## 3 Results
### The Influence of the Degree of Electrodes Asymmetry on the NP Synthesis with the UDP
According to the SEM images of electrode ends shown in Figure 11, one can observe the structure of their typical surface after the prolonged treatment by SDs. Since craters are found only on the cathode, we can logically assume that the anode is covered with NPs synthesized on the cathode. Thus, NPs synthesized from the cathode material are partially transferred to the anode and partially carried away in the form of aerosol by the transport gas. Herewith, the average size of craters turns out to be much smaller for the UDP than for the ODP (Figure 7c,d and Figure 11c,d). Moreover, the average crater's dimensions were found to be larger at the area of spark channel location on the cathode (Figure 11c) opposite the growing protrusion on the anode than at the cathode area aside the spark channel location (Figure 11d). Figure 10: Time dependences of the modal size of NP agglomerates and their concentrations during the electrodes wear by the UDP for electrodes ends arranged (**a,d**) in parallel and at the angles of (**b,e**) 3\({}^{\circ}\) and (**c,f**) 10\({}^{\circ}\). A protrusion formed on the anode with the UDP was studied in optical and scanning electron microscopes (Figure 12). The photo in Figure 12a shows the localization of the spark channel in the discharge process at the stage of growth of the protrusion from the upper electrode (anode). Moreover, before the main protrusion growth begins, many clusters of NPs are gather on the anode in the form of micro-protrusions, which are located in a ring at a distance of about 2 mm from the center of the anode (in Figure 12b inside the dashed circle). When one of those protrusions begins to dominate, the growth of remaining protrusions stops, except for a single one quickly moving to the cathode, closing the interelectrode gap with a conducting bridge. Localization of the SD channel from a growing protrusion is similar to the previously observed SD processes with the pin-to-plane electrode configuration for pin-cathode [37] and pin-anode cases [38]. Figure 11: SEM images of typical surfaces of the (**a**) anode and (**b**) cathode after prolonged processing in the SD with the UDP and size distributions of craters on the cathode (**c**) at the area of spark channel location opposite the growing protrusion on the anode and (**d**) at the area aside the spark channel location for the case of the initial parallel arrangement of electrodes’ ends. The microstructure study of the growing conductive protrusion (Figure 12c) in SEM allows us to conclude that in the central part of the protrusion the stacking of dendrite-like NP agglomerates is dense (Figure 12f) and on its periphery, the NP agglomerates assemble in the form of clusters with a loose stacking (Figure 12d,e). At the same time, many dendrite-like agglomerates in the central part of the protrusion are molten, probably due to their participation in the transfer of the SD current to the anode (Figure 12f). In experiments with the UDP, we analyzed the dispersed composition of aerosol NPs by TEM and measured the mass loss of the cathode and anode. According to a series of TEM images of NPs (for example, Figure 13a-c) for each of the three variants of the electrodes' ends arrangement, the mass-size distributions of NPs (Figure 13d-f) were determined. It indicates that with an increase in the degree of electrodes asymmetry the NP agglomeration increases. In the case of electrodes parallel ends, a large number of isolated NPs were observed (Figure 13a). In the case of the electrodes' ends arrangement at the angle of 3\({}^{\circ}\), the particles mainly formed small agglomerates. At the angle between ends of 10\({}^{\circ}\), the energy release density in the discharge became sufficient for the fusion of NPs with the necks' formation specific for strong aggregation. At the same time, in all cases, a great number of NPs with sizes of 2-5 nm were observed, which did not make a significant contribution to the mass distribution.