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RESEARCH PAPER
Charging efficiency of nanoparticles in needle-to-plate
chargers with micro discharge gaps
Wenming Yang &Rong Zhu &Liangqi Wang &
Beiying Liu
Received: 8 December 2018 /Accepted: 28 May 2019
#Springer Nature B.V. 2019
Abstract Simple chargers, based on needle-to-plate
configuration with micro gap distance of several hun-
dred micrometers, for the direct charging of aerosol
particles, have been designed and constructed. Charging
efficiency and particle deposition loss (electrostatic and
diffusional) of this type of chargers have been evaluated
systematically for particles (polystyrene) with diameters
ranging from 28.9 to 287 nm, at different applied volt-
ages, different aerosol flow rates, and different gap
distances. The charging efficiency increases with parti-
cle size, applied voltage, and mean residence time of
aerosol particles in the charger, while decreases with gap
distance. As a consequence of the small effective vol-
ume of the chargers, a negligible diffusional loss of
particles was obtained. The electrostatic loss of charged
particles within the chargers was relatively large for
smaller particles at higher applied voltage, but a charg-
ing efficiency of 82% was achieved for 28.9-nm parti-
cles using the charger with a gap distance of 150 μm.
The chargers can charge nanoparticles effectively and
are easy to be integrated into minimized devices such as
sensor chips.
Keywords Charging efficiency.Direct charging .Micro
channel .Aerosol nanoparticle .Instrumentation
Introduction
Particle charging is an important process in the field of
aerosol science and technology. For instance, electrostatic
precipitation requires particles to be charged as efficient as
possible to eliminate airborne particles (Lawless and
Sparks 1988). In the electrical detection of particles, parti-
cle charging even plays a key role in the inversion of
aerosol properties from sensed signals (Li et al. 2009;
Yang et al. 2017a). There are several mechanisms by
which aerosol particles acquire net charges, among which
the unipolar diffusion charging can achieve relatively
higher charging efficiencies (Kruis and Fissan 2001;
Kwon et al. 2007; Alonso and Huang 2015).
Self-sustaining gas discharge with a needle-to-plate
configuration is one of the most common ways to gen-
erate unipolar ions (Mizeraczyk et al. 2016; Intra and
Tippayawong 2011; Zheng et al. 2016;Liaoetal.2018).
This type of technique was originally developed as an
ion generator by Whitby (1961) which had an efficiency
of up to 100% for converting the discharge current into
free ions. Thereafter, a conical chamber served as a
charging space was assembled to the original design to
form an indirect aerosol charger (Whitby and Clark
1966). In this design, ions generated by the gas dis-
charge around a needle were drawn into the chamber
where aerosol particles become charged mostly by dif-
fusion. From then on, measures were taken to reduce
JNanopartRes (2019) 21:125
https://doi.org/10.1007/s11051-019-4572-8
W. Yan g (*):B. Liu
School of Mechanical Engineering, University of Science and
Technology Beijing, Beijing 100083, China
e-mail: wenming_y@126.com
R. Zhu (*):L. Wang
State Key Laboratory of Precision Measurement Technology and
Instruments, Department of Precision Instruments, Tsinghua
University, Beijing 100084, China
e-mail: zr_gloria@mail.tsinghua.edu.cn
particle losses inside or (and) improve the charging
efficiencies of this type of chargers. Jaworek and
Krupa (1989) used an AC electric field rendering an
oscillatory movement of charged particles when they
passed through the charger, which prevents the particles
from precipitating on one of the electrodes. Medved
et al. (2000) added a sheath flow in the direction of the
needle to sweep the ions into the mixing chamber.
Turbulent jets were formed in their chargers and a
charging efficiency of 27% for NaCl particles with a
diameter of 10 nm was obtained as a result of the
relatively longer mixing time. A needle-to-ring structure
(Choi and Kim 2007) and its doubled version with a
twin needle configuration (Marquard et al. 2006a,b)
were also designed, where the direction of aerosol flow
was perpendicular to the ion injection flow. However,
perpendicular or opposing direction between ion and
aerosol flow may lead to severe electrostatic losses of
charged particles (Chen and Pui 1999). A charging
efficiency of 35% for particles with 10 nm in diameter
was achieved when ions were injected at an angle of 45°
with aerosol flows (Qi et al. 2007). An additional ion
trap zone was added on the original combination of
discharge zone and mixing zone for this type of chargers
so as to be used in different applications (Park et al.
2009;Parketal.2010).
In other indirect chargers with needle configuration, the
sonic orifices in Whitby type chargers were replaced by
metal meshes and an additional relatively weak electric
field was applied to drive ions through the meshes and into
charging zones (Qi et al. 2008;LiandChen2011;Cao
et al. 2017). This type of chargers can be made relatively
smaller in size and the charging efficiency attains 35% for
10-nm particles and 74% for 50-nm ones.
As for the direct charger using needle configuration,
particle charging takes place in the same area with that
where discharge occurs. Sierra et al. (2003)proposeda
prototype in which aerosol flow was confined to the
place near the needle and particles were charged near the
tip area. This structure was later modified in order to
improve its performance (Alguacil and Alonso 2006). In
the improved version, aerosol inlet was arranged on the
lateral wall of the cylindrical charger and particles were
expelled from the charger imminently after they become
charged. As a consequence of this improvement, the
charging efficiency attained 40% for 10-nm particles.
Intra and Tippayawong (2010), Intra et al. (2017)devel-
oped similar chargers as well. Configurations such as
point-to-point and multipoint-to-plane were also applied
mainly in electrostatic precipitators to charge submicron
particles directly (Chang et al. 2015;Leetal.2013).
Direct charging in channels with smaller gap dis-
tances of needle-to-plate configuration was developed
in recent years. Chua et al. (2009)microfabricatedan
ionizer with a discharging gap distance of 1.1 mm and
combined it with a separator. Park et al. (2010) and Lee
et al. (2011) fabricated a chip-type charger whose sharp
silicon tip was made using MEMS process. Together
with a micro virtual impactor, this charger could be
assembled into a particle detection chip (Kim et al.
2008). ZnO nanowires were used as the needle electrode
for gas discharge by several researchers (Yang et al.
2013,2016a;Parketal.2015), where the sustaining
voltage was lowered to hundreds of volts. Yang et al.
(Yang et al. 2017b) fabricated a self-sustaining dis-
charge structure consisting of a tungsten needle and a
plate. The height of the discharge gap fell in between
100 and 400 μm. This structure was also used as the
charging component in an integrated aerosol sensor
(Yang and Zhu 2014;Yangetal.2017a; Zhang et al.
2016,2017).
As mentioned above, chargers with micro channels
for charging were able to be integrated into small
aerosol separation and measuring devices easily.
However, the charging performance of these chargers
has not been evaluated systematically until now. The
purpose of the present work is to measure the extrinsic
charging efficiency and particle deposition loss of the
needle-to-plate chargers with hundreds of micron
electrode gaps for nanometer particles under different
conditions. The charging efficiency is one of the main
performance criteria of electrical aerosol chargers
(Marquard et al. 2006a,b).
Methods
Charger
Figure 1schematically shows the charger and its
electrical connections. The whole system consists
of the charger, a high-voltage (HV) module (Lslai,
LSL957), an oscilloscope (Tektronix, TDS201), and
two resistors. Positive polarity discharges in needle-
to-plate configurations were ignited in the charger.
A tungsten needle with a tip diameter of about
40 μm was used as the anode (as that shown in
Fig. 2), and the plate was a blank printed circuit
125 Page 2 of 10 J Nanopart Res (2019) 21:125
board (PCB), which was covered with a layer of
36-μm-thick copper whose diameter was 5 mm.
The tungsten needle was inserted into a SMA con-
nector and the insulator in the connector was
moved to ensure the alignment between the tip of
the needle and the outer edge of the connector. A
Teflon spacer with a specific thickness was placed
between the connector and the copper electrode to
form the discharge gap. The copper electrodes were
cleaned using alcohol, acetone, and then deionized
water ahead of their integration into the charger to
remove oil and grease. The purpose of this process
is to prevent the formation of particles from con-
taminants. The overall dimensions of the charger
are about 16 × 12 × 14 mm (length × width ×
height). The length and width of the channel in
thechargerare11mmand5mm,respectively.
Chargers with four gap distances gof 150 μm,
200 μm, 300 μm, and 400 μm were fabricated, respec-
tively. The HV module can supply a DC high-voltage up
to 3 kV (25 W), which was automated via a LabVIEW
program to vary the potential in a controlled and repeat-
able manner. The oscilloscope (internal resistance
10 MΩ) is in parallel with the resistor R
2
(1 kΩ)to
monitor the occurrence of the self-sustaining discharge.
R
1
(98.4 kΩ) connected in series with the HV module is
a current limiting resistor to protect the module.
Set-up for charging efficiency measurements
Experiments were carried out using the set-up sketched
in Fig. 3. Polystyrene (PS) particles (Thermo Scientific,
Waltham, MA, USA) as the testing samples were gen-
erated by nebulizing PS suspension (1.9 μg/mL) in an
aerosol generator (Topas, ATM-220). Compressed air
wasusedasthecarriergas.Theliquidaroundthe
original droplets ejected from the generator was re-
moved by drying with a diffusion dryer (Topas, DDU-
570) to produce solid-particle aerosol. The desired par-
ticle concentration at the downstream of the dryer was
realized by adding a dilution flow through a tee, and the
dilution flow rate was controlled by a mass flow con-
troller (MFC, Sevenstar, CS230). The aerosol flow was
then passed through the first ion trap to remove charged
particles and ensure that the particles to be measured are
all electrically neutral. A voltage of 100 V was applied
on the ion trap. Then, a portion of the particles were
sampled in order to measure the particle concentration
entering the charger N
in
by a condensation particle
counter (CPC, PALAS, CPC-200) after they pass
through valve 2. Another portion entered into the test
charger. The particles leaving the charger were finally
passed through the second ion trap before being counted
with the CPC so as to remove all the charged particles.
The volumetric flow rate entering into the charger was
controlled and measured by the regulation of valve 1
and the rotameter (with light plastic floater, 0–500 mL/
min). After this flow rate was set to the desired value, the
rotameter would be removed from the flow circuit to
eliminate its effect on the aerosol flow. The rate of the
flow passing through the charger was relatively low
(lower than 325 mL/min in this study) and it was not
sufficient for the normal working of CPC (at least1.5 L/
min), so a bypass clear air flow (supplement gas in
Fig. 3,1.1L/min)withaspecificvolumetricratewas
High-voltage module
R
1
R
2
Oscilloscope
Teflon spacer
SMA connector
Tungsten needle
Copper electrode
on PCB
+
_
PC
g
Inlet Outlet
(gap distance)
Fig. 1 Schematic of the charger (not in scale)
Fig. 2 Photo of a tungsten needle used in the charger
JNanopartRes (2019) 21:125 Page 3 of 10 125
provided and mixed with the aerosol flow in a Y-junc-
tion. In order to minimize the errors coming from the
diffusion losses to the wall of the system, the tubes
between the inlet of the chargers and the upstream tee,
and that between this tee and the CPC were made as
short as possible.
The outlet concentration with the charger on, N
out, ON
,
and the outlet concentration of uncharged particles with
charger on, NUN
out;ON , were measured. N
out, ON
was mea-
sured when the voltage of the charger was on, but that of
the ion trap was off. NUN
out;ON was measured when both
voltages were on. The inlet concentration N
IN
was mea-
sured through the route Btee + valve 2^,wheretheparticle
loss in the valve was ignored. For given particle size,
volumetric flow rate, gap distance of the charger, and
discharge voltage, the extrinsic charging efficiency was
calculated as
η¼Nout;ON−NUN
out;ON
NIN
ð1Þ
The electrostatic loss of charged particles inside the
charger was calculated as
ξ¼NIN−Nout;ON
NIN
ð2Þ
Several sets of experiments were carried out at varying
particle diameters, discharge voltages, and aerosol flow
rates through the test charger. The volumetric flow rates
and the corresponding flow speed and average residence
time for chargers with different discharge gap distances are
given in Table 1. PS nanospheres with nominal diameters
of 30, 50, 100, 200, and 300 nm were used in the exper-
iments. The direct product of the atomizer is an aerosol of
droplets which may contain no sphere (surfactant or dis-
persant only) or more than one sphere. When the droplets
dry, the resulting particles may be agglomerates of surfac-
tant or several PS spheres. So before conducting the fol-
lowing experiments, the size and concentration of the PS
aerosol particles at the inlet of the charger were measured
using an UCPC (TSI 3776) when there was no dilution gas
flows. Their results are shown in Table 2. The geometric
mean diameters deviated not much from the nominal ones
which indicate that most of the aerosol particles consisted
only one PS sphere. Therefore, the effect of multi-sphere or
no-sphere particles was neglected during the characteriza-
tion of the chargers.
The ion trap downstream of the charger was removed
during the measurement of electrostatic losses and mean-
while, additional pure gas flow with a rate that was the
same as the supplement gas flow was provided at the
upstream of the charger inlet. For each set of operating
conditions, measurements were repeated at least three
times. The reproducibility of the results was fairly good
in all the cases, with relative errors all below about 15%.
Results and discussion
Current-voltage relationships of the chargers
For different gap distances of chargers, the discharge
currents for various applied voltages are shown in Fig.
Fig. 3 Experimental set-up for the measurement of charging efficiency
125 Page 4 of 10 J Nanopart Res (2019) 21:125
4. Voltages lower than 1.8 kV were used in our exper-
iments to prevent the occurrence of spark-over phenom-
enon. During these experiments, the output voltage sup-
plied by the module was decreased continuously after
the self-sustaining discharge was ignited in order to
lower the discharge current. The discharge could be
sustained until some critical sustaining voltages were
attained. Due to the limited power of the module, the
currents of the four chargers attained nearly a same
value when the applied voltages were increased up to
1.8 kV.
Undesired particles may form from erosion or
sputtering of the electrodes when gas discharge occurs
(Romay et al. 1994). We have examined this possibility
by driving a clean and dry gas flow through the charger
when the discharge was turned on. Before this, the
particle concentration was first measured with the char-
ger turned off to determine the background counts of the
clean air supply. For a new charger, there was initially
detected particles with concentrations of less than 10
3
/
cm
3
at typical voltages (0.8–1.8 kV), but they vanished
after several times of usage. These particles may come
from the sputtering of non-mental substances on the
electrode surfaces. Furthermore, for still larger voltages
or at some accidental moments during the sustaining of
discharge, the spark-over phenomenon occurred, the
discharge current fluctuated in an uncontrollable man-
ner, and a very high ion number concentration detected
(higher than 10
6
/cm
3
). Under this circumstance, parti-
cles are generated by spark discharge (Tabrizi et al.
2009). Therefore, the discharge was monitored in a
real-time way to avoid the influence of this effect in
the following experiments.
Effect of applied voltage
The applied sustaining voltage of the discharge is an
important factor that influences the charging efficiency
and electrostatic losses because both the ion number
concentration and electric field depend on it. For differ-
ent particle diameters, the extrinsic charging efficiency
at various applied voltages is given in Fig. 5. Higher
efficiency was obtained for larger particles since they
were much easier to adhere ions due to diffusion charg-
ing. In most cases, the charging efficiency increases
with the applied voltage. This should be induced by
the higher number concentration of ions for larger ap-
plied voltage. However, if we use equation (Alonso
et al. 2006;Yangetal.2016b)
Nion ¼I
eZEA ð3Þ
Tabl e 1 Volumetric flow rates (mL/min) for chargers with different discharge gap distances g
Flow speed (m/s) Average residence
time (ms)
Flow rates
(g=400 μm)
Flow rates
(g=300 μm)
Flow rates
(g=200 μm)
Flow rates
(g=150 μm)
2.70 1.85 325 244 163 122
1.88 2.67 225 169 113 84
1.04 4.81 125 94 83 47
Tabl e 2 Parameters for PS nanoparticles used in the experiments
Nominal diameter
(nm)
Geometric mean
diameter (nm)
Aerosol concentration
at the inlet of the
charger (× 10
5
/cm
3
)
30 28.9 1.7
50 45.4 1.8
100 104.1 1.6
200 209.3 1.2
300 287.0 1.3
0.6 0.8 1.0 1.2 1.4 1.6 1.8
1
2
3
4
5
6
7
Discharge current (mA)
Voltage (kV)
150 m
200 m
300 m
400 m
Fig. 4 Current-voltage characteristics of the self-sustaining dis-
charge in chargers with different gap distances
JNanopartRes (2019) 21:125 Page 5 of 10 125
to estimate the average number concentration of ions in
the discharge gap based on the measured discharge
current, where Iis the discharge current, ethe elemen-
tary charge, Zthe ion mobility, Ethe average electric
filed, and Athe surface area of the grounded electrode,
non-monotone changes of ion number concentration
with the applied voltages were obtained, as that shown
in Fig. 6. It may be the highly non-uniform distribution
of ions underneath the needle that causes this error
estimation. So this equation is not suitable for calculat-
ing the average ion number concentration in the dis-
charge gap for this geometry or for this type of self-
sustaining discharge.
A previously published model (Yang et al. 2018)was
used to calculate the ion concentration in the discharge
gap using Comsol Software. The resultant positive ion
concentration distribution at the mid-line location of the
discharge gap is given in Fig. 7. The ion concentration
decreases several orders of magnitude with the distance
away from the needle tip. Due to its non-uniform distri-
bution, there is a certain portion of the gap space that does
not maintain sufficient ions (ion concentration is less than
10
10
/m
3
) to charge aerosol particles. Furthermore, the
higher the applied voltage, the more ions at the same
location, which can be used to explain the changes of
charging efficiency with applied voltages. We have tried
to improve the ion distribution (ions are more diffusional
in the radial direction) in the discharge gap by replacing
the solid fill of plate electrodes by an annulus scheme, but
it induced sparks and released new particles very easily.
0.8 1.0 1.2 1.4 1.6 1.8
0.3
0.4
0.5
0.6
0.7
0.8
Charging efficiency
Voltage (kV)
287 nm
209.3 nm
104.1 nm
45.4 nm
28.9 nm
Fig. 5 Effect of applied voltage on charging efficiency for differ-
ent particle diameters (gap distance 200 μm, velocity of flow
2.70 m/s)
0.6 0.8 1.0 1.2 1.4 1.6 1.8
1x10
21
2x10
21
3x10
21
4x10
21
5x10
21
6x10
21
Estimated ion concentration(/m
3
)
Voltage (kV)
400 m
300 m
200 m
150 m
Fig. 6 The estimated ion number concentration in the discharge
gap using Eq. (3)
0.0 0.5 1.0 1.5 2.0
10
13
10
14
10
15
10
16
10
17
10
18
10
19
10
20
10
21
Ion number concentration (/m
3
)
r (mm)
0.8 kV
1.0 kV
1.4 kV
1.8 kV
Fig. 7 Distribution of positive ion number concentration in the
discharge gap (gap distance 200 μm,atthemid-linelocationofthe
discharge gap)
1.0 1.5 2.0 2.5
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Charging efficiency
Velocity of aerosol flow (m/s)
287 nm
209.3 nm
104.1 nm
45.4 nm
28.9 nm
Fig. 8 Effect of velocity of flow on charging efficiency for
different particle diameters (applied voltage 1.4 kV, gap distance
400 μm)
125 Page 6 of 10 J Nanopart Res (2019) 21:125
Effect of velocity of flow
The velocity of aerosol flow affects the residence time of
particles in ion environment and thereby influences the
charging efficiency. A shorter aerosol residence time in the
charger should result in lower electrostatic losses but,
meanwhile, it also reduces the particle charging probabil-
ity. In this study, three velocities were used (as that given in
Tab le 1) and their effects are shown in Fig. 8.Asthe
velocity of flow increases or the mean aerosol residence
time reduces, the charging efficiency decreases sharply to a
low level, especially for smaller particles.
Moreover, the velocity of flow affects the discharge
as well. Too fast of flowing gas will quench the dis-
charge. This is because the flowing gas blows away the
drifting electrons such that there is no sufficient supply
of electrons to knock on the grounded plate and then
secondary ions are not enough to sustain the discharge.
This is also a shortcoming for this type of direct charg-
ing devices.
Effect of gap distance
In fact, this is also the effect of ion concentration distri-
bution in the discharge gap. Several sets of results are
shown in Fig. 9. Higher charging efficiency can be
obtained bysmallergap distance. When the gap distance
becomes larger, the proportional volume of space that is
capable of maintaining enough ions to ensure diffusion
150 200 250 300 350 400
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Charging efficiency
Gap distance ( m)
280 nm
209.3 nm
104.1 nm
45.4 nm
28.9 nm
Fig. 9 Effect of discharge gap distance on charging efficiency for
different particle diameters (applied voltage 1.8 kV, velocity of
flow 1.88 m/s)
0 50 100 150 200 250 300
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Charging efficiency
Diameter of nanoparticles (nm)
150 m
200 m
300 m
400 m
0 50 100 150 200 250 300
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Charging efficiency
Diameter of nanoparticles (nm)
150 m
200 m
300 m
400 m
Fig. 10 Effect of particle diameter on charging efficiency fordifferent gap distances (applied voltage 0.8 kV, velocity of flow 1.88 m/s (left)
and 2.7 m/s (right))
4 8 16 32 64 128 256
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
This study
(1)
(2)
(3)
(4)
(5)
(6)
Extrinsic charging Efficiency
Particle diameter (nm)
Fig. 11 Comparison of the extrinsic charging efficiency between
the present charger (g=150 μm, applied voltage 0.8 kV, flow
velocity 1.88 m/s) and some other typical needle-type chargers.
The numbers in the legend correspond to the following references:
(1) Sierra et al. (2003, flow rate 2 lpm), (2) Alonso et al. (2006,
applied voltage 3.2 kV), (3) Qi et al. (2007, total volumetric
flowrate 5 lpm), (4) Qi et al. (2008, flow rate 0.3 lpm), (5) Li
et al. (2011, flow rate 3 lpm, discharge current 2 μA, applied
voltage 0.6 kV), (6) Alonso and Huang (2015)
JNanopartRes (2019) 21:125 Page 7 of 10 125
charging will reduce and the ion concentration at the
same radial position will decrease as well. For the char-
ger with a gap distance of 150 μm, charging efficiency
of 82% was obtained for particles with 28.9 nm in
diameter.
Effect of particle diameter
Figure 10 shows the experimentally measured extrinsic
charging efficiencies at two flow rates as a function of
particle diameter. As can be seen, the charging efficien-
cy increases with particle size, but attains more or less
some constant values for different gap distances. A
comparison is made between the extrinsic charging
efficiency obtained in this study and that reported in
former investigations for needle-type chargers. This is
given in Fig. 11. All of them belong to the unipolar
chargers. Data were available only for a small range of
particle diameter for some chargers. Notice that the
studied charger has comparable performance with that
of Qi et al. (2008) for the particles around 50 nm in
diameter. But it gave a lower performance for particles
with their diameter smaller than 50 nm. This is a result
of non-uniform distribution of ions, although their con-
centration is higher in some space of the gap. However,
its simple structure and easiness to be integrated into
minimized devices render its possible applications in
aerosol monitor with limited space allowed.
Particle loss in chargers
Diffusional loss of PS particles in the chargers has been
evaluated under all conditions by turning off both the
voltage of the charger and that of the ion trap. No
obvious discrepancy between the concentration at the
inlet and the outlet of the chargers was detected for
particles with different diameters. Due to the small size
of the charger, the flowing distance of aerosol particles
within them is relatively short, such that the diffusional
loss is very little for this type of charger.
The electrostatic loss of particles was evaluated ex-
perimentally and calculated using Eq. (2). Its results as a
function of particle size and applied voltage are shown
in Fig. 12. As expected, the electrostatic loss increases
with increasing voltage and decreasing particle size.
This result is the same as that found by Huang and
Alonso (2011). The reason is that smaller charged par-
ticles have a relatively larger electrical mobility than
larger ones.
Conclusions
The performance of chargers with needle-to-plate con-
figuration for charging nanometer aerosol particles has
been evaluated systematically. The gaps for discharge
and particle charging in the chargers are with hundreds
of micros in height. This type of charger is simple,
fabricated, and integrated into minimized devices easily.
The charging efficiency increases with particle size,
applied voltage, and mean aerosol residence time in
the charger, while decreases with gap distance. This type
of charger has no detectable diffusional loss for the
particle size used in the experiments. Electrostatic loss
of particles inside the charger also increases with parti-
cle size and applied voltage. Although the extrinsic
charging efficiencies of the present chargers are lower
than conventional unipolar chargers with relatively larg-
er size, their simple structure and easiness to be integrat-
ed into minimized devices render its possible applica-
tions in aerosol monitor and removal with limited space
allowed.
Funding information This work was supported by the State
Scholarship Fund from China Scholarship Council and the Fun-
damental Research Funds for the Central Universities under Grant
FRF-TP-17-011A2.
Compliance with ethical standards
Conflict of interest The authors declare that they have no con-
flict of interest.
0 50 100 150 200 250 300
0.0
0.1
0.2
0.3
0.4
0.5
1.8 kV
1.4 kV
1.0 kV
0.8 kV
Electrostatic loss (%)
Diameter of nanoparticles (nm)
Fig. 12 Electrostatic loss as a function of particle diameter and
applied voltage (gap distance 200 μm, velocity of flow 1.88 m/s)
125 Page 8 of 10 J Nanopart Res (2019) 21:125
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