## 1 Introduction  
Pesticides and fertilizers are widely used in agricultural production due to their substantial agronomic and economic benefits [1]. The effective adhesion and diffusion of pesticide droplets on the surface of leaves can improve the efficiency of pest control [2]. However, pesticide droplets can easily fall off and poorly adhere because of the fluff and wax on tea leaves used to protect the buds against environmental damage and reduce transpiration [3]. Ineffective spraying occurs when pesticides insufficiently deposit on the hydrophobic surface of tea leaves [4]. Although increasing the spray quantity is preferred to improve the pesticide solution deposition in tea gardens, it can lead to serious pesticide wastage. As a consequence, excessive pesticides will increase the risk of residual pesticides in tea and cause environmental pollution [5]. Recently, spray additives have played an important role in the reformation of pesticide formulation and the control of crop pests. [PERSON] et al. [6] pointed out that spray adjuvants can significantly improve the wettability of droplets and increase deposition on target crops by altering the physical properties of pesticides, thus effectively reducing pesticide consumption [7, 8]. Furthermore, as the major spray additives used in tea protection, surfactants [9, 10], emulsified oils [11], and polymers [12, 13] are widely used to promote diffusion by reducing the surface tension of droplets and the leaf-droplet interfacial tension caused by a special molecular adsorption property [14]. In recent decades, many scholars have focused on the effect of different additives on the spray deposition of various plant targets. For example, [PERSON] et al. [15] experimentally evaluated six different surfactants and oil adjuvants to improve foliar spray deposition. The results indicated that adjuvant concentration is essential for adequate deposition compared to other components. However, inadequate concentrations may not achieve the diffusion effect, which is required for the improvement of deposition volume and quality. [PERSON] et al. [16] quantitatively examined the retention and coverage of 10 commercially available tank-mix adjuvants on pea (_Pisum sativum_ L.) and barley (_Hordeum vulgare_ L.) leaves at different growth periods. Surfactants, especially silicone, caused the largest increase in spray coverage on both surfaces of pea and barley leaves compared with the emulsifiable oil and film former. Meanwhile, a study evaluated the dispersion and evaporation of single 300 mm diameter droplets amended with four spray adjuvants at different concentrations on four types of surfaces (abaxial and adaxial leaflet surfaces, petiole, basal stem) of soybean (_Glycine max_ (L.) Merr.). The results showed that droplet-wetted areas increased with the increase in adjuvant concentrations nonlinearly and differed on the four surfaces: the abaxial surface had the largest wetted area, followed by the adaxial surface, the petiole, and then the basal stem [17]. Furthermore, [PERSON] et al. [18] compared the wettability of pesticide droplets deposited at different positions (paraxial and distal to the veins, midvein, and secondary vein) on eucalyptus leaves and concluded that the wetted area was a minimum on the midrib, and the wetted area on the secondary vein was slightly larger than that on the interveinal area. Adjuvants have positive effects on the wetting behaviors of pesticides, which can be attributed to chemical and physical characteristics. Furthermore, adjuvants can change the physicochemical properties of pesticide solutions, such as viscosity and surface tension [19]. The mechanism of the wetting state has been widely discussed by investigating the behavior of surfactant adsorption under equal conditions [14]. [PERSON] et al. [20] found that the surface tension of liquids substantially decreased with the increase in methylated silicone due to its superior surfactant properties.