filtration efficiency (99.995%) for nanoparticles at
a low pressure drop (100 Pa). The hybrid membrane has high porosity, small pore size, and a
large specific surface. Generally, microporous
fibrous filters, despite their higher filtration efficiencies, have a higher particle retention ratio and
show a higher tendency for clogging [47]. Also,
the smaller fiber diameter is associated with a
smaller pore size due to the decrease in pore
aperture with fiber diameter, improving the
interception-collision of nanoparticles with the
nanofiber and increasing capture efficiency [48].
The nanofiber packing density (a) depends on
fiber density and filter thickness. High a values
correlate with effective particle capture. Some
workers have investigated the effect of a on the
filtration efficiency of nanofilters. Leung et al.
[35] found a good relationship between the packing density and the filtration efficiency for PEO
fibers. By increasing a from 3.9 Â 10
À3 to
36 Â 10
À3 , filtration efficiency increased from
15% to 90%. At the same time, the most penetrating particle size (MPPS) decreased from 140 to
90 nm (Fig. 4c). Choi et al. [38] found that a
laminated nanofiber with high packing density
has a lower quality factor owing to its high pressure drop when compared to a mixed fiber
(nanofiber 780 nm/microfiber 11.4 mm). Furthermore, reducing the fraction of nanofiber in a
microfiber filter enhanced the quality factor at a
low packing density. Therefore they suggested
that an optimal ratio would be to have a nanofiber
mixing fraction of 5% in mass in order to decrease
pressure drop and increase the quality factor for
nanoparticle capture.
According to Eq. (10), a larger fiber thickness
can be useful for improving the performance of
nanofiber filters. By increasing the fiber thickness,
the basis weight and the pore size of fibers are also
increased, elevating filtration efficiency. However, Kim et al. [49] noted that if the fibers are
too thick, the filtration efficiency is reduced due to
the increased pressure drop. Wang et al. [50]
found that a thick filter contributed to higher
energy consumption and larger pressure drop.
Research by Zhang at al [51] showed that a multilayer PAN fibrous membrane can be more effective for obtaining a high quality factor compared
to a single thick layer nanofiber. Leung et al. [35]
concluded that the effect of fibers’ thickness on
the MPPS is less than the effect of fiber packing
density. The work of Yun et al. [52] demonstrated
that the pressure drop across an electrospun PAN
filter (270 nm) increased linearly from 63 to
220 Pa as the fiber thickness increased from
0.004 to 0.02 mm. However, they reported that
all fibrous filters for a given particle size had the
same single-fiber efficiency values, indicating
thickness-independent quality factors.
Hung et al. [53] found that larger fiber basis
weights enhanced filtration efficiency nonlinearly.
However, the increased basis weights decreased
the quality factor due to an increased pressure
drop (Fig. 4d). Also, this study found that the
multi-structure nanofiber membrane produced in
the electrospinning process with low basis weight
showed better performance than a single-structure
nanofiber membrane with a high basis weight.
Besides the filter structure, there are many filtration parameters capable of significantly changing filter performance. The air velocity across the
filter and particle size distributions play a key role
in this regard [35]. Many studies find that a higher
face velocity results in a lower filtration efficiency.
Wan et al. [37] found the filtration efficiency of a
polysulfone (PSU) membrane fibrous filter without TiO 2 decreased from ~100% to 94% as the
face velocity increased from 15 to 90 L/min, while
filtration did not decrease significantly for a filter
coated with TiO 2 nanoparticles at 5% by weight
(Fig. 4e). In the fibrous filters, the pressure drop is
proportional to the face velocity in accordance
with the experimental results of this study
(Fig. 4f). Thus, the results suggested low quality
factors at higher face velocity. Similarly, Wang
et al. [54] have tested a polyvinylidene fluoride
(PVF)/polytetrafluoroethylene (PTFE) nanoparticle filter for face velocities of 2–16 cm/s. They
found the filtration efficiency dropped from
~100% to 95% for pure PVF and a slight decrease
to 98% for PVF with 5 weight percent PTFE.
The relationship between particle size and filtration efficiency is well established. Many studies have shown that filtration efficiency is very
dependent of the particle size because of its effect
on the efficacy of the capture mechanisms. For
Airborne Nanoparticles: Control and Detection
97
a low pressure drop (100 Pa). The hybrid membrane has high porosity, small pore size, and a
large specific surface. Generally, microporous
fibrous filters, despite their higher filtration efficiencies, have a higher particle retention ratio and
show a higher tendency for clogging [47]. Also,
the smaller fiber diameter is associated with a
smaller pore size due to the decrease in pore
aperture with fiber diameter, improving the
interception-collision of nanoparticles with the
nanofiber and increasing capture efficiency [48].
The nanofiber packing density (a) depends on
fiber density and filter thickness. High a values
correlate with effective particle capture. Some
workers have investigated the effect of a on the
filtration efficiency of nanofilters. Leung et al.
[35] found a good relationship between the packing density and the filtration efficiency for PEO
fibers. By increasing a from 3.9 Â 10
À3 to
36 Â 10
À3 , filtration efficiency increased from
15% to 90%. At the same time, the most penetrating particle size (MPPS) decreased from 140 to
90 nm (Fig. 4c). Choi et al. [38] found that a
laminated nanofiber with high packing density
has a lower quality factor owing to its high pressure drop when compared to a mixed fiber
(nanofiber 780 nm/microfiber 11.4 mm). Furthermore, reducing the fraction of nanofiber in a
microfiber filter enhanced the quality factor at a
low packing density. Therefore they suggested
that an optimal ratio would be to have a nanofiber
mixing fraction of 5% in mass in order to decrease
pressure drop and increase the quality factor for
nanoparticle capture.
According to Eq. (10), a larger fiber thickness
can be useful for improving the performance of
nanofiber filters. By increasing the fiber thickness,
the basis weight and the pore size of fibers are also
increased, elevating filtration efficiency. However, Kim et al. [49] noted that if the fibers are
too thick, the filtration efficiency is reduced due to
the increased pressure drop. Wang et al. [50]
found that a thick filter contributed to higher
energy consumption and larger pressure drop.
Research by Zhang at al [51] showed that a multilayer PAN fibrous membrane can be more effective for obtaining a high quality factor compared
to a single thick layer nanofiber. Leung et al. [35]
concluded that the effect of fibers’ thickness on
the MPPS is less than the effect of fiber packing
density. The work of Yun et al. [52] demonstrated
that the pressure drop across an electrospun PAN
filter (270 nm) increased linearly from 63 to
220 Pa as the fiber thickness increased from
0.004 to 0.02 mm. However, they reported that
all fibrous filters for a given particle size had the
same single-fiber efficiency values, indicating
thickness-independent quality factors.
Hung et al. [53] found that larger fiber basis
weights enhanced filtration efficiency nonlinearly.
However, the increased basis weights decreased
the quality factor due to an increased pressure
drop (Fig. 4d). Also, this study found that the
multi-structure nanofiber membrane produced in
the electrospinning process with low basis weight
showed better performance than a single-structure
nanofiber membrane with a high basis weight.
Besides the filter structure, there are many filtration parameters capable of significantly changing filter performance. The air velocity across the
filter and particle size distributions play a key role
in this regard [35]. Many studies find that a higher
face velocity results in a lower filtration efficiency.
Wan et al. [37] found the filtration efficiency of a
polysulfone (PSU) membrane fibrous filter without TiO 2 decreased from ~100% to 94% as the
face velocity increased from 15 to 90 L/min, while
filtration did not decrease significantly for a filter
coated with TiO 2 nanoparticles at 5% by weight
(Fig. 4e). In the fibrous filters, the pressure drop is
proportional to the face velocity in accordance
with the experimental results of this study
(Fig. 4f). Thus, the results suggested low quality
factors at higher face velocity. Similarly, Wang
et al. [54] have tested a polyvinylidene fluoride
(PVF)/polytetrafluoroethylene (PTFE) nanoparticle filter for face velocities of 2–16 cm/s. They
found the filtration efficiency dropped from
~100% to 95% for pure PVF and a slight decrease
to 98% for PVF with 5 weight percent PTFE.
The relationship between particle size and filtration efficiency is well established. Many studies have shown that filtration efficiency is very
dependent of the particle size because of its effect
on the efficacy of the capture mechanisms. For
Airborne Nanoparticles: Control and Detection
97