(264e) Fragmentation and Bounce of Nanosized Agglomerates Due to the Impaction

The impaction behaviour of aerosol
particles has applications in fields such as powder processing, nuclear safety,
and health research. Two major phenomena may occur during the impaction of
agglomerated particle onto a surface. The first one is the plastic deformation
of the agglomerate which may lead to fragmentation of the agglomerate if there
is enough impaction energy available. The second process is the bounce of the
particles whenever the adherence between the particle and surface can be
overcome by the energies stored to elastic deformations.

To achieve better understanding on how
these processes work and affect each other, the fragmentation and the bounce
should be observed simultaneously. In this study, these processes were
investigated for two agglomerate materials with varying agglomerate properties.

A method to study the simultaneous fragmentation and bounce
of the particles was developed and it is described in detail by Ihalainen et
al. (2012). The basic principle of the method was as follows. The aerosols in
study were first size classified by their electrical mobility with a
Differential Mobility Analyzer (DMA) to achieve relatively monodispersed
aerosol population. This population was then introduced to a single stage Micro
Orifice Uniform Deposit Impactor (MOUDI) in which the particles impact onto the
impaction plate due to the inertia of the particles. The impaction velocity
depended on the aerodynamic diameter of the particle and the air flow
conditions at the impactor. To be able to estimate this impaction velocity,
computational fluid dynamics simulations (CFD) coupled with Lagrangian method
to analyze the particle trajectories were made. The sample flow from the MOUDI
was collected to a low pressure sampling chamber which enables the analysis of
the bounced with both online and offline methods. The deposited particles were
collected both from the impaction plate and bounced fraction and analyzed using
a Transmission Electron Microscopy (TEM). In addition to the TEM, the bounced
particles were characterized with a scanning mobility particle sizer (SMPS).
The mass fraction of the particles which bounce was estimated from the SMPS
data using size dependent effective density. The effective density of the
particles was evaluated using a combination of aerosol particle mass analyzer
(APM) and SMPS.

The agglomerated particles of TiO₂ and Fe were
generated using atmospheric pressure chemical vapor synthesis (APCVS; Lähde et
al, 2011). Titanium tetra isopropoxide was used to generate TiO₂
agglomerates and iron penta carbonyl to produce iron agglomerates. The
properties of the agglomerates were manipulated during the generation to find
out how these changes affected the fragmentation and bounce. The properties
varied for TiO₂ particles were the primary particle size and the
degree of sintering, i.e. the bridging between the particles. The average primary
particle sizes used here were 27 and 16 nm. The oxidation state of the iron
particles was varied by changing the amount of oxygen available during the
particle formation phase. The oxygen concentrations used during the generation
were 21 and 0.05 %.

The size distributions of the bounced particles for each
particle type after impaction at about 150 m/s are presented in Figure 1. It
shows that the original monodispersed size distribution has transformed to a wider
distribution of smaller particles due to the fragmentation. Fig. 1 also shows
that not all of the particles did break-up and some fractions of the particle
populations remained intact during the impaction. This was pronounced at lowest
impaction velocities for the TiO₂ particles with higher degree of
sintering and for the iron oxide particles. It can also be observed from Fig. 1
that the changing the agglomerate properties affected the fragment size.
Increasing of TiO₂ agglomerates sintering increased the size of the
fragments meaning that fewer bonds per agglomerate broke up. This was due to
the sintering which made the bonds between the primary particles stronger. The
reduction of the primary particle size also resulted in larger fragments however
the change was not as significant as in the case of sintering. The fragments of
the iron oxide particles were larger than in the case of TiO₂ at the
same velocities and the oxidation level of the iron oxide particles was not
found to have a significant effect on the size of the fragments.

The mass fractions of the fragmented and bounced particles
increased as the impaction velocity increased in almost all of the cases. With
TiO₂, the smaller primary particle size reduced the bounce fraction
compared to the case with larger primary particle size. There was no
significant difference in bounced mass fractions between the iron oxide cases.

Figure 1. The number size distributions of the intact and
the bounced particles at similar impaction conditions.

This
work was carried out in support of the international ARTIST II program which is
acknowledged for funding this work. The authors express their gratitude to
those members of the Laboratory for Thermal-Hydraulics at PSI and of the Fine
Particle and Aerosol Technology Laboratory at the University of Eastern Finland that participated in the experimental work and construction of the facility.

Ihalainen, M., Lind, T., Torvela, T., Lehtinen, K.E.J.
and Jokiniemi, J. (2012) Aerosol Sci. Technol. 46:9, 990-1001.

Lähde, A., Kokkonen, N., Karttunen, A., Jääskeläinen,
S., Tapper, U., Pakkanen, T.A., Jokiniemi, J (2011) J. Nanopart. Res. 13,
3591-3598.