# Characterization and Optimization of a Conical Corona Reactor for Seed Treatment of Rapeseed
## 3 Results and Discussion
### Treatment of Rapeseed
Yet, the duty cycle also affects the discharge temperature. Since most seeds should not be manipulated at high temperatures to prevent damage (for long time exposure), the temperature evolution caused by duty cycle variation needs to be investigated. In a study about the effects of seed germination conditions on the germination of _Pinus bungeana_ seeds, [PERSON] et al. (2020) [41], reported the importance of temperature. Increasing the temperature to which seeds are exposed benefits the seed germination process until an optimum value, after which it decreases promptly [41]. Figure 6 exhibits temperature curves assessed for different voltage duty cycles after 3 min of plasma exposure: for the gas temperature and the temperature measured at the seed coat. Three minutes of treatment was chosen as a middle point from the gas temperature assessment (Figure 3). For this exposure time, the gas temperature rose linearly with an increase in duty cycle reaching up to 70 \({}^{\circ}\)C for D = 90%. Differently, the estimated temperature at the seed coat increased initially with a similar pace but saturated for higher duty cycles, reaching a maximum of 50 \({}^{\circ}\)C for 90% of the voltage duty cycle. Therefore, even though the overall temperature remarkably rose inside the reactor after 3 min of plasma treatment, the seeds were not excessively heated. When plasma is applied to a material's surface, its characteristics are modified [42]. Following the advance of plasma agriculture studies, the modifications caused by plasma on the surface of seeds have been investigated [43]. Several studies reported chemical modification of the seed coat after exposure to plasma. [PERSON] et al., 2017 [44], reported that plasma treatment can oxidize the seed coat and attach new nitrogen radicals. [PERSON] et al., 2021 [19], observed an increase in oxygen-carbon ratio in wheat and barley seeds after treatment using a DBD reactor operating with argon. Some studies also reported physical changes after plasma treatment. [PERSON] et al., 2018 [45], reported the formation of cracks in the seed coat caused by treatment using low-pressure RF nitrogen plasma. As a result of physical and chemical changes in the seed coat, its wettability may also change. This can be accessed via contact angle measurements. Studies, such as [PERSON] et al., 2016 [46], and [PERSON] et al., 2018 [47], had already reported reductions in apparent contact angle in seeds after treatment using cold atmospheric discharges. [PERSON] et al. (2015) [31],  
Figure 5: Influence of the number of seeds on the power density for zero (empty reactor) to around 1015 seeds (Petri dish bottom completely filled). reported a maximum reduction of 30.4% in apparent contact angle in rapeseed treated with low-pressure plasma. Other studies also observed a reduction in the apparent contact angle in treated rapeseed seeds, such as a reduction of 20% in [16] and a maximum reduction of 41.57% in [27]. [PERSON] et al. (2020) [30], reported no significant reduction in the water contact angle in rapeseed treated indirectly using plasma processed air. Figure 7 presents the WCA measurements of rapeseed seeds treated with the funnel reactor using different voltage duty cycles. Seeds used in this study displayed a hydrophobic surface with a WCA of around 110\({}^{\circ}\). After plasma treatment for 3 min, the seed surface wettability increased for all used voltage duty cycles. The seed coat became hydrophilic, and the contact angle value decreased for higher duty cycle percentages. For D = 10%, the water contact angle dropped to around 65\({}^{\circ}\) (reduction of 37.7%) and this value reduced even more for D = 90%, where a contact angle of 40\({}^{\circ}\) (reduction of 64.2%) was obtained. Interestingly, only a small wettability recovery of around 20% was observed after one week (data not shown), where the fastest recovery period happened in the first two days. Suggesting that plasma modifications persisted on the seed coat for longer periods as investigated in [19].