Effects of monomer shape on the formation of aggregates from a power law monomer distribution

J Perry,L S Matthews,J Kimery,T W Hyde

doi:10.1088/1367-2630/15/7/073026

Abstract

The coagulation of dust aggregates is an important process in many physical systems such as the Earth's upper atmosphere, comet tails and protoplanetary discs. Numerical models which study the aggregation in these systems typically involve spherical monomers. There is evidence, however, via the polarization of sunlight in the interstellar medium, as well as optical and LIDAR observations of high-altitude particles in Earth's atmosphere (70–100 km), which indicate that dust monomers may not necessarily be spherical. This study investigates the influence of different ellipsoidal monomer shapes on the morphology of aggregates given various distributions of monomer sizes. Populations of aggregates are grown from a single monomer using a combination of ballistic particle–cluster aggregation and ballistic cluster–cluster aggregation regimes incorporating the rotation of monomers and aggregates. The resulting structures of the aggregates are then compared via the compactness factor, geometric cross-section and friction time.

Highlights

The coagulation of dust aggregates is an important process in many physical systems such as the Earth’s upper atmosphere, comet tails and protoplanetary discs
Independent of the monomer sizes and distributions, aggregates comprised of spherical monomers, on average, have a radius which grows faster with mass than do aggregates comprised of either type of ellipsoidal monomers, as measured by the slope of the fit line to the data
The simplest reason for this is that ellipsoidal shapes can pack more efficiently than spheres, as is reflected in the larger compactness factors seen for ellipsoids

Summary

Introduction

The coagulation of dust aggregates is an important process in many physical systems such as the Earth’s upper atmosphere, comet tails and protoplanetary discs. Indications of non-spherical monomers may be found in the Earth’s upper atmosphere, at altitudes of [70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100] km, where tons of meteoric material is ablated by heat and friction during each rotational period (Gabrielli et al 2004) This ablated material, termed meteoric smoke particles, has been conjectured to play an important role in atmospheric processes, such as the nucleation of ice particles, and in polar summer mesospheric echoes (Keesee 1989). Aggregates were built using monodisperse monomers under both ballistic particle–cluster aggregation (BPCA) and ballistic cluster–cluster aggregation (BCCA) collision regimes and the resulting morphologies were examined Aggregates of these two types of monomers were seen to differ structurally as they grew to large size. This study expands upon this previous work in two ways: (i) additional aspect ratios are explored, including oblate ellipsoids in addition to prolate ellipsoids and (ii) aggregates are built from polydisperse monomer populations, reflecting size distributions found in astrophysical environments (Mathis et al 1977, Martin and Whittet 1990)

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Conclusion