## 3 Results and Discussion  
### Characterisation of the DBD  
The voltage applied to the electrodes and the discharge current were measured using voltage and current probes. Ten thousand measurements were recorded over a 2.0 \(\times\) 10\({}^{-4}\) s time period. This corresponds to five recorded cycles of the sinusoidal voltage and discharge current, which are illustrated by way of example for experiment 1 of the experimental array (Figure 5).  
The applied voltage and the discharge current in Figure 5 are phase-shifted by approximately 90 degrees. The voltage follows the current. The reason for these two effects are the capacitive properties of the electrode configurations. The discharge current has also a sinusoidal shape, but in addition, several high frequency peaks are superimposed at the maximum amplitude. The sinusoidal part of the discharge current is the displacement current, which is created by the changing polarisation of the applied electrical field, which causes the displacement of electric particles in the nitrogen gas and the dielectric. The displacement current flows independently of whether the plasma is ignited or not.  
The high frequency peaks superimposed on the current signal originate from the ignited plasma. They mark the increased current flow while the gas loses its dielectric properties and becomes conductive due to the high amount of free charge carriers in the plasma. The individual peaks are very narrow, which corresponds to a short burning time of the individual filaments in the nanosecond range. This clearly indicates that the discharge regime is filamentary.  
The observable difference of the current peak size on each half of the sinusoidal waveform is a result of the asymmetric electrode configuration, which combines a flat electrode with a sawtooth electrode. It is assumed that the sawtooth electrode is the cathode during the negative polarisation of the current. The seed electrons, which are accelerated from the cathode to the anode, would then receive more kinetic energy during the negative polarisation at the beginning of their acceleration, because the electrical field is strongly increased near the points of the teeth in the sawtooth electrode due to the edge effect. As a result of the increased kinetic energy, these seed electrons can cause more ionisation, which increases the current because more free electrons travel from the cathode to the anode. In the reverse case, when the seed electrons start near the flat electrode, which is then the cathode during the positive polarisation, their kinetic energy is not additionally increased by an edge effect. Thus, the current peaks are lower during the positive polarisation. Discharge powers estimated for each set of conditions in the experimental array are listed in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline
**Number of Experiment** & **Discharge Power [W]** & **Average Water Contact Angle, Degrees** \\ \hline
1 & 5.5 & 80.4 \\
2 & 5 & 61.5 \\
3 & 5.3 & 67.6 \\
4 & 5 & 74.7 \\
5 & 5 & 71.7 \\
6 & 6.8 & 76.9 \\
7 & 8.5 & 79.8 \\
8 & 6.7 & 75.8 \\
9 & 4.6 & 72.2 \\
10 & 5.4 & 68.3 \\
11 & 6.8 & 71.5 \\
12 & 5 & 70.7 \\
13 & 5.1 & 78.0 \\
14 & 5.2 & 69.2 \\
15 & 4.6 & 64.1 \\
16 & 8.5 & 67.3 \\ \hline \end{tabular}
\end{table}
Table 2: Estimated discharge power and average water contact angles for BOPP film treated in nitrogen.  
Figure 5: Measured voltage applied to the three-tooth electrode configuration and the resulting discharge current for experiment 1 of the experimental array.