## 3 Results and Discussion
### Contact Angle of Bifenthrin Droplets with Adjuvants on Tea Leaves
Among the four adjuvants, the contact angle of droplets with trisiloxane organosilon Silwet L-77 decreased the most significantly with the change in concentration. According to the points corresponding to the initial time in Figure 5a,b, compared with the other three additives, the initial contact angle was the largest when the additive amount was small but decreased rapidly when the amount was more than 0.05%. The other three additives at the minimum concentration (0.01%) significantly decreased the contact angle of droplets at 0 s (65 \(\pm\) 2.5\({}^{\circ}\)), but the decrease in the contact angle was not obvious, as the concentration increased at this moment. This result could be because the trisiloxane surfactant molecules with low concentration diffused inside the droplet and accumulated on the surface of the droplet at a slower rate than the other three additives. Figure 4: Deposition state of distilled water droplets on the surfaces of (**a**) fresh bud and (**b**) young leaf photographed by the CCD camera. Figure 5: Contact angles of 10% bifenthrin diluent with different concentrations of four adjuvants on the surface of fresh bud (a,c,e,g) and young leaf (b,d,f,h). Moreover, Silwet L-77 and nonionic surfactant 6501 significantly decreased the contact angle of droplets on the surface of tea leaves over time, but there were differences between the above two adjuvants. The droplets mixed with Silwet L-77 rapidly expanded within 5 s, and the range of the contact angle greatly changed with the concentration. For instance, the pesticide droplets could only expand to a contact angle of about 60\({}^{\circ}\) at the 0.01% concentration, indicating partial wetting. Moreover, the contact angle could be rapidly reduced to 0\({}^{\circ}\) at the 0.2% concentration, indicating complete wetting. The concentration at this moment exceeded the critical wetting concentration (CWC) of Silwet L-77, a concentration above which a transition from partial wetting to complete wetting occurred at spreading over moderately hydrophobic surfaces [38]. This concentration was different in the leaves of the two growing stages due to the different surface morphology of the fresh bud and young leaf of tea. Similarly, some previous studies on rough surfaces also reported a similar phenomenon [33; 39]. The contact angle of bifenthrin droplets with 6501 also rapidly decreased with time, reaching 47 \(\pm\) 1.5\({}^{\circ}\) at a low addition amount (0.01%). As the concentration increased to 0.2%, the contact angle gradually decreased but did not decrease to an extreme state (0\({}^{\circ}\)) like Silwet L-77. The diffused trend of droplets with JFC and Greenwet 720 was similar. The droplets spread significantly within 15 s, after which the contact angle value decreased slightly, and the shape of droplets remained stable. The wetting phenomenon of adjuvants is related to their ability to reduce the surface tension of the mixed solution. The increased Silwet L-77 can rapidly reduce the surface tension of the mixture to the lowest value, leading to its extreme wetting and spreading ability. The critical micelle concentration (CMC) of Silwet L-77 surfactant was lower than that of the other kinds of trisiloxane surfactants in the past determination [20], indicating that lower surface tension can be achieved through a small addition. The relatively higher surface tension of the bifenthrin mixture of Greenwet 720 and JFC at each concentration (Table 2) resulted in the equilibrium of droplets at larger contact angles than the other two surfactants.