by allowing them to form supercooled droplets and ice crystals at −20 and −40°C, respectively. Electron microscopy revealed that soot residuals from ice crystals were more compact (roundness ∼0.55) than those from supercooled droplets (roundness ∼0.45), while nascent soot particles were the least compact (roundness ∼0.41). Optical simulations using the discrete dipole approximation showed that the more compact structure enhances soot single scattering albedo by a factor up to 1.4, thereby reducing the top-of-the-atmosphere direct radiative forcing by ∼63%. These results underscore that climate models should consider the morphological evolution of soot particles due to cold cloud processing to improve the estimate of direct radiative forcing of soot.

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“…A study by Hansson and Ahlberg (1985) used a DMA to investigate the effect of shape on mobility diameter using agglomerates of Polystyrene latex (PSL) spheres. They concluded that particle alignment in the electric field of the DMA plays an important role for PSL doublets and triplets, but not for compact quadruplets and quintuplets.…”

Section: Introductionmentioning

confidence: 99%

On the Effect of Particle Alignment in the DMA

Zelenyuk

Imre²

2007

Aerosol Science and Technology

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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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Dynamic shape factors of sphere aggregates in an electric field and their dependence on the Knudsen number

Cited by 20 publications

References 15 publications

The electrical mobilities and scalar friction factors of modest-to-high aspect ratio particles in the transition regime

The electrical mobilities and scalar friction factors of modest-to-high aspect ratio particles in the transition regime

Morphology of diesel soot residuals from supercooled water droplets and ice crystals: implications for optical properties

On the Effect of Particle Alignment in the DMA

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