Droplet impingement behavior analysis on the leaf surface of Shu-ChaZao under different pesticide formulations

Highlights

• The effects of the three different pesticide formulations were assessed.

• A CLSVOF-based interface trace approach was proposed.

• Droplet impingement behaviors on the leaf-surface of Shu-Chazao were analyzed.

Abstract

Spray deposition in agriculture is of particular importance to apply pesticides to plant because poor efficiency leads to reduced biological efficacy, environmental contamination and even economic losses. Spray deposition on leave surfaces is associated with impingement dynamics behaviors of pesticide droplets. But how this impact affects the foregoing deposition is an intriguing subject of research, notably for the deposition on tea leave-surfaces. In this study a tea leaf-surface of a real tea tree, i.e. Shu-Chazao was selected as an impingement target of the pesticide droplet. A Couple Level Set & Volume of Fluids (CLSVOF) interface tracking method was proposed to characterize the impingement dynamics behaviors of the three commonly used pesticide droplets (such as chlorothalonil, dimethoate and malathion) on the leaf-surface and thus assess the effects of the different pesticide formulations on the spray deposition. Four key factors, including liquid phase pattern, surface wettability, pressure and velocity distributions were investigated, respectively, along the transverse and longitudinal directions of the leaf-surface. The calculated predictions provide a reasonable match with the published data. With our study, the CLSVOF interface trace modeling is demonstrated to have great potential for in-depth study of the impingement dynamics behaviors of the pesticide droplets on the tea leaf-surface. The simulation results can contribute to spray efficiency improvement of the tea plants in China.

Introduction

The challenges in the spray application process have increased in complexity over recent years: consumers desire the highest quality of produce, however, regulators insist on safety (to the consumer from residues) and risk reduction (to the operator, environment or ecosystem) (Weiss et al., 2001, IliasMalgarinos et al., 2014). Hence, efficient and sustainable plant protection requires that the various stages with pesticides, e.g. deposition, retention, uptake, and translocation should be optimized by either reducing detrimental ecological effects and retains or by improving both biological efficacy and economic viability of the growers (IliasMalgarinos et al., 2014). As a matter of fact, however, many spray programs widely used in the agricultural industries appear to provide lesser control of pests than might be expected from laboratory trials (Zwertvaegher et al., 2014). This may basically be due to inadequate canopy penetration and foliage coverage, which are believed to be strongly associated with the spray retention (Zwertvaegher et al., 2014). More generally, the better spray retention is dependent of the better spray deposition, and the above depositing action is sufficiently influenced by the impingement dynamics behaviors of the pesticide droplets (Dorr et al., 2008). Hence, a careful study of the underlying mechanism on impingement behavior of the pesticide droplets on the leave-surfaces is essential to maximize the pesticide efficacy.

From the viewpoint of fluid mechanics, the aforementioned impact dynamics behavior of pesticide droplets can be depicted as follows: as a pesticide droplet hits a leaf surface of a plant, the droplet’s vertical momentum leads to radial flow, in which the impact kinetic energy and surface energy of the droplet are converted to the surface energy of the splat and the viscous dissipation energy; subsequently, the droplet undergoes an oscillatory motion with gradually decaying amplitude till it reaches a sessile state(Kim and Chun, 2001, SubhasishMitra et al., 2013). At this point, for the droplet four dynamics behaviors may possibly appear:(1) spreading, (2) receding, (3) rebounding or splashing and (4) deposition, as shown in Fig. 1. In detail, firstly, the kinetic energy of the droplet causes it to spread out across the surface till the maximum spread diameter, when all the available kinetic energy is converted to the potential energy. Next, the droplet recoils due to surface tension. Note that in the process of spreading and recoiling phases the energy loss of droplet takes place. Finally, If the energy loss is so low, that is, the impact kinetic energy can overcome the viscous resistance, the droplet may splash or rebound off the leaf; however, if the loss is too much, the droplet deposits (Spillman, 1984, Forster et al., 2012).

Tea, a unique traditional agricultural product in China, is a popular kind of healthy, green and natural drink with the people around the world. Same as the other plants, chemical pesticides are commonly and widely used for the growth of tea trees, the more significance and importance of which are to protect the damage from disease and pest and consequently improve tea qualities (Cheng, 2012). But, pesticides are toxic, and there is an increasing demand to reduce their use. Unfortunately, current over-standard for pesticide residue is the biggest challenge for pesticide applying to tea trees (Cheng, 2012). The excessive pesticide residues not only lower the tea quality but also harm the people health and what is the most importance is to significantly pollute the natural environment, e.g. pollute the soil and contaminate groundwater. Therefore, it is very important and imperative to improve the tea quality by optimizing the spray retention.

Currently the droplet dynamics characteristics on surface are widely investigated in the industrial engineering fields, e.g. self-cleaning applications, micro-assays and micro fluidic devices (SubhasishMitra et al., 2013, An and Lee, 2012, Yarnold, 1938, Miwa et al., 2000). Due to expensive field measurements of specific crop/environment combinations in the practical spray applications, the mathematical and computational methods have been occasionally tried predicting the drop-impact-related processes to help quantify and optimize the complexities of the agrichemical spray retention by plants. For example, Forster et al. Forster et al. (2010) developed an empirical model for initial adhesion and spray retention by individual plants, in which they determined the dominant parameters on the physical properties of the spray droplet and leaf surface characteristics. An analytical investigation of the interpolation technique was made by Boukhalfa et al. Boukhalfa et al. (2014) and Massinon et al. Massinon and Lebeau (2013), who claimed that for the optimal spray retention, the droplets impacting the plant surface must retain on the plant and thus the volume percentage of the adhering droplets should be maximized. Note that the interpolation technique is a classical mathematics approach to estimate unknown data values by using known data values (Cobos and Peetre, 1991).

Focusing to CFD (Computational Fluid Dynamics) modeling, in order to understand the underlying physical mechanisms on the impingement of a single liquid droplet onto a solid surface, in the engineering field there exist a large number of different numerical studies, the commonly used models of which include Volume of Fluid (VOF), Level Set, and Couple Level Set & Volume of Fluids (CLSVOF) Strotos et al., 2011, Caviezel et al., 2008, Yokoi et al., 2009. For the three numerical approaches, VOF is being most widely used in the agricultural engineering field. To the best of the authors’ knowledge, however, previous research results, especially the VOF-based predictions cannot be found in literatures on the impingement dynamics behaviors of the pesticide droplets on the tea leaves. In fact VOF has one of the biggest advantages that it is unable to accurately compute such important properties as the curvature and the normal to the interface and hence trends to incur inaccurate or wrong simulations (Olsson and Kreiss, 2005). The leaf-surface does have so complicated micro-structural topography, as shown in Fig. 2, Fig. 3.

More generally, pesticide formulations have strong effects on surface tension and dynamics viscosity of droplets, which in turn affect the impingement momentum, as well as, the conversion between kinetic energy, surface energy and viscous dissipation energy. Consequently, they sufficiently affect the dynamics behaviors of the droplet on the leaf surfaces (Spillman, 1984). Currently, CLSVOF modeling seldom appears to be available in the agricultural engineering field, let alone in the spray retention on the tea leaf-surface. And also another important reason is that CLSVOF model, coupling the LS (Level Set) and VOF, not only contains their common advantages but also overcomes their disadvantages(Olsson and Kreiss, 2005). The detailed descriptions will be presented further in the Section 2.3.2. Hence, in order to optimize spraying parameters, here we propose CLSVOF-based interface tracking approach to characterize the impingement dynamics behaviors of the different pesticide droplets on the tea leaf-surface and thus assess the effects of the varying formulations on the spray deposition by tea foliage. To achieve this, a leaf surface of Shu-Chazao, a real tea tree living in central and south China, is act as the target for the droplet interception and impaction model (Fig. 2), and the three commonly used pesticides (such as chlorothalonil, dimethoate and malathion) are chosen. Note that tea leaf-surface is hydrophilic.