Bacteria Sterilization/Inactivation Applications

Various working gases flowed between the parallel plates, including N₂ (99.99%), O2 (99.99%) and compressed air (produced from an oil-less compressor). The flow rates were controlled by manually adjustable flowmeters. For clarity of presentation, all of

the results presented in this paper were performed under the conditions of 30 kHz (power supply) and 10 slm (flow rate). The petri dishes which contained the bacteria were transported by a pre-programmed moving stage. For the treatment of the bacteria, the distance between the bottom edge of the planar DBD and the bacteria varied in the range of z=4-20 mm. The moving speed of the petri dishes was kept at 1 cm/s and the number of passes of the DBD jet pass varied in the range of 1-18. Note that “a pass” is defined as the motion of the APPJ traveling back and forth over the petri dish. Resulting residence time over the petri dish was 0.1 s for a pass under the designated speed, assuming the jet width is 1 mm at the impinging point. In addition, the gas temperatures in the post-discharge jet region were measured using a K-type thermocouple, which was fixed on a moving stage. The testing conditions of the E. coli and B. subtilis inactivation have been summarized in Table 2-6.

2.3.2.2 B. subtilis Spore

Various working gases flowed between the parallel plates, including compressed air (produced from an oil-less compressor) and CF₄/compressed air (2%). The flow rates were controlled by manually adjustable flowmeters. For clarity of presentation, all of the results presented in this paper were performed under the conditions of 30 kHz (power supply) and 5 slm (flow rate). The petri dishes which contained the bacteria were transported by a pre-programmed moving stage. For the treatment of B. subtilis spore, the distance between the bottom edge of the planar DBD and the bacteria was fixed at 14 mm. The moving speed of the petri dishes was kept at 1 cm/s and the number of passes of the DBD jet pass varied in the range of 1-18. Resulting residence time over the petri dish was 0.1 s for a pass under the designated speed. In addition, the gas temperatures in the post-discharge jet region were measured using a K-type thermocouple which was fixed on a moving stage. The testing conditions of B. subtilis

spore inactivation have been summarized in Table 2-7.

Chapter 3

Characterization of the DBD APPJ

In this chapter, the measurements of the characteristics of the planar DBD system are presented in detail. They include: 1) Visualization of the post-discharge jet region; 2) electrical properties; 3) Optical properties and 4) Ozone concentration, which are described in the following in turn.

3.1 Visualization of the Post-Discharge Region

Figure 3-1 shows the typical images of the post-discharge region for N2 and N₂+0.06% O₂ discharges under the conditions of 50 slm, 60 kHz and 500 W, which were taken after 0.2-2 s of exposure. For a pure nitrogen discharge, the plume extended over a longer distance (~ 2.5 cm) and had a yellow-orange color. When the plume became even shorter (~ 1 cm) it had a blue color with 0.06% of trace oxygen addition.

These color changes could be clearly explained from the OES measurement, as shown in Figure 3-8b, which shows the optical emission spectra in the visible range obtained in the post-discharge region. It was readily apparent that as 0.06% oxygen was added: the blue lines were greatly enhanced, which led to the observed blue plume. Also, it was interesting to observe that as 1.6% oxygen was added, most of the optical emissions in the visible range diminished, which resulted in an invisible plume.

Figure 3-2 shows the bottom view of the discharge region for N₂, O₂, and compressed air plasmas, respectively, under the conditions of 10 slm, 30 kHz and 300 W taken after 0.2 s of exposure. The image of the nitrogen discharge was a very bright and uniform blue color, which was mainly caused by the fluorescence of short-lived

excited nitrogen (^N2

(

^B3

_∏

^g

)

^{and N2}

(

^C3

∏

^u

)

⁾ [Akishev et al., 2008]. The oxygen plasma was a typical electronegative discharge, but its image was much darker than other gas discharges that contained nitrogen. In addition, the micro-discharges were clearly seen in the images of the compressed air and oxygen discharges, which were essentially filamentary-like. This also resulted in rapidly oscillating discharge currents as shown in Figures 3-7a and 3-7b.

Figure 3-3 shows the side view of the post-discharge regions for N₂, O₂ and compressed air discharges, respectively, under the conditions of 10 slm, 30 kHz and 300 W. For the pure nitrogen case, the colorful plume was ~ 1.5 cm in length, with a yellow to orange color, which was mainly caused by the fluorescence of the N2 first positive lines metastable nitrogen N B²( ³

∏

_g)→N²(A³

∑

_u⁺) (500-700 nm). For the discharges generated by pure oxygen and compressed air, the plumes were hardly visible because most of the metastable nitrogen was quenched by the oxygen. In addition, the color was clearly confirmed from the OES measurements, as shown in Figure 3-10, which is described in detail in later section.

3.2 Electrical Properties

3.2.1 Current and Voltage Waveforms

Figure 3-4 shows the typical measured input voltage (60 kHz) and discharge current waveforms under the conditions of 50 slm (flow rate of N₂) and 500 W (the output power from the power supply). The pulsed width was approximately 3 µs. The results showed that the discharge current increased rapidly as the voltage exceeded some value (~4 kV) and decreased rapidly as the voltage further increased to some level. The rapid decrease of the current was mainly due to the charge accumulation on the dielectric

surface, which is a well-known mechanism of DBD to prevent the discharge from arcing. It was also interesting to observe that the current changed direction from positive to negative as the applied voltage reached the maximal value (~8.5 kV). After the pulse, the filament current oscillated between positive and negative values at a lower voltage, mainly because of the displacement current (large rate of change of applied voltage).

The measured peak current for N₂ was approximately 0.45 A for 25 cm² of the discharge area, which was equivalent to ~0.018 A/cm² in terms of current density. Note this corresponds to a typical value of current density of the so-called Townsend-like discharge [Choi et al., 2006].

Figures 3-5(a)-(c) show the typical measured distorted sinusoidal voltage (30 kHz) input voltage to the electrodes and the discharge current waveforms produced in the DBD using pure N₂, pure O₂, and compressed air of 10 slm, in which the output power from the power supply was kept constant (300 W). It was clear that the discharge of N2

(see Figure 3-5a) was nearly homogeneous based on the current waveform, while those of pure O2 (see Figure 3-5b) and compressed air (see Figure 3-5c) were filamentary-like.

These can also be further verified from Figure 3-2, which illustrates a series of discharge images of these gases. The measured peak current was approximately 400 mA for 25 cm² of the discharge area, which was equivalent to ~16 mA/cm², which is slightly less than 18 mA/cm² when the driving frequency is 60 kHz, as presented earlier.

One-dimensional fluid modeling for nitrogen DBD driven by a sinusoidal voltage waveform [Choi et al., 2006] and by the same quasi-pulsed (distorted sinusoidal) waveform [K.-W. Cheng et al., 2010] have both shown that it is a typical Townsend-like discharge for gap distance of 1 mm, in which the electron number density is much less than that of the ion number density (N₂⁺ and N₄⁺) and very abundant long-lived metastable nitrogen are generated. Details of the fluid modeling of nitrogen [K.-W.

Cheng et al., 2010] are beyond the scope of the current study and are skipped here for

brevity. Discharges using pure O2 and compressed air were more filamentary-like than pure N₂ discharges, as confirmed from the rapid oscillating currents in Figures 3-5b and 3-5c, and also the images of the micro-discharge in Figure 3-3.

3.2.2 Power Absorption Estimation Based on Lissajous Figures

Figure 3-6 shows the typical Lissajous figure obtained for the same test conditions as shown in Figure 3-4. The shape of the Q-V curve was a distorted parallelogram [Wagner et al., 2003] as observed in a DBD driven by a sinusoidal AC power source.

The results showed that as the voltage reached the peak value (~8.5 kV), the maximal effective charge (~350 nC) across the electrodes was obtained, which extinguished the discharge (zero current) caused by the shielding effect, as observed in Figure 3-4. As the voltage continued to decrease, the effective charge began to decrease, as had been expected. The estimated plasma absorbed power in this case was 175 W with an efficiency of about 35%.

Figure 3-7 shows the typical Lissajous figure obtained for the same test conditions as in Figure 3-5. Accordingly, for the corresponding N₂, O₂ and compressed air discharges, the estimated absorbed plasma power was 88.6W (28%), 86.4W (28.8%) and 83.5W (27.8%), respectively, where the percentage in each parenthesis represents the ratio of absorbed power to input power. Note that the input power was directly read from the power supply.

3.3 Optical Properties

3.3.1 Species Identification

3.3.1.1 Nitrogen-Based DBD APPJ

Figures 3-8(a)-(b) show the measured optical emission spectra in the range of 180-900 nm in the post-discharge region. In the discharge region, the addition of 0.06%

O2 reduced the production of ^N2

(

^C3

∏

^u

)

^[N²+ →e N²

(

C³

∏

u

)

+e] because of the reduced electron number density due to the high electron affinity of oxygen. Hence, in the post-discharge region, the emission line intensity for the N₂ second positive [ ^N2

(

^C3

∏

^u

)

^→N2

(

^B3

∏

^g

)

^{+hυ (337.1 nm)} ] decreased accordingly (see Figure 3-8a).

However, the addition of 0.06% O2 increased the amount of ground-state NO and, thus,

( )

NO A²

∑

^{+ and NO B}

(

²

∏ )

^{[through NO+N2}

(

^A3∑u+

)

^{→NO A}

(

^2∑+

)

^and

(

u

) ( )

NO+N² A³

∑

⁺ →NO B²

∏

]. Hence, in the post-discharge region, the NO-γ, the NO-β and the N₂ first positive (Lewis-Rayleigh afterglow) emission line intensities increased through [ ^{NO A}( ²

^∑

⁺)^{→NO+hυ (180−260nm) ], [ NO B}( ²

∏

)^{→NO+hυ (260 380 − nm) ] and}

[^N+N+N2→N2

(

^B3∏^g

)

^and N2

(

B³∏g

)

→N2

(

A³∑u⁺

)

+hυ (580 nm) ], respectively. In the above, the increase of the N2 first positive line intensity (580 nm) was mainly caused by the increase of N atoms through the reaction channels in the discharge region, such as

1

2 2( _u)

e+N →N a′∑⁻ +e ^and N a2( ′¹

∑

_u⁻)+NO→N+O+N2 with 0.06% addition of O₂. As more O2 was added, all line emission intensities decreased because of the reduced plasma (thus, electron) intensity in the discharge region [Golubovskii et al., 2002;

Brandenburg et al., 2005; Golubovskii et al., 2004; Guerra et al., 2001; Dilecce et al., 2007].

In addition, the emission intensity of 236.6 nm was the highest among these emission lines, which was similar to earlier observations [Iwasaki et al., 2007]. For a clear picture of how the UV emission intensity (e.g. 236.6 nm) varied with its position in the post-discharge region and the concentration of oxygen addition into the nitrogen

discharge, we have summarized the measurements in Figure 3-9. Figure 3-9 shows that UV emission generally increased with increasing amounts of trace oxygen addition and reached a maximal value with only 0.06% oxygen addition. Note that the NO-γ emission was appreciable even with the “pure” nitrogen case since there are always impurities (oxygen) in commercial nitrogen bottles. Further addition of O2 (> 0.06%) greatly reduced the UV emission, which was probably caused by the very high electron affinity of oxygen that can appreciably reduce the amount of excited nitrogen [N₂(A³

∑

_u⁺)]. In addition, the UV emission generally decreased rapidly with increased downstream distance from the channel exit, probably because of dramatically decreasing amounts of excited nitrogen as it was quenched by entrained ambient air.

Note detailed reaction channels related to nitrogen, oxygen or mixture of both are summarized in Table A-1 for reference. However, these could be clarified using some simulation techniques such as fluid modeling coupling with a flow solver, which is currently in progress in our group.

3.3.1.2 Air-Based DBD APPJ

3.3.1.2.1 Pure Air DBD

Figure 3-10 shows the optical emission spectra in the UV and VIS regions measured in the post-discharge region. UV emission was greatly reduced because of the strong electro-negativity of O2 gas, as mentioned earlier. Detailed plasma chemistry using more detailed measurements or simulations would be a worthy topic of study in the near future. In addition, in the range of 450-550nm, emission of the nitrogen discharge was much stronger than for either pure oxygen or compressed air, which caused the yellow-orange plume as observed in Figure 3-3.

3.3.1.2.2 Air/Carbon Fluorine Mixture DBD

Figure 3-11 shows the optical emission spectra in the 180-380 nm region measured in the discharge region with air and CF4/air (2%) DBDs. The 180-280 nm UV emission almost disappeared because of the strong electro-negativity of O₂ gas for both cases. In addition, in the range of 300-380nm, the peak lines of emission were similar to the compressed air and CF₄/air (2%) discharges. Obviously, there are no lines related to F atom and CFx (x=1-3) fragments in the OES data mainly because the excitation energy of F atoms is very high (between 14.37 and 14.75 eV) and it is impossible for the low-energy electrons to excite the F atoms. However, there have been some studies showing that it was possible to find CF₂ fragment in the range of 240-360 nm if the argon and small amount of CF4 are used as the discharge gases of a DBD [Fanelli F, 2009]. This was probably caused by the abundant metastable argon having higher excitation energy (11.55-11.72 eV), which dissociated the CF4 more easily than by the electrons themselves. However, more studies are required to understand the observed phenomena.

3.3.2 Gas Temperature Measurements

Figure 3-12 shows the measured gas temperature distribution along the centerline (z=3-20 mm) (i.e., 8-25 mm from the end of the electrodes) in the post-discharge jet region for N2, O2 and compressed air discharges. Note gas temperatures were measured using the thermocouples, not by the OES technique as mentioned in Chapter 2. It is because the corresponding emission intensities in the post-discharge region were too weak for measuring the rotational gas temperatures. Also with addition of large quantity of oxygen, the related emission lines related to nitrogen diminished because of the high electron affinity of the oxygen. The results showed that the gas temperatures for all the

cases decreased rapidly in the downstream direction. For example, the temperature of the pure nitrogen case dropped from 71°C (z=3mm) to 27°C (z=20mm), which was essentially at room temperature. In addition, gas temperatures for the APPJ containing more nitrogen were higher than those containing more oxygen. This was attributed to the much higher power absorption of neutral nitrogen caused by the ohmic heating effect which positive nitrogen ions collide with neutral background gas molecules as compared to the oxygen case since the ion density is higher in the nitrogen case. In general, the low gas temperatures in the plasma jet region observed in this study were highly suitable for inactivating bacteria without the danger of damaging bio-medical devices, which are often made of polymer-like materials.

3.4 Concentration Measurements of Ozone

3.4.1 Measurements using Ozone Monitor

Figures 3-13 and 3-9 show the concentrations of O₃ and typical NO-γ UV emission (236.6 nm, photon energy: 5.2 eV), respectively, as a function of downstream distance and O2/N2 (%) in the plasma jet region. The resolution distance of the measurement of O₃ concentration distribution and NO-γ UV distribution was 2 mm. The distributions of O₃ showed a maximal value at the downstream location of z≈10 mm for various ratios of O₂/N₂. In addition, UV emission was very strong in the near jet region (z up to 10 mm), especially when O₂/N₂≈0.05%. It is well known that ozone can effectively absorb UV emissions in the range of 220-280 nm [Wameck 2008]. Thus, an appreciable amount of oxygen radicals [ O₃ +hυ →O P(³ )+O₂] was generated in the near jet region (z < 10 mm). Note that the NO-γ UV emissions were probably caused by the collision of ground NO species produced in the discharge with the abundant and long-lived

metastable N₂ [N₂(A³

∑

_u⁺)] in a nitrogen-based discharge [Iwasaki et al., 2008]. Higher intensity NO-γ UV emissions did not indicate a higher amount of ground NO species since it was also proportional to the amount of metastable N₂, which was strongly dependent on the amount of oxygen addition and downstream location in the plasma jet.

Another possible mechanism for improving the hydrophilic property caused by the existence of metastable N₂ [N₂(A³

∑

_u⁺)] has been described in Chapter 4.

3.4.2 Infrared Absorption Spectra of the Post-Discharge Region

Figure 3-14 shows the infrared absorption spectra of the post-discharge regions for N2, O2 and compressed air discharges in the 800-3600 cm^-1 range as measured by an in-situ FTIR technique. The strong absorption peak of 1055 cm^-1 for ozone was clearly observed in compressed air and O2 discharges because of the abundant oxygen species in the discharges. Note ozone was generated in the discharges, and carried downstream since it is relatively long lived. Table 3-1 shows the absorption peak value of 1055 cm^-1 for ozone in the post-discharge regions for N₂, compressed air and O₂ discharges in the range of z=4-20 mm. The results showed that very high levels of ozone were produced in the post-discharge regions of compressed air and oxygen cases. Observed abundant ozone played a key role in the inactivation of the bacteria as described in Chapter 6.

3.5 Summary of Chapter 3

Characterizations of the proposed planar DBD APPJ system are summarized as follows:

(1) Direct visualization and OES spectra aided to our understanding the changes in

plume color.

(2) OES measurements showed that abundant metastble nitrogen was generated in the post-discharge jet region, which was important in later applications. However, addition of oxygen species reduced the amount of metastable nitrogen tremendously because of reduced plasma density due to electro-negativity of the oxygen.

(3) Gas temperature measurements showed that gas temperatures were low enough in the post-discharge jet region for most of the applications studied in the thesis.

(4) Measurements showed that addition of oxygen enhanced the amount of ozone, which were important in bacteria inactivation.

Chapter 4

Hydrophilic Modification of the PP Film Using Nitrogen-Based DBD APPJ

In this chapter, hydrophilic modification of the PP film using the developed planar nitrogen-based DBD APP system has been described in detail. Details of the experimental setup and characteristics of the discharge have been shown in Chapter 2 and Chapter 3, respectively, and are skipped here for brevity. Several diagnostic techniques, including contact angle measurements, aging effect, AFM analysis and XS analysis, were employed to study the effect of plasma jet treatment on the hydrophilic property and their corresponding results are described in the following in turn. Some important findings have been summarized at the end of this Chapter.

4.1 Contact Angle Measurements

4.1.1 Stationary Conditions

Figure 4-1 shows the distribution of the measured contact angles of the PP film as functions of the downstream distance (z=2-20 mm) and the ratio of O₂/N₂ (%) (0.004%- 1.6%) immediately after the plasma treatment. Note that the measurement of the contact angle of the untreated sample was 103°. The results were obtained by averaging 3-5 measurements over a region of 10 mm by 10 mm of PP film with ±3°. The results indicated that the favorable operating conditions (contact angles < 30°) included: 1) z=6-9 mm and O2/N2 <0.008%; 2) z=2-10 mm and 0.04%<O2/N2 <0.3%; and 3) z=0-6 mm and 0.6%<O₂/N₂ <1.0%. The above observations could be explained by the

distributions of ozone and UV emission in the near jet region. It was interesting to learn that the lowest contact angle (23°) occurred at the ratio of O2/N2 near 0.06% at z=2 mm in Region 2, which was very close to the exit of the DBD. This could be attributed to the strong UV emission (200-300 nm) near this condition (see Figure 3-9), where the ozone could efficiently absorb and convert into an oxygen radical [Iwasaki et al., 2007], and then easily react with the surface, as will be demonstrated later. In addition, UV emissions in the range of 200-300 nm also played an important role in breaking the chemical bonds of C-H (4.2 eV) and C-C (3.8 eV) since, for example, the photon energy of 236.6 nm was 5.74 eV. In Region 1, the ozone concentration had a maximal value at z=6-9 mm (as shown in Figure 3-14) due to the entrainment of oxygen from the ambient, where the UV emission was still appreciable in order for the ozone to convert efficiently into an oxygen radical. In Region 2, both ozone concentration and UV emission were appreciable, which led to the production of a significant amount of oxygen radicals. In addition, as mentioned earlier, strong UV emission also played an important role when the PP film was non-stationary, as will be discussed next. In Region 3, the ozone concentration was very high (40-80 ppm) (see Figure 3-14), although UV emission became relatively low in the downstream region after z=10 mm (see Figure 3-9).

4.1.2 Non-Stationary Conditions

The measured contact angles for the non-stationary films have been summarized in Tables 4-1 and 3-3 at z=2 mm. Table 4-1 lists the contact angles for various O₂/N₂ ratios (0-1.6%) at a non-stationary speed of v=0.5 cm/s, while Table 4-2 summarizes the data at various non-stationary speeds for PP film (0.5-8 cm/s) for four typical O₂/N₂ ratios in which the contact angles were in the range of 26-39°, as shown in Table 4-1. Only the cases of z=2 mm for the non-stationary films have been presented here, since we found

that the contact angles became very large after this distance (e.g. >70° at z=4 mm with

that the contact angles became very large after this distance (e.g. >70° at z=4 mm with

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