22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Hydrodynamics of helium APPJ and interactions with materials
L. Wang, W. Ning, S. Jiai, Y. Zheng and Z. Shi
Xi’an Jiaotong University, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an, P.R. China
Abstract: A ‘V-type’ schlieren system was built up to investigate the hydrodynamics of a
helium atmospheric pressure plasma jet (APPJ). With maximum flow rate of 3 standard
litre per minute (slm), the helium flow was laminar inside the tube. After injecting into
ambient air, the combined effect of buoyancy and binary diffusion decide the length of
helium channel and the occurrence of turbulence. When discharge was ignited, the laminar
region shrunk with voltage rising, while the jet length increased. The maximum length of
the jet was limited by the length of laminar region. When the jet interacted with organic
glass, the tip of the plume was diffusive, resulting in uniform patterns. When the jet
interacted with silicon, the discharge was reinforced and the contact area was contracted.
Filament discharge was observed at high voltage with silicon, which would cause burning
damage to the silicon surface. The flow patterns of interaction with different materials
were similar, which indicated the intensity of discharge hade minor effect on the neutral
flow.
Keywords: hydrodynamics, schlieren, APPJ, material interaction
1. Introduction
Great efforts were dedicated the investigations of the
atmospheric pressure plasma jet (APPJ) in the last few
decades for its significant potential applications in the
area of material industries and bio-medical treatments [1].
Generated in the open air, APPJ is weakly ionized
plasma, nevertheless plentiful and energetic particles were
found to be carried be the jet plume. Meanwhile, the
current carried by the jet was in the order of 1 mA, and
the neutral temperature was around the room temperature,
which were apt to avoid local electric shock or
overheating, making APPJ especially preferable to treat
live tissue and other electric/heat-sensitive materials. In
our previous researches [2, 3], APPJ was used to strip the
photoresist material deposited on silicon substrate. It was
concluded that Ar or Ar-contained APPJ was more
efficient in photoresist stripping, while slight damage in
the silicon surface was co-exited; He APPJ was less
effective, nevertheless it was more uniform and gentle
thus leading to better stripping effects.
In this report, hydrodynamics of the He APPJ was
firstly investigated.
Based on experimental and
computational results [4, 5], the jet plume was confined to
the helium flow channel where the helium concentration
was higher than certain critical value. It was worth noting
that this value was obtained on condition that laminar
helium flow was formed in the channel. As a matter of
fact, the helium flow channel was defined by flow rate,
buoyancy and electro-hydrodynamic force, thus it was apt
to turn into turbulence.
The flow patterns were
virtualized by schlieren images. Additionally, the
interaction between the APPJ and materials (organic glass
and silicon) were studied.
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2. Experiment Setup
The He APPJ was generated in a double-ring co-axial
dielectric structure, detail descriptions could be found in
[6]. A ‘V-type’ schlieren system was set up as illustrated
in Fig. 1. A continuous LED lamp was used as spot light
source, which was then focused by a 10 cm diameter
parabolic mirror. The jet plume was penetrated by the
reflected light, which was focused at the knife edge and
then captured by a single lens reflex (SLR) camera
(Canon EOS 700D) with 1/4000 s shutter time. Another
horizontally placed SLR camera (Nikon D5100) was used
to record the plume shapes. When interacting with
downstream materials, the latter camera was elevated to a
position with ~30o depression angle against the
interaction point.
Helium Plasma
Jet
Diaphragm
LED
Knife Edge
Parabolic Mirror
Materials
SLR Camera #2
SLR Camera #1
Fig. 1. Diagram of experimental setup.
1
3. Results and Discussions
3.1. Hydrodynamics of the jet plume
Schlieren graphs of the helium flow injecting into
ambient air without discharge was illustrated in Fig. 2.
The tube was vertically downwards thus the buoyancy
was against to the flow velocity. With inner diameter of
the tube being 4 mm, the maximum flow velocity (m/s)
could be expressed as V max = 8.33Q, where Q was the
flow rate in slm (standard litre per minute). The flow
patterns inside the tube should be laminar by judging
from the Reynolds number, which was calculated to be
about 136 even at 3 slm flow rate. Schlieren images
confirmed the speculation. On the other side, the
buoyancy was significant in the helium channel because
of the large density ratio between air and helium
(ρ air /ρ helium ≈ 7.2). It could be inferred from Fig. 2 that
the helium channel ceased at a point where the flow
momentum was balanced by buoyancy, then it was bend
upwards and diffused into ambient air. Turbulence was
observed at the end of flow channel as flow rate
exceeding 2 slm. This was supposed to be caused by the
binary diffusion between air and helium. As a matter of
fact, the diffusion resulted in the perturbation and
reduction of buoyancy. The maximum length of the
helium channel was fitted with the blue-dot curve in Fig.
2. The increment of length was not linear with flow rate,
which was also attributed to the reduction of buoyancy
caused by binary diffusion.
As shown in Fig. 3d, the plume length increased to
21 mm, meanwhile the length of laminar region reduced
to about 21 mm at 15.6 kV. This observation was
consistent with Oh et al. [7], who concluded the
maximum visible plume length was limited to the laminar
region. Additionally, the turbulence became more violent
with voltage rising, and the jet plume was slightly
flickering, indicating unstable discharge. Gas heating and
ion momentum transfer were two possible mechanisms of
the experiment observations, and further investigations
was necessary for more profound understanding.
0
1
Laminar
region
2
3
4
5
6
7
8 cm
(a)
(b)
0
(c)
1
(d)
Scale /cm
2
Fig. 3. He flow patterns at fixed flow rate of 2 slm with
discharge on. The embedding graph showed the visual
shape of the jet plume. (a) V p-p = 7.2 kV; (b)
V p-p = 9.4 kV; (c) V p-p = 11.2 kV; (d) V p-p = 15.6 kV.
3
4
5
6
Curve of
maximum length
7
8
0.5
1.0
1.5
2.0
Q [slm]
2.5
3.0
Fig. 2. Visualization of He flow into ambient air without
discharge.
Fig. 3 showed the flow patterns and visual plumes with
discharge on. Ignited at 7.2 kV (peak-peak value, the
same hereinafter), the visual plume was about 15.5 mm,
while the helium channel extended to 61 mm, which was
about 10 mm longer than the case without discharge as
shown in Fig. 2. On the contrary, the length of laminar
region (as labelled in Fig. 3) shrunk by about 5 mm. With
the rising of voltage till 11.2 kV, the visible plume length
slightly increased, while the laminar region reduced
obviously. Then significant transition occurred as voltage
exceeding certain value in the range of 11.6 kV to 12 kV.
2
3.2 Interaction with materials
The downstream materials were placed 10 mm beneath
the tube orifice. The flow rate was fixed at 2 slm. Visual
images of the jet plume interacting with materials was
shown in Fig. 4. It could be inferred from Fig. 3 and Fig.
4 that the jet near the orifice was indifferent to
downstream materials, while the lower part connected
with materials was changed. When the jet reached
organic glass, the plume tip was diffusive along the
surface, as shown in Fig. 4a. The contact area was
~ 2 mm diameter stable circle when voltage was lower
than 11.6 kV, and transformed to more bigger area as
voltage exceeding 12 kV. As mentioned in Sec 3.1,
significant evolution was occurred in the discharge during
this voltage range. The discharge at 13.6 kV and 16 kV
was more intense and flickering, which might be in favour
of higher efficiency, but also be potential to cause partial
damages.
Compared with organic glass, the interaction with
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7.3 kV
9.8 kV
13.6reinforced
kV
16in
kVthe
silicon
was more
intense. 10.6
ThekVjet was
damage on the silicon surface. Despite their difference on
10 mm
No discharge
5 mm
10.6 kV
(a)
7.2 kV
9.4 kV
10.4 kV
14.2 kV
7.6 kV
9.8 kV
13.6 kV
(a)
16 kV
16 kV
7.2 kV
9.4 kV
14.2 kV
16 kV
10.4 kV
(b)
(b)
Fig. 5. Schlieren images of jet plume interacting with
downstream materials. (a) Organic glass; (b) Silicon.
Fig. 4. Visual graphs of jet plume interacting with
downstream materials. (a) Organic glass; (b) Silicon.
vicinity of contact area by speculating from the luminance.
On the other side, the contact area was more contracted
than the one with organic glass, as shown in Fig. 4b.
When applied voltage was lower than 12 kV, stable
discharge was formed. The diameter of the contact area
was estimated to be around 1.2 mm. When voltage rose
beyond 12 kV, filament discharge was observed. The
discharge was so fierce that the silicon was left with burn
trace, which should be avoided in photoresist stripping.
Fig. 5 illustrated the flow patterns when the jet
interacted with materials. When discharge was off, the
main helium channel was surrounded by the floated
helium atmosphere with diameter being about 12 mm.
When discharge was ignited, the helium was blown to the
right side, regardless of the voltage rising. Explanation of
this phenomenon required further investigations.
Comparing the flow patterns of organic glass and silicon,
they were highly consistent with each other, nevertheless
the discharge on silicon surface was more intense. This
indicated that the discharge intensity had minor effects on
the neutral flow.
4. Conclusion
Hydrodynamics of helium APPJ was investigated by
schlieren images. Laminar flow was formed inside the
tube. After injecting into ambient air, both buoyancy and
binary diffusion between helium and air affected the flow
patterns. With discharge on, the laminar region shrunk
while jet length increased. It was found that the
maximum of the jet length was limited by the length of
laminar region. When interacting with organic glass, the
jet was diffusive on the dielectric surface, while it was
contracted when interacting with silicon. The silicon
would enhance the discharge, and filament discharge was
observed at high applied voltage, which left burning
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discharge intensity, the flow patterns of interacting with
dielectric or silicon remained similar, which indicating the
discharge intensity had minor effects on the neutral flow.
5. Acknowledgements
This research was supported by the Fundamental
Research Funds for the Central Universities of China.
6. References
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[3] W. Ning, L. Wang, M. Fu and S. Jia. in: 21st
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