关键词: 水稻, 表面张力, 临界胶束浓度, 接触角, 乳突

Abstract: 【Objective】In order to provide a basis of mechanisms controlling retention of pesticide droplets on target leaf, characterization of rice leaf surface and behavior analysis of single droplets of trisiloxane surfactant (Silwet-408) solutions on rice leaf surface with different inclination angles were studied.【Method】Scanning electron microscope (SEM) was used for observation of rice leaf surface characteristics and the critical surface tension (CST) of rice leaf was determined by Zisman method. Surface tension of Silwet-408 solutions at concentrations of 0, 3.91, 7.81, 15.63, 31.25, 62.50, 125.00 and 250.00 mg·L^-1 was measured and the critical micelle concentration (CMC) of Silwet-408 was also measured according to the change of surface tension of Silwet-408 solutions. Then the contact angle of a single droplet on the rice leaf surface with 3 inclination angles was determined by contact angle meter.【Result】SEM images showed that 3 types of hairs and densely covered papillae were observed on both the adaxial and abaxial sides of rice leaf. Densities of papillae, with significant difference between the adaxial and abaxial rice leaf surface, were ((12.4±0.7)×10³) and ((7.6±0.8)×10³)/mm², respectively. In contrast, no significant differences in stomatal length or stomatal density were found. The CMC of Silwet-408 was 78.5 mg·L^-1 and surface tension value of correspondingsolution at CMC was 20.77 mN·m^-1. The estimated CST values of the adaxial and abaxial rice leaf surface were 29.90 and 31.22 mN·m^-1, respectively. Among the measured Silwet-408 solutions, the droplets of solutions at lower concentrations (0, 3.91, 7.81 mg·L^-1) rolled off rice leaf with different inclination angles on condition that their surface tensions were more than the CST of rice leaf and Silwet-408 concentrations were less than the CMC. Surface tensions of solutions at concentrations 15.63, 31.25, and 62.50 mg·L^-1 were less than the CST of rice leaf and Silwet-408 concentrations were less than the CMC. Droplets of solutions at concentrations 15.63 and 31.25 mg·L^-1 adhered to rice leaf with lower inclination angle (30°). On the contrary, droplets rolled off rice leaf with higher inclination angle (60°). Droplets of solutions at 62.50 mg·L^-1 adhered to rice leaf and significant differences existed in decreasing speed of θ variation and wetting hysteresis. The droplets of solutions at higher concentrations of 125.00 and 250.00 mg·L^-1 adhered to rice leaf with different inclination angles on condition that their surface tensions were less than the CST of rice leaf and Silwet-408 concentrations were more than the CMC. After 40 s, no significant differences were observed in decreasing speed of θ variation and wetting hysteresis. Analysis results of advancing and receding angles on rice leaf surface with different inclination angles showed that advancing angles (θ_a) were larger than receding angles (θ_r). Within 40 s, both θ_a and θ_r decreased gradually.【Conclusion】The higher hydrophobicity of rice leaf is mainly ascribed to densely covered wax papillae on rice leaf surface and it may be related to hair length and stomatal density of rice leaf. Rice leaf surface is low energy. When surface tensions of Silwet-408 solutions are less than the CST of rice leaf surface and the concentrations of Silwet-408 are more than the CMC, droplets would show better adhesion on rice leaf surface with different inclination angles and wetting. Because of larger surface tension, droplets of low concentration solutions on rice leaf surface with different inclination angles are easier to roll off. That θ_a of droplets of Silwet-408 solutions on rice leaf surface with different inclination angles are always larger than θ_r illustrates wetting hysteresis. The phenomenon of wetting hysteresis indicates that rice leaf surface is rough and roughness is closely related to e.densely covered papillae on rice leaf surfac

Key words: rice, surface tension, critical micelle concentration (CMC), contact angle, papilla