# Influence of Multiple Factors on the Wettability and Surface Free Energy of Leaf Surface
## 3 Results and Discussion
### The Influence of Solid Surface on Wettability  
In order to investigate the effect of solid surface on wettability, four types of solid surfaces and four kinds of reagents were selected as solid surface and test liquid, respectively, which has been described in detailed in Section 2.1. Figure 7 shows the SEM images of rice and rape leaves at 1000 and 5000 magnification times, respectively.  
Figure 6: The variation of wettability with the increase of droplet falling height.  
The non-smooth surface structure of most plant leaves, such as furs, spines, protrusions, and other appendages, is a direct factor affecting the hydrophobicity of leaves [37]. From the view of the microstructure of leaves, the surface of rice leaves is rough with a regular arrangement of beaded microprotrusions (Figure 5a), whose average diameter is around 5-8 \(\upmu\)m [38]. Some studies showed that those microprotrusions are the mixture of wax on the leaf surface and silicon oxide with strong hydrophobicity [39], while the leaf surface is covered with dense acerose arrangement at the magnification of 5000 times (Figure 5c). In addition, the surface of rape leaves is relatively smooth and the waxy morphology of the leaves is irregularly arranged with the shape of blocks. The wax around the stomata is in the form of small rods [40], and there are filamentous appendages around the stomata (Figure 5d). In general, some results in current research indicated that the wettability of leaf surface with less waxiness and sparse villus distribution is better.  
Figure 8 compares the wettability of four solid surfaces tested with different PPP. It can be seen that the hydrophilicity of four solid surface from high to low is WSP, wax, rape leaf, and rice leaf. Specifically, when using water for experiment, only the contact angle with WSP was less than 90\({}^{\circ}\) and appeared as hydrophilic, while the water droplet with rice leave, rape leave, and wax appeared as hydrophobic. Besides this, the wettability of rice leaves changed from hydrophobic to hydrophilic while using three PPP to replace water droplet, among which the wettability of rice leaves was greatly improved by using pymetrozine and decreasing the contact angle to about 80\({}^{\circ}\).  
Figure 8: The variation of contact angle with the variation of solid surfaces and PPP.  
Figure 7: High magnification SEM of rape and rice leaves (adaxial surface): (**a**) rice of 1000 times; (**b**) rape of 1000 times; (**c**) rice of 5000 times; (**d**) rape of 5000 times.  
For the influence of PPP types on the wettability of solid surface, the contact angle between WSP and three kinds of PPP was from 50\({}^{\circ}\) to 60\({}^{\circ}\). The results of wettability between rice, rape, and wax on the three PPP showed that the contact angle of 25% suspending agent pymetrozine was slightly lower than that of 30% microemulsion prochloraz, and both of them were obviously superior to 20% wettable powder tricyclazole. It is concluded that the solubility of tricyclazole as a wettable powder is poor, whose solution is a mixture of powder and water and there is sediment in the solution after resting. Thus, the effect of improving wettability is not as good as that of suspending agent and microemulsion.