Bounce Behavior and Regulation of Pesticide Solution Droplets on

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Agricultural and Environmental Chemistry

Bounce Behavior and Regulation of Pesticide Solution Droplets on Rice Leaf Surfaces Li Zheng, Chong Cao, Lidong Cao, Zhuo Chen, Qiliang Huang, and Baoan Song J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02619 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Journal of Agricultural and Food Chemistry

Bounce Behavior and Regulation of Pesticide Solution Droplets on Rice Leaf Surfaces Li Zhenga, Chong Caob, Lidong Caob, Zhuo Chena, Qiliang Huangb,*, Baoan Songa,*

a

State Key Laboratory Breeding Base of Green Pesticide and Agricultural

Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Research and Development Center for Fine Chemicals, Guizhou University, Guiyang 550025, China b

Key Laboratory of Integrated Pest Management in Crops, Ministry of Agriculture,

Institute of Plant Protection, Chinese Academy of Agricultural Sciences, No. 2 Yuanmingyuan West Road, Beijing 100193, China

* Author to whom correspondence should be addressed Bao-An Song: [email protected] Qiliang Huang: [email protected]

Tel: 86-851-83620521. Fax: 86-851-83622211 Tel: 86-10-62890876. Fax: 86-10-62890876. 1

ACS Paragon Plus Environment


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Abstract: Pesticide spray droplets can damage ecological environments and negatively

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affect biodiversity if they reach non-target areas. Effective retention of pesticide droplets

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on plant surfaces is an important challenge. In this study, a high-speed camera was utilized

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to visualize the bounce behavior of droplets of different pesticide solutions on rice leaf

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surfaces. We explored the addition of surfactants (SAAs) to different pesticide solutions

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and altered a pesticide solution system to prevent or regulate droplet bounce behavior.

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Experimental results indicate that the addition of SAAs to a pesticide solution can inhibit

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the bouncing of droplets on rice leaf surfaces. Additionally, a water-in-oil (EO) emulsion

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can not only significantly inhibit droplet rebound on a superhydrophobic surface, but also

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quickly and automatically spread pesticide droplets to maximize the wetting area.

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Therefore, this work effectively improves the utilization of pesticides and reduces

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environmental pollution.

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Keywords: Superhydrophobic surface; Bounce behavior; Pesticide solutions; unmanned

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aerial vehicle (UAV); water-in-oil (EO) emulsion; Polymer emulsifier

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Introduction

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Pesticides play an important role in controlling agricultural pests and ensuring the

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high quality of agricultural products.1 Most pesticide products are applied via spraying.

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However, it has been reported that the effective utilization rate of traditional pesticide

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formulations is typically less than 10%,2 with approximately 0.1% of pesticides actually

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reaching their target.3 Modes of pesticide loss include evaporation, spray drift, splash or

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shatter, bounce off and so on.4–7 Bounce is the main loss mode, significantly reducing the

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efficiency of deposition and retention of droplets on plant surfaces.8–11 Spray droplets can

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damage ecological environments and negatively affect biodiversity if they reach non-target

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areas.12 Maximizing the deposition and retention performance of spray droplets on the

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surfaces of target plant leaves plays an important role in achieving optimal biological

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activity and minimizing negative side-effects on ecological environments.

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The behavior of droplets on the surfaces of plants is complex. It depends on the fluid

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properties of the sprayed liquid, plant interface geometry, and physicochemical properties.

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In recent years, the impact of droplets on superhydrophobic surfaces has attracted increased

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attention from researchers.13–14 Modifying droplet deposition by changing fluid properties

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is a common approach to increasing pesticide retention. One method is to add a surfactant

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(SAA) to a spray solution to promote the spreading of droplets on surfaces by reducing

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surface tension.15 However, SAA liquids still slide and bounce at tilted angles.16 Another

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method requires altering the rheological properties of a fluid by adding a small amount of 3



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a polymer additive to a spray solution.17–19 Bergeron et al. found that the addition of a small

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amount of poly-(ethylene oxide) (PEO) significantly inhibited droplet bounce without

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affecting fluid properties.20 However, despite considerable research on polymer additives,

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the physical mechanisms at play have not yet been conclusively elucidated.21

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The surface properties and structures of plant leaves also influence droplet retention.

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When a droplet hits the surface of a plant, there is a tendency for the droplet to splash or

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rebound and land on the ground, even for low-kinetic-energy drops. especially in

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superhydrophobic surface. This problem is magnified by the fact that leaves are covered

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with a superhydrophobic surface in the form of a highly crystalline wax layer.22–23 Such

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surfaces maintain their ability to repel penetrating droplets under dynamic conditions.24–25

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Wu et al. hypothesized that micron-sized mastoid structures and nanostructures lead to

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superhydrophobic properties in rice leaves, while third-order prismatic structures create

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energy barriers in orthogonal directions,26 leading to anisotropic sliding phenomena.27–29

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Maher reported an in situ precipitation method that compensates for the hydrophobicity of

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surface defects and captures droplets during impact.30 Song demonstrated that adding a

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small amount of sodium bis(2-ethylhexyl) sulfosuccinate vesicle surfactant significantly

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suppresses droplet rebound on superhydrophobic surfaces.16

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Another factor that affects spray deposition is the application system, which can

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significantly affect the characteristics of the spray. The process of pesticide spray

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application is very complex, particularly when considering the rapid development of 4


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unmanned aerial vehicles (UAVs) for plant protection in recent years. Compared to

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conventional spraying methods, the volume per unit area has decreased sharply in UAV

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applications. For pesticides without dilution or small levels of dilution, it is very difficult

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to prevent bouncing by utilizing spray additives and such modified solutions often cannot

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meet application requirements. Additionally, no countries require test data for the

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evaluation of liquid properties when registering pesticides and users typically follow the

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recommended concentrations provided by manufacturers. Therefore, it is of great

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theoretical and practical value to study how pesticides are lost when bouncing off their

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targets and explore methods to control this loss. Considering this situation, we previously

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prepared and characterized stable water-in-oil (EO) emulsions of isoprothiolane. Through

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the selection of SAAs and optimization of formulations, deposition performance was

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studied further. Thus far, few new results have been reported in this field.

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In this study, for rice leaf surfaces, which represent a typical superhydrophobic plant

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interface, we compared the bounce behaviors of different pesticide solutions by utilizing a

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high-speed camera to visualize the impacts of droplets on the surfaces of rice leaves. We

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explored the addition of SAAs to different spray solutions and possible changes to pesticide

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solution systems through formulation optimization (water-in-oil (EO) emulsions) to

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prevent or regulate droplet bounce behavior. This work can guide the future use of

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pesticides to reduce loss and prevent pollution.

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Materials and methods 5



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Plants

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Rice (Nanjing 11) was cultivated from seeds in individual pots containing

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vermiculite/fertilized soil under outdoor conditions. The leaves were tested after growing

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for approximately four weeks. To keep the leaves fresh and maintain the authenticity of the

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experiment, the leaves were not removed from the plants. Additionally, we worked

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exclusively on dry surfaces in this study.

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Materials

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The preparation of solutions is described in Table 1. The 75% tricyclazole WP was

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purchased from Jiangsu Changqing Agrochemical Co., Ltd. The 75% tricyclazole WG was

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purchased from Zhejiang Sega Science and Technology Co., Ltd. The 20% fenoxanil SC

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was purchased from Jiangsu Changqing Agrochemical Co., Ltd. The 2% kasugamycin AS

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was purchased from Hebei Boken agriculture Co., Ltd. The 40% isoprothiolane EC was

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purchased from Sino-Agri Leading Biosciences Co., Ltd. The 20% isoprothiolane EO

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(water-in-oil) emulsion was prepared in our laboratory. 43 The organosilicone synergist

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GY-S903 and GY-UTMAX SAAs were purchased from Beijing Grand AgroChem Co.,

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Ltd. The polymeric emulsifier A-7 was purchased from the Dauni Research Center of

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Advanced Science & Technology Co., Ltd.

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High-speed imaging (drop impact apparatus)

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As shown in Fig. 1, our system consisted of a high-speed digital video camera, digitally

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controlled syringe pump, imaging system, and LED light source. The digitally controlled 6


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syringe pump (µSP6000, Syramed) with constant speed generated droplets from a precision

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flat-tipped syringe needle with a 0.06-mm internal diameter (34 gauge). The initial

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diameter of a spherical droplet was D0 = 2 ± 0.3 mm. The liquid flow rate was sufficiently

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low to obtain a nil initial drop speed (0.05 mL/min). The droplets fall perpendicular to the

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rice leaves from the same initial height, ensuring that each droplet would have the same

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impact velocity (V = 0.25 m/s). A high-speed camera (Photron, Fastcam Apx-Rs) was

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utilized to record the impacts of droplets at 9000 frames per second with a resolution of

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512 x 384 pixels on the rice leaf surfaces. The camera was equipped with a long-distance

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microscope (Hirox OL-35, Tokyo, Japan). Shadow images were analyzed by utilizing the

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Image-Pro Plus software to track droplet boundaries and quantitatively study the droplet

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bounce process.31 All experiments were conducted at 25 °C with a relative humidity of

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51%.

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Surface tension measurements

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The surface tensions of the pesticide solutions (Table 1) were measured based on the

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Wilhelmy plate method utilizing a DCAT 21 tensiometer (Data Physics, Germany) at 298

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± 0.1 K. Prior to each measurement, the platinum plate was sterilized under an alcohol

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flame after being cleaned with deionized water and ethanol. The surface tension of water

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was utilized to calibrate the tensiometer and ensure cleanliness of the plate and glassware.

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The samples were measured until their surface tension values remained constant, indicating

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that equilibrium had been reached. Three consecutive measurements were taken for each 7



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sample and the standard deviations were no more than ± 0.20 mN/m.32

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Contact angle measurements

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By utilizing the OCA 20 contact angle meter (Data Physics, Germany) at 298 ± 0.1 K

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with a relative humidity of approximately 65% based on the sessile drop method, the

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contact angles of the pesticide solution (Table 1) droplets on the rice leaf surfaces were

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measured. In our experiments, 3 uL droplets were syringed and immediately deposited on

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the leaf surfaces (within 360 s of depositing the drop). Measurements were repeated three

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times by depositing additional drops at new locations.

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Wetting and spreading

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A VHX-2000 three-dimensional microscope was utilized to observe the surface

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morphology and wetting process of the EO solutions with depth-of-field and super-

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resolution imaging modes at a magnification range of 0.1–5,000 times.

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Results and discussion

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Comparison of the bouncing behaviors and regularities of different traditional

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solutions

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In this experiment, we compared the impacts of droplets of different traditional

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solutions on the surfaces of rice leaves. This experiment revealed the dynamics of drops as

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they collided with rice leaves, expanded, and subsequently rebounded.

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As shown in Fig. 2a, the solution droplets for two different dilutions of kasugamycin

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AS (AS3,000/AS10,000) impacting superhydrophobic rice leaves exhibited complete rebounds 8


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in spherical or pancake shapes. The same bouncing behavior was observed four times

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(Supplementary Movie 1). When the kinetic energy depleted, the droplets stopped

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bouncing. Tricyclazole WP (WP500/WP1,500) solution droplets stopped after only three

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bounces. Tricyclazole WG (WG500/WG1,500) showed similar behavior to tricyclazole WP.

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The diluted Fenoxanil SC1,500 solution was opaque and only two rebounds occurred. As the

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solution concentration increased (SC5,000), the rebound height decreased and only one

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rebound occurred. Based on these results, one can see that different solutions have different

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degrees of bounce behavior on rice leaf surfaces.

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Because of the low speed of the droplets, the shear force of the air on the droplets

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while they were falling was negligible, meaning the droplets always maintained a regular

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spherical shape before impact.33 Because of their inertial force, the droplets spread out on

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the leaf surfaces in the shape of a hat after impact. The droplets spread outward by

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overcoming surface tension and frictional force from the leaf surfaces. The kinetic energy

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of the droplets was gradually converted into surface energy for the gases and liquids. A

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portion of this energy was utilized to overcome viscous dissipation.34 As the kinetic energy

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of the droplets decreased and the surface energy increased to a maximum, the droplets

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reached their maximum spreading radii and the outermost droplet speeds decreased to

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zero.35

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When droplet spreading diameter reaches its maximum value, the kinetic energy of

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the droplets is converted into a gas-liquid interface in a non-equilibrium state. Under this 9



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non-equilibrium static pressure, the droplets retract.28 During the retraction process, the

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surface energy of the droplets is converted back into kinetic energy and dissipation energy,

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which is utilized to overcome dynamic friction and the viscous force of the leaf surfaces.36

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This increased kinetic energy accelerates droplet retraction speed. The free surface of the

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droplet begins to flow and a sleek bowl-type convex liquid column forms at the center of

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the liquid film. The inertial force causes the convex liquid column to extend upward and

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continuously elongate. Kinetic energy is then converted into gravitational potential energy.

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At this time, the viscous force of the superhydrophobic surface attracts the convex liquid

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column and prevents the droplet from leaving the hydrophobic surface.37 When the length

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of the convex liquid column is maximized, the droplets separate away from the surface.

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When all the kinetic energy is converted into gravitational potential energy, surface energy,

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and dissipated energy, the length of the convex liquid column reaches its maximum, after

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which all remaining energy is converted into kinetic energy. The convex liquid column

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falls again and eventually reaches a new equilibrium. This process continues until the

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energy is completely dissipated and the drop settles on the surface.38

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Fig. 2b describes the bounce behavior of droplets on rice leaves in greater detail and

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with greater accuracy based on the time evolution of the spread factor (ξ(t) =D(t)/D0).

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Before impact, the drops are almost spherical with diameter D0. The spread factor ξ(t) is

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utilized to characterize an impact (spreading and retracting) and D(t) represents the contact

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line diameter as a function of time.39 This means it records changes in the diameter of a 10


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droplet over time. All droplets spread to their maximum diameter and then retract.

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Compared to the stage in which inertia dominates the expansion behavior during the first

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3 ms, the flow rate during retraction is nearly an order of magnitude slower. This decreased

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retraction speed can be quantified by comparing the Reynolds and Weber numbers of pure

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water droplets during the expansion and retraction stages: Reexpansion /Reretraction

Bounce Behavior and Regulation of Pesticide Solution Droplets on

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