because of favorable properties including small
pore size and high specific surface area. These
properties enable the slip effect of airflow through
the fibers, reducing the drag force on the airstream
as it passes nanofibers and subsequently strengthening the different filtration mechanisms on the
filter [26]. Normally the surface of a fiber blocks
the airflow, diverting airstreams, triggering turbulence and viscosity, and necessitating power and
pressure drop for the air to flow past. However,
when the fiber diameter similar to or below the
size of the mean free path of air molecules (just
under 70 nm), normal continuum flow behavior
no longer applies. Molecules can fly past the fiber
without interrupting the momentum of the flow;
this is called “slip flow.” As detailed below,
because of slip flow, nanofiber filters hold the
promise of yielding high QF filtration for
nanoparticles.
Air filtration using fibrous filters is a promising
method for separating nanoparticles from an airstream while allowing air molecules to pass
[29]. Filters manufactured with materials including plastic, cellulose, glass, and carbon are the
most common [24]. The fibers are made into a
soft flexible mat, with fiber orientations primarily
perpendicular to the streamline [30]. These mats
provide a high filtration efficiency for ultrafine
particles along with minimal aerodynamic resistance and thus find many applications such as
industrial and indoor cleaning purification, respirators, and car cabins [23, 30].
A few mechanisms are important for trapping
nanoparticles from their air: interception, inertial
impact, diffusion, and electrostatic deposition.
Their relative importance will vary depending on
gas velocity, particle and fiber size, particle charging, surface static charge, and so on. The total
filtration efficiency for nanoparticles is the sum
of the filtration efficiencies of particles by each
mechanism (Table 2). Sound classical descriptions of fibrous filter performance can be found
in many sources (see, e.g., [23, 24, 30]).
Particles larger than around 300 nm are subject
to inertial separation from a curved flow streamline, leading to impact with an obstruction. Such
particles are unable to adjust to an abruptly changing streamline in the vicinity of a fiber.
The diffusion constant increases as the size of
particles decreases. For particles smaller than
ca. 100 nm, diffusional separation of particles from
the continuum flow streamline can be the dominant
mechanism leading to impaction. Every filter has a
minimum in efficiency for a characteristic particle
size called the most penetrating particle size
(MPPS), which typically falls between 100 and
300 nm. The removal efficiency of filters at the
MPPS determines the filter grade. In addition there
are two criteria, surface chemistry and the electrostatic potential of fibers, which play significant roles
in determining filtration efficiency. For example,
polar functional groups at the filter surface may
lead to strong adsorption of nanoparticles.
The relative roles of the mechanisms depend
on the Knudsen number (K n ) as [26]:
K n ¼
2l
d f
ð9Þ
where d f is the diameter of fiber and l is the mean
free path of gas molecules. The mean free path is
ca. 66 nm at the standard state of air, obtained as:
l ¼
RT
ffiffi ffi
2
p
N a p d
2
m p
ð10Þ
where R is the gas constant, T temperature, N a
Avogadro’s number, d m the collision diameter of
an air molecule (3.7 Â 10
À10 m), and p the air
pressure [31].
There is a relationship between the airflow
regime, as defined by the Reynolds number,
across the filter and the fiber diameter in all the
airborne particle filtration systems. Particles can
be removed from an airstream in the continuum
flow regime (K n < 0.001, generally for milliscale
fibers),
the
slip
flow
regime
(0.001 < K n < 0.25, generally for microscale
fibers),
the
transition
flow
regime
(0.25 < K n < 10, for nanoscale fibers), and the
free molecular regime (K n > 10, for ultrafine
scale fibers), as shown in Fig. 2a, b [32]. In the
continuum flow regime, the mean free path of the
gas molecules is much smaller than the diameter
of the fiber. Most filtration systems use fibers
operating in the continuum flow regime. The
Airborne Nanoparticles: Control and Detection
91
pore size and high specific surface area. These
properties enable the slip effect of airflow through
the fibers, reducing the drag force on the airstream
as it passes nanofibers and subsequently strengthening the different filtration mechanisms on the
filter [26]. Normally the surface of a fiber blocks
the airflow, diverting airstreams, triggering turbulence and viscosity, and necessitating power and
pressure drop for the air to flow past. However,
when the fiber diameter similar to or below the
size of the mean free path of air molecules (just
under 70 nm), normal continuum flow behavior
no longer applies. Molecules can fly past the fiber
without interrupting the momentum of the flow;
this is called “slip flow.” As detailed below,
because of slip flow, nanofiber filters hold the
promise of yielding high QF filtration for
nanoparticles.
Air filtration using fibrous filters is a promising
method for separating nanoparticles from an airstream while allowing air molecules to pass
[29]. Filters manufactured with materials including plastic, cellulose, glass, and carbon are the
most common [24]. The fibers are made into a
soft flexible mat, with fiber orientations primarily
perpendicular to the streamline [30]. These mats
provide a high filtration efficiency for ultrafine
particles along with minimal aerodynamic resistance and thus find many applications such as
industrial and indoor cleaning purification, respirators, and car cabins [23, 30].
A few mechanisms are important for trapping
nanoparticles from their air: interception, inertial
impact, diffusion, and electrostatic deposition.
Their relative importance will vary depending on
gas velocity, particle and fiber size, particle charging, surface static charge, and so on. The total
filtration efficiency for nanoparticles is the sum
of the filtration efficiencies of particles by each
mechanism (Table 2). Sound classical descriptions of fibrous filter performance can be found
in many sources (see, e.g., [23, 24, 30]).
Particles larger than around 300 nm are subject
to inertial separation from a curved flow streamline, leading to impact with an obstruction. Such
particles are unable to adjust to an abruptly changing streamline in the vicinity of a fiber.
The diffusion constant increases as the size of
particles decreases. For particles smaller than
ca. 100 nm, diffusional separation of particles from
the continuum flow streamline can be the dominant
mechanism leading to impaction. Every filter has a
minimum in efficiency for a characteristic particle
size called the most penetrating particle size
(MPPS), which typically falls between 100 and
300 nm. The removal efficiency of filters at the
MPPS determines the filter grade. In addition there
are two criteria, surface chemistry and the electrostatic potential of fibers, which play significant roles
in determining filtration efficiency. For example,
polar functional groups at the filter surface may
lead to strong adsorption of nanoparticles.
The relative roles of the mechanisms depend
on the Knudsen number (K n ) as [26]:
K n ¼
2l
d f
ð9Þ
where d f is the diameter of fiber and l is the mean
free path of gas molecules. The mean free path is
ca. 66 nm at the standard state of air, obtained as:
l ¼
RT
ffiffi ffi
2
p
N a p d
2
m p
ð10Þ
where R is the gas constant, T temperature, N a
Avogadro’s number, d m the collision diameter of
an air molecule (3.7 Â 10
À10 m), and p the air
pressure [31].
There is a relationship between the airflow
regime, as defined by the Reynolds number,
across the filter and the fiber diameter in all the
airborne particle filtration systems. Particles can
be removed from an airstream in the continuum
flow regime (K n < 0.001, generally for milliscale
fibers),
the
slip
flow
regime
(0.001 < K n < 0.25, generally for microscale
fibers),
the
transition
flow
regime
(0.25 < K n < 10, for nanoscale fibers), and the
free molecular regime (K n > 10, for ultrafine
scale fibers), as shown in Fig. 2a, b [32]. In the
continuum flow regime, the mean free path of the
gas molecules is much smaller than the diameter
of the fiber. Most filtration systems use fibers
operating in the continuum flow regime. The
Airborne Nanoparticles: Control and Detection
91