## 3 Results and Discussion
### Adhesion Tension and Work between Bifenthrin Droplets and Tea Leaves  
The wetting process of pesticides on the surface of tea leaves can be divided into adhesion and spreading [41]. Whether pesticide droplets can effectively adhere to the surface of tea leaves is the key to the automatic wetting process. Moreover, not only the effective attachment but also the rapid diffusion of droplets is required to enhance pesticide efficacy, achieving the large coverage area and ideal plant protection effect. The change value of the free energy \(\Delta G\) during the wetting process can be calculated as [42]:  
\[|\Delta G|=\gamma_{\rm sg}+\gamma_{\rm lg}-\gamma_{sl}=W_{a} \tag{2}\]  
\(W_{a}\) represents the adhesion work, which is the minimum work required to pull the contacted solid and liquid away from the interface. The adhesion work reflects the binding capacity at the solid-liquid interface. The larger the \(W_{a}\) value, the stronger the solid-liquid  
Figure 6: Relationship between \(\cos\theta\) and the surface tension of liquid. Each point represents the corresponding value of the four adjuvants at different concentrations. The straight line is the fitted image, and the specific equation is marked in the upper right corner of each figure: (**a**) Silwet L-77; (**b**) 6501; (**c**) JFC; (**d**) Greenwet 720.  
bond. The adhesion tension reflects the wetting ability between solid and liquid, which can be calculated as:  
\[\beta=\gamma_{\rm sg}-\gamma_{sl} \tag{3}\]  
The larger the value is, the better it is for wetting. The adhesion tension \(\beta\) and adhesion work \(W_{a}\) can be deduced from the relationship between the equilibrium contact angle and three interfacial free energies in Equation (1):  
\[\left\{ \begin{array}{l}\beta=\gamma_{lg}\cos\theta\\ W_{a}=\gamma_{lg}(1+\cos\theta)\end{array} \right. \tag{4}\]  
The adhesion tension and adhesion work can be used to evaluate the wettability and adhesion of the adjuvant droplets when the surface tension of the mixture and the contact angle of droplets on the surface of tea leaves are available. The relationship between wetting and adhesion in adjuvant solutions can be obtained through this method [43]. The trends of the adhesion tension and work of bifenthrin droplets with different adjuvarants on the surfaces of the fresh bud (a) and young leaf (b) changing with the concentration are shown in Figure 7. The adhesion tension value was negative when no adjuvarants were added, indicating that the bifenthrin diluent was difficult to wet on the surface of tea leaves. Adding a small amount of adjuvant (0.01%) can rapidly increase the adhesion tension to a positive value and improve the wetting and spreading behavior of droplets. In addition, the adhesion work decreased with the increasing additive concentration, indicating that the adsorption between the droplets and tea leaves decreased with the improved wettability of the bifenthrin solution; that is, the pesticide can be easily lost from tea leaves. Figure 7: Adhesion tension and adhesion work of bifenthrin droplets with different adjuvarants on the surfaces of the fresh bud (**a**) and young leaf (**b**) vary with the concentration. Therefore, the adhesion between droplets and tea leaves should also be considered in selecting adjuvants to improve the retention of pesticides. The bifenthrin droplets with organosilicon surfactant Silwet L-77 can rapidly spread on the surface of young leaves in a short time (Figure 8). However, it can easily cause the loss and waste of the pesticide due to the small adhesion work between the droplets and tea leaves. The bifenthrin diluent containing surfactant 6501 and JFC can show better wettability and adhesion on the surfaces of the fresh bud and young leaf, with a lower cost than Silwet L-77, resulting in better economics for large-scale pesticide spraying operations in tea gardens. Moreover, the adhesion tension and work tend to be stable when the concentration is greater than 0.1%.