# Influence of Multiple Factors on the Wettability and Surface Free Energy of Leaf Surface
## 1 Introduction  
An efficacious and efficient spray application is an important goal to increase the benefits of plant protection products (PPP) used for crop pest and disease control [1]. The ultimate purpose of using different spray application techniques is to ensure an adequate deposition of active components of PPPs on target crops [2]. The PPPs are sprayed on the surface of the crop with a water or oil solution, and the wettability of leaf surface, adhesion force, spreading and retention behavior of PPP solution on leaf surface are the key factors affecting the exertion of PPP efficacy [3]. Generally, leaf surface wettability, which directly reflects the leaf surface hydrophilicity [4], is the result of the interaction between the surface tension of liquid and the properties of leaf surface. The leaf surface wettability affects the water absorption, gas exchange, pathogen infection, and other functions of the plant leaf [5]. Improving the effective utilization of PPPs, reducing at the same time the pesticide losses, is the main goal in the field of chemical spray application for pest and disease control [6]. The effectiveutilization rate of PPPs generally refers to the proportion of PPPs that play the role of pest and disease control in the total amount of PPPs. When applied with conventional spray technique, the effective utilization rate of PPPs is only 20-30% and only a small portion could be deposited on the target crops [7]. During the process of droplets falling on the surface of plant leaves, some droplets will adhere to the leaf surface, while some other droplets will rupture, rebound, and slip off [8]. The wettability of droplet on leaf surface is closely related to the properties of leaf surface and PPP; the formulation of PPP can be varied according to the specific crop, thus the wettability of leaf surface should be clarified initially. To specifically quantify the wettability, contact angle and surface free energy (SFE) are the most commonly used indicators [9]. Contact angle is a quick and convenient indicator to evaluate the wettability between test liquid and solid surface [10]. When the contact angle is \(0^{\circ}\), the liquid completely infiltrates the solid surface; when the contact angle is \(180^{\circ}\), the liquid is spherical on the solid surface and the liquid completely un-infiltrated the solid surface; when the contact angle is between \(0^{\circ}\) and \(90^{\circ}\), it is considered to be hydrophilic; when the contact angle is between \(90^{\circ}\) and \(180^{\circ}\), it is considered to be hydrophobic [11]. However, different scholars have defined different boundaries between contact angle and wetting range according to the study purpose [12]. Moreover, SFE belongs to one of the basic thermodynamic properties of solid surface and it does not change with the variation of test liquids [13]. Generally, the hydrophobic surface tends to be low energy while the hydrophilic surface tends to be high energy [14]. On one hand, some researchers have indicated that the wettability of plant leaf surface is closely related to the content and morphological distribution of wax on a leaf surface [15], density, texture, and distribution of leaf villus [16], and the number of stomata [17]. [PERSON] et al. [18] found that some hydrophobic leaves tend to have high-density wax distribution, while the wettability of the leaves with more wax content is relatively small. [PERSON] et al. [19] studied the wettability of rape leaf, and their results suggested that the fresh rape leaves exhibited hydrophobicity but with a high adhesion force. Besides this, [PERSON] et al. [20] supposed that the compound structure of mastoid and waxy layer was the fundamental reason affecting the wettability of lotus leaves. The research of [PERSON] et al. [21] pointed out that the hierarchical structures of rice leaf surfaces was the basic mechanism of rice leaf hydrophobicity. Moreover, [PERSON] et al.