# SDBD Flexible Plasma Actuator with Ag-Ink Electrodes: Experimental Assessment
## 3 Results and Discussion  
Figure 3a-c provide representative oscillograms of the actuator driving voltage, DBD current, and plasma emission integrated over ultraviolet-near infrared (UV-NIR) wavelengths, respectively. Clearly, the main ionization and excitation processes are related to the rising part of the driving voltage, whereas much weaker current and emission are observed during the falling part. This fact has been mentioned and studied extensively in other DBD-based setups, showing that the propagation of cathode- and anode-directed streamers is involved [23]. During the rising part of the voltage, primary electrons propagate towards the anode, promoting ionization and excitation processes. The qualitative similarity between the current and emission impulses (Figure 3b,c) is underlined. At the same time, the resultant heavy positive ions are accelerated towards the dielectric-covered cathode, transferring momentum and energy to the surrounding air molecules, leading eventually to the flow field patterns that are illustrated below. On the other hand, during the falling part of the voltage, we probably deal with anode-directed streamers. According to this assumption, since the exposed electrode is now negatively driven, electrons are repealed towards the dielectric-covered electrode. They are following the field lines and diverging from each other giving a diffused and weaker excitation. The impulsive form of the current (Figure 3b) and the optical emission (Figure 3c) are in accordance with the conventional photographs of the DBD pattern shown in Figure 4. The induced plasma is associated with filamentary DBDs. The number and the electron density of the filaments along the flat electrodes increase for higher voltage amplitude, but a homogeneous glow discharge is never reached with the voltage amplitudes used here. It is considered that the electric field is quite elevated due to the electrode design (thickness of a few micrometers only). Thus, important fluid perturbations are anticipated because of electrohydrodynamic forces [2; 24; 25; 26]. Schlieren images are in line with this statement (discussed later). Based on the above observations, it becomes apparent that the power is principally delivered to the actuator during the positive cycle of the driving voltage. Figure 5 shows that, under the present experimental conditions, the actuator consumes a mean power of less than 10 W. An increase at the voltage amplitude of just 2 kV\({}_{\text{PP}}\) leads to power doubling (Figure 5a versus Figure 5b). However, the consumption remains practically unchanged over at least 24 h of operation (accumulative). The latter is an implication of a quite reliable actuator. Figure 3: Representative waveforms of (**a**) the high voltage, (**b**) DBD current, and (**c**) wavelength-integrated optical emission over two periods. The projection of the optical fiber acceptance cone on the actuator surface corresponds to a circle of 12.5 mm in radius, centered on the point (x = 0, y = 0, z = 0). However, despite the consistency of the actuator in terms of power consumption, both dielectric and electrode degradation are observed as the operation time accumulates. Figure 6 demonstrates this aging effect, as it is exhibited on the top surface of the actuator. The photo in Figure 6b refers to 24 h of accumulative operation and the worst-case stress scenario (i.e., 16 kV\({}_{\text{pp}}\)). The actuator of Figure 6b remains functional, but the quantification of its material degradation and its possible efficiency deterioration might be the task of another study. A better insight into the species contributing to the plasma-induced flow field may be gained by the OES spectrum of Figure 7a. Although a much wider wavelength range has been examined (see Section 2 for grating specifications), only N\({}_{2}\)(SPS) and N\({}_{2}^{+}\)(FNS), shown in Figure 7a, are detectable.