LEP
5.3.1000
Plasma physics: Surface treatment
Related Topics
Arc discharge, glow discharge, electron avalanches,
Townsend breakthrough mechanism, streamers, microdischarges, dielectric barrier discharge (DBD), surface energy,
contact angle (CA), contact angle measurement.
Principle
Different samples are exposed to a dielectric barrier discharge
in air at atmospheric pressure. The plasma induces both
chemical and physical reactions on the sample surface altering the surface structure and thus the surface energy. The
contact angle of water on the sample surface is observed in
the exposed and in the unexposed region to assess the effect
of the plasma on the surface energy.
Equipment
Plasma Physics Operating Unit
09108.99
1
Plasma Physics Experimental Set
09108.10
1
Plasma Physics Sample Set
09108.30
1
Transfer pipette, 2-20 µl
47141.01
1
Disposable tips
for microliter pipettes, 1-200 µl
47148.01
1
Ethanol, 1 l
31150.70
1
Vernier caliper
03010.00
1
Aqua demin.
Additional material for optical contact angle measurement:
Millimeter paper
Support material
Illumination
Web Cam
measure Dynamics Software
14440.61
1
Tasks
Various samples are to be treated with a plasma for different
periods of time. The effect of the treatment on the contact
angle of water on the surface is to be observed by drop size
measurement or by web cam photography.
Set-up and procedure
– Connect and arrange the plasma Physics Operating Unit
and the Plasma Physics Experimental Set as seen in Fig. 1.
– Always clean metal, glass and PVC samples with water,
dishwashing detergent and a lint-free clean cloth.
– Rinse with (demineralized) water and dry with a clean cotton cloth, for example dish towel, polishing the surface
while drying.
– Clean other polymer samples with alcohol and a lint-free
clean cloth.
– The wooden samples are single-use only but may be used
in different regions several times.
– Check uniform surface wettability by breathing on the
sample – the surface should cloud uniformly and no structures of former surface treatment should be visible.
– Set the transfer pipette counter to 020, that is drop size of
2 µl, and put on a fresh pipette tip.
– Push the pipette button to noticeable stop, immerse tip in
(demineralized) water and release button, then 2 µl water
are inside the tip.
– To push the drop out of the pipette push the button so far
that the inner water meniscus is exactly at the pipette tip.
– Set the drop gently down on the sample surface, practice
setting down round drops of uniform size.
– Measure diameter of area of drop contact to sample surface with caliper without touching the drop with the caliper.
– Treat sample with plasma for different time at different
regions of the sample, you may mark the regions with a
ohp pen.
– Measure drop diameters in treated regions.
– If drop not round: estimate average size.
– Calculate contact angle and draw diagrams treatment
duration – contact angle for different sample materials.
Fig. 1: Fundamental set-up
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Plasma physics: Surface treatment
Alternative 1: Area of contact drop to sample surface measurement with web cam
– Mount the web cam that it views top-down (Fig. 2).
– All-sided diffuse illumination is advisable for contrast of the
drop's edge.
– Place sample with drop and a cut-out strip of millimeter
paper into the focus under the camera.
– Camera settings: maximal contrast and black and white
operation.
– Save pictures as *.jpg.
– Start measure dynamics software, select "File" > "Open
picture…".
– Select in the drop-down menu file type "JPeg picture
(*.jpg)" and open the desired file.
– Select "Measure" > "Calibration".
– Fit the calibration line with the mouse over the picture of
the millimeter paper, see Fig. 4.
– Click on the legend and enter distance and unit.
Fig. 4: Calibration line
Fig. 5: Circle to fit the drop picture
Fig. 2: Alternative 1
Fig. 6: "Display/Paint objects" window
–
–
–
Fig. 3: Alternative 2
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Select "Display" > "Paint" and use the circle button (Fig. 6).
Adjust the properties of the circle with the upper right button from the "Display/Paint objects" window right of the
"Play" button.
Set "Width" to 1 pixel, select a well visible color and no filling, see Fig. 7.
Note down circle diameter and area.
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEP
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Plasma physics: Surface treatment
–
–
–
Use "Display" > "Paint" to do so.
Adjust the properties of the circle with the upper right button from the "Display/Paint objects" window right of the
"Play" button (see Figs. 6 and 7).
Set "Width" to 1 pixel, select a well visible color, no filling
and a partly translucent "Alpha channel" slide bar setting
with the slide bar near the middle.
Fig. 8: Angle measurement
Fig. 7: Window to adjust properties
Alternative 2: Direct contact angle measurement with web cam
– Place the sample with drop on it right in front of the web
cam and focus well on the drop, drop centered in the field
of view.
– Tilt the camera only few degrees downwards.
– Illuminate a spot on a plane well behind the sample, the
spot size at most the field of view of the camera (Fig. 3).
– Keep the rest of the room dark so the drop appears dark
in front of an illuminated background to minimize picture
misinterpretation through glare and reflexes.
– Best camera settings are black and white, maximal contrast, high brightness and exposure.
– Save pictures as *.jpg.
– Start measure dynamics software, select "File" > "Open
picture…".
– Select in the drop-down menu file type "JPeg picture
(*.jpg)" and open the desired file.
– Select "Measure" > "Measure angle".
– Move the yellow marks with the mouse to desired positions and read out the contact angle, see Fig. 8.
– To improve accuracy you may draw circles to fit the profile
of the drop and its reflection on the sample surface – the
tangent of a circle is better visible than that of the drop, the
intersection of the circles defining the horizon – and you
may evaluate two tangents to each side (Fig. 9).
Fig. 9: Angle measurement with help of circles
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
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5.3.1000
Plasma physics: Surface treatment
Theory
Plasma, the fourth state of matter
Plasma is often referred to as the fourth state of matter after
solid, liquid and gaseous. A plasma is a gaseous state of matter in which part or all of the species (atoms or molecules) are
ionized. The plasma may consist of neutrals, free electrons
and positively charged ions. In the presence of electronegative
gas particles the electrons may get captured by them so that
negative ions may also be present and the number of free
electrons reduced.
A plasma can be characterized by it's degree of ionization.
The degree of ionization X is defined as X = ni / (ni + na)
where ni is the number density of ions and na is the number
density of neutral atoms. Also particle density (or pressure)
and temperature play main roles in the description of plasma
behavior.
Ionizing processes as heat, chemical reactions, ionizing radiation or electrical fields may lead to plasma formation. Some
plasmas are named according to the process that generate
them, for example "low pressure radio frequency (RF) plasma".
Widely known plasmas are
– the photosphere of the sun exited by heat,
– flames exited both by chemical reactions and heat,
– the ionosphere of the earth exited by UV, soft X-ray and
energetic proton irradiation,
– the mercury vapor glow discharge inside a common fluorescent lamp exited by low frequency high voltage electric
fields
– or the lightning from a thunderstorm as an example of an
electric arc.
Since mobile charge carriers are present, plasma is an electric
conductor and the static electric field inside the plasma is
usually weak. Strong static electric fields may be present
though in sheaths around electrodes, where the reactions
forming the plasma take place.
Non-thermal and thermal plasma
Depending on pressure and mechanism of plasma generation
the plasma may not be in thermal equilibrium, so different
species may have strongly different mean energies, sometimes also referred to as different temperatures, though the
term temperature is only precisely defined in thermal equilibrium.
For example in case of weakly ionized electric plasma at low
gas pressure in the hPa range as in glow discharges the gas
ions have nearly ambient temperature while the electrons gain
energy from the electric fields until they are able to excite or
ionize the present gas particles. The excitation or ionizing
energies can be in the range of several eV. According to thermodynamics in an ideal gas per degree of freedom the average energy in thermal equilibrium is E = 1/2 kB T with the
Boltzmann constant kB. The energy of two eV per degree of
freedom of electrons would correspond to a temperature of
T = 1 eV / kB = 1.602-19 CJ / 1.38-23 J/K = 11600 K, so the
electrons may be considered "hot" compared to the ions at
about 300 K. Such a glow discharge is structured in regions of
high field where plasma formation takes place and others of
low field – different regions having different luminosities. So a
glow discharge is an example of a non-thermal plasma.
In contrast to that in a thermal plasma like an arc pressure and
degree of ionization are high enough that electrons and ions
efficiently exchange their energy and have the same temperature of a few thousand K. The arc discharge is not structured
in a way like a glow discharge.
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Mobility, diffusion and drift
The strong deviation of electron and ion behavior depends on
the strong differences in mobility and momentum exchange in
collisions.
The mobility of particles in a gas decreases with their mass
and the mass of the light H atom is already about 2000 times
the mass of an electron. Also higher pressure decreases the
mobility since the pressure determines the mean free path.
The motion of a charged particle in a gas under influence of a
static electric field is an average directed drift. The drift velocity is the mobility times the electric field strength. In gases at
breakdown conditions the directed drift velocity of electrons is
in the range of 105 ... 106 m/s, the velocity of ions is in the
range of 50 ... 500 m/s.
In a collision between a gas ion and another atom or molecule
the masses of the particles are of the same order of magnitude. After the collision the probability is high that the momentum is evenly distributed between the collision partners. So if
a ion gains some energy from the electric field it will loose it
probably again in the next collision. This is not so for the electrons in a plasma of low ionization degree that are mainly
reflected from the much heavier gas particles with few
momentum transfer at each collision unless they can excite or
ionize the collision partner. Only few energy is transferred due
to rotational and vibrational state transitions during collision
betwenn electrons and molecules.
Because of their higher mobility the electrons in a glow discharge shield the electric fields while the ions spread their
energy efficiently in collisions unless pressure is that low that
their mean free path is as long as the dimensions of the electric field distribution.
Reactions induced by non-thermal plasma
In a non-thermal plasma the energetic electrons and ionized or
exited gas particles can activate a lot of desired chemical and
physical reactions while a sample surface exposed to the
plasma is not heated to a great extent. The energy transfer
from the electric source is mainly to the electrons and thus
excitation and not to excessive heating so the electrical effection of chemical reactions by plasma can be very efficient. At
low gas pressure use of high frequency electric fields and
electrodeless configurations are possible. So non-thermal
plasma is used for plasma chemical vapor deposition, etching
and sputtering at low pressure and in high purity environments, especially in semiconductor or optics fabrication.
Unluckily no glow discharge will establish at atmospheric
pressure but electric breakdown will lead under these conditions to formation of an electric arc, a thermal plasma, if sufficient field strength of DC, AC or high frequency is applied. On
the other hand it is desirable to have non-thermal plasma at
atmospheric pressure at hand so no expensive and unhandy
vacuum equipment is necessary for the possibility of chemical
reactions or surface treatment provided by non-thermal plasma. So understanding the plasma formation process may
help.
Electron avalanches
With electric plasma generation, that is electrical gas breakdown, charge carrier multiplication provides for the ionization.
To reach a significant degree of ionization a randomly present
charged particle needs to produce in average more than one
other charged particle before recombining with another particle or an electrode. Otherwise the electric field will sweep
away the present charge carriers and any discharge will die
out.
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEP
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Plasma physics: Surface treatment
The main starting process of charge multiplication in gases is
an electron avalanche because electrons gain most energy
from the electric field due to highest mobility. An initial electron
may be set free by omnipresent random ionizing radiation or
by a positive gas ion recombining at a surface under emission
of Auger electrons. There may be a retardation time before the
starting electron appears delaying discharge formation. In an
electron avalanche an initial electron produces after inelastic
scattering in average more than one more free electron and a
positively charged ion. Such an avalanche spreads in field
direction with the electron directed drift velocity and laterally
with the undirected thermal electron diffusion velocity so the
outer form of the avalanche is drop-shaped, a round
avalanche head and a pointy tail.
High and low charge density discharge behavior
What happens after development of an electron avalanche
depends on the gas conditions and field distribution. In the
following the situation between parallel metal plates some millimeters to centimeters apart with applied AC voltage is considered.
a) If the pressure is low, then the charge density of the
avalanche is small enough not to significantly distort the
applied field. The avalanche reaches the anode after some
nanoseconds where the electrons get absorbed leaving a
trace of slower ions behind which now travel to the cathode. When the ions impinge on the cathode after some
microseconds they release with a certain probability secondary electrons by Auger effect when recombining with
the metal's conduction band electrons. The secondary
electrons seed in turn new electron avalanches. Since the
reaction between ions and electrode surface determine the
rate of electron creation, this type of discharge is sensitive
to electrode materials. After less than a millisecond a plasma has formed between the metal plates with a positive
space charge due to the slower positive ions in front of the
cathode strengthening the field there and lowering it in the
rest of the discharge volume. This cathode fall region contracts to a size such that avalanche multiplication
becomes optimal there. This is called Townsend breakdown and the result of this process is a glow discharge.
b) If the pressure is high the free paths are so small that the
resulting charge densities of an avalanche generate an
electric field strength in the range of the field strength that
initiated the avalanche. The separating electrons and ions
form a dipole that weakens the field inside the avalanche
region and amplifies it to the head and tail of it. The field
strength increases there to an extent that electrons set off
by photons emanating from the avalanche start new
avalanches with increased probability. Several avalanches
connect to a streamer, a thin weakly ionized plasma channel. Once the streamer interconnects the plates after some
nanoseconds, current can begin to flow between the
plates heating the plasma and forming an arc. Thermal
emission occurs where the arc touches the electrodes. The
streamer formation propagates at least with the electron
drift velocity. For long distances as in lightning the electrons in the head of interconnected streamers can be
accelerated to energies above 100 eV where they have
again a reduced reaction cross-section with gas molecules. They are called run-away or beam electrons and can
form a leader discharge which leaves a thin highly ionized
channel before connecting the electrodes.
Providing low charge density discharges at atmospheric pressure
So if at atmospheric pressure a non-thermal plasma is
desired, arc formation has to be avoided because streamers
will form if break-through conditions are reached or streamer
formation has to be avoided altogether.
One method is the dielectric barrier discharge (DBD) where a
dielectric between the electrodes provides for an AC field
strong enough for breakthrough and streamer formation but
inhibits large current flow and thus arc formation. So the
streamers lead to microdischarges that conduct current until
there is enough charge gathered on the dielectric surface that
the external field is shielded. Since the mobility of charges
sideways on the dielectric surface is low, many parallel
microdischarges take place until the field is lowered over the
whole electrode surface. So the discharge is filamented and
the filaments tend to arrange in an equidistant pattern and
tend to stay located because the patterned charge distribution
left on the dielectric seed new microdischarges in the same
pattern after field reversal.
Streamer formation may be inhibited to some extent in a DBD
if after a discharge cycle enough seed electrons remain everywhere on the dielectric to initiate so many avalanches that a
uniform but relatively low ionization is achieved and a so
called atmospheric pressure glow discharge (APG) forms.
Another method to produce non-thermal atmospheric pressure discharges is to use micro-structured electrodes (MSE).
There the electrode distance is that small that breakthrough
occurs with only a few hundred volt between the electrodes
and the avalanche length small enough to avoid high charge
densities capable of streamer formation.
Technical applications of non-thermal athmospheric pressure
plasmas
Technically DBDs are employed to large scale in ozone generators (with glass tubes as dielectric) and also for surface treatment to improve printability or wettability for example in the
textile industry. A future application might be cleaning of
exhaust gases from toxic compounds.
In a DBD in air at ambient pressure besides ions of air molecules neutral radicals like ozone and nitrogen oxides and their
ions are produced which then can react with a sample surface. The surface itself can produce reactive radicals if bombarded with electrons and ions and the radicals from the DBD.
Altogether surface compounds can be changed in a DBD
leaving polar groups as bound oxygen and the like on a former unpolar surface or the surface layer structure can get disturbed by insertion of faults which may lead to deformed
structures with increased dipole moment. The surface energy
is altered and the changed hydrophilicity alters the wettability.
Technically the trend from unpolar solvents to water-based
systems for surface coating requires better wettability for
unpolar surfaces so improved hydrophilicity is desired.
The surface treatment experiment
In the present experiment the plasma is created by a
(200 ± 5) Hz AC (15000 ± 500) kV field from a source delivering maximally 2 mA. The field is applied over a 1 mm dielectric barrier consisting of alumina ceramic (Al2O3 or corundum)
and a gap of 2.5 mm in air at ambient pressure. One electrode
is metal powder inside the alumina tray and the other is the
aluminium ground plate of the Plasma Physics Experimental
Set.
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
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Plasma physics: Surface treatment
A sample can be placed into the space between alumina tray
and ground plate and can be exposed to the usually filamentary discharge for durations between 0.2 s and 60 s. A change
in the surface energy of the sample is detected by the measurement of the contact angle of a water droplet on the sample surface on the untreated surface and after different plasma treatment durations.
Wetting and contact angle
The cosine of the contact angle is according to Young's equation for partial wetting equal to the ratio of surface energy difference between solid-air- and solid-liquid surface to surface
energy of liquid-air surface
cosa sS sLS
sL
(1)
with sL the surface energy of the liquid-air surface, sS the surface energy of the solid-air surface, and sLS the surface energy of the liquid-solid surface, the surface energy also called
surface tension. This equation can be derived by considering
the forces. The liquid-air-solid contact line can move on the
sample surface only parallel to the surface and will move until
the force tangential to the sample surface is zero, the tangential components of the forces cancel each other. See Fig. 10.
The surface energy is equal to a force per length tangential to
the surface at a rim of the surface.
Three cases are commonly distinguished: complete, partial or
no wetting.
In case of complete wetting the contact angle is zero, the
solid-air surface energy exceeds the sum of liquid-air and liquid-solid surface energies and a drop is spread over the whole
surface.
Partial wetting occurs, if the surface energy of the solid-air
interface is less than the sum of liquid-air and solid-liquid surface energies but greater than the solid-liquid surface energy.
A defined contact angle below 90° occurs.
In these two cases the surface energies of both drop and solid
are lowered by adhesion of the drop.
Fig. 10: Forces on the meeting line of the phases,
cos a | FL | = | FS | – | FLS |
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| FL II | =
If the solid-air surface energy is less than the solid-liquid surface energy the contact angle will exceed 90° and the surface
is called unwettable by the liquid. A drop will roll off the surface then when the surface has a slope. Contact angles
exceeding 90° can be observed though and the liquid-solid
surface energy is always positive and less than the liquid-air
surface energy, since adhesion to something is always better
than to vacuum due to always present London forces or vacuum radiation pressure. The surface energy of the solid is
raised with the presence of the drop, only the drop's surface
energy is lowered so the whole system can be still thermodynamically stable.
The interaction between condensed matter and air is assumed
to be negligible and the same as to vacuum.
The wettability of a surface can be raised by plasma treatment. The plasma leaves polar groups on the surface raising
the dipole and hydrogen bond interactions between surface
compounds thus raising the surface energy and strengthening
the surface-water interactions lowering the liquid-solid surface energy.
Evaluation
In case of drop size measurement the contact angle can be
determined from the drop size and the known volume of the
drop. Let the drop volume V on the pipette be set to
2 µl = 2 mm3. In case the drop is small enough so gravitational distortion of the drop form plays no role and the surface is
uniform enough so the drop is round then the drop will form a
spherical segment on the sample surface with a specific contact angle between sphere surface and sample surface
(Fig. 11). The segment of a sphere with sphere radius R has
segment radius a and height h and the volume
V = π h (3a2 + h2)/6 = π h2 (R – h /3 )
(2)
and the contact angle
a = arcsin(a / R)
(3)
Fig. 11: Drop with contact angle a, height h, sphere radius R
and diameter 2a
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEP
5.3.1000
Plasma physics: Surface treatment
In the experiment the drop diameter 2a is observed so the
dependence of a on a is of interest. The left part of eq. (2)
delivers for h the cubic equation
h3 + 3a2h – 6V/π = 0
with the solution
3
2
u
a2
3
2
u
with u 2
3
3
V a6 a V b
p
p
B
and from the theorem of Pythagoras
R
a2 h2
.
2h
Since the formulas are unhandy and the achievable accuracy
low, the value of the contact angle a may be read out of the
graph of Fig. 12. The data were created using spreadsheet
calculation to evaluate the formulas.
Fig. 13 shows typical measurement results for the drop size
method after plasma treatment of the samples for time t.
The absolute volume of a drop from the pipette is not so precisely defined and for a drop radius near the value for 90 ° is
the relative angle error greatest so that falsely angles above
90 ° appear. For example some of the drop volume may have
rested on the tip resulting in a falsely too high angle value.
The behavior of a water drop on wood is changed in the way
that after plasma treatment the drop gets soaked into the
wood more quickly the longer the plasma was applied. The
wetting seems to be completely and directed along the fibers.
In case of the polymers the method is too inaccurate to really
order the polymers according to their surface energy without
using at least a second test fluid. It can be seen that plasma
treatment raises the surface energy but not to such an extend
that complete wetting with water occurs.
From the literature value of their surface energy one would
expect complete wetting with water both for glass and aluminium, which is always covered with it's oxide in air. But both
have chemisorbed water already on their surface after contact
with normal room atmosphere. SiO2 in glass and Al2O3 form
links to hydroxyl groups during contact with water which in
turn form hydrogen bonds to water molecules. So these surfaces pick up water immediately after production and their
surface energy is in turn already lowered. Plasma treatment
seems to exchange some of the hydroxyl groups and complete wettability is the result.
The filamentary structure of the discharge over aluminium
leads to not uniform surface energy in the treated region and
the drops spread not to a circle. This may reduce the reliability of the evaluation with the drop diameter method.
Fig. 12: Contact angle in dependence on drop radius
PHYWE series of publications • Laboratory Experiments • Physics • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
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Plasma physics: Surface treatment
Table 1 shows literature values for the surface energy at 20 °C.
Table 1: Surface energy values
Material
surface energy
[mJ/m2]
dispersive contribution
to surface energy
[mJ/m2]
polar contribution
to surface energy
[mJ/m2]
Al2O3
5900 - 7300
n. a.
n. a.
Glass
560
n. a.
n. a.
Water
72.8
n. a.
n. a.
PVC
41.5
39.5
2.0
PE
35.7
35.7
0.0
Polyester (PC)
34.2
27.7
6.5
PP
30.1
30.2
0.0
PTFE
20.0
18.4
1.6
Fig. 13: Measurement results for the contact angle of water on different sample surfaces after plasma exposure of duration t
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