Multiscale approach of spray retention on superhydrophobic plant surfaces

The retention of most plant protection products on plants leaves is a key to their efficient use. In particular for monocots, problems observed in black-grass (Alopecurus myosuroides Huds.) weeding efficiency or the struggle against septoria in wheat may originate from insufficient retention of the active ingredients (a.i.) on this type of plants. Poor retention may even promote the emergence of resistance in the target population. Among the underlying factors related to these plants, some characteristics are detrimental to a good retention of a.i., namely their reduced leaves size, the superhydrophobicity of their leaf surfaces resulting from the presence of epicuticular waxes and hairs, as well as their erected architecture. Parameters related to the treatment, such as the spraying technique, the volume per hectare and the physico-chemical properties of the sprayed mixture are also involved and contribute to the variability of volumes of a.i. retained by each plant. The operating choices of a treatment must therefore be optimized according to the target properties in order to maximize retention and reduce variability in retained volumes by each plant or even leaf. A mixture applied by agricultural nozzles impacts the plant in the form of droplets of various sizes, velocities and incidence angles. Two important phenomena determine the fate of each droplet: the possible adhesion to the surface as a function of the surface wetting regime and its possible fragmentation depending on the ratio of impact and surface energies. The combination of these two phenomena results in various types of impacts that unequally contribute to the overall retention by a plant. Spray application trials are often performed in the laboratory on small monocotyledon plant, such as barley at early growth stage, to test tracer or active ingredient retention for various formulations and additives. Unfortunately, results from such trials suffer from high variability rendering very laborious any identification of the overriding factors and the contribution of each underlying mechanism in the main process. An alternative approach is to focus on the physico-chemical properties of the formulation and of the leaf surface to characterise retention efficiency. Despite the efforts to develop predictive models of the fate of droplets based on physical and chemical properties, the complexity of leaf surfaces at different scales and diversity of formulation types still adversely affect the predictive quality of this type of approach. Moreover, the variability of retention existing between plants during a spray treatment remains poorly studied. The main goal of this thesis is to develop a general methodology to improve the understanding of application methods of foliar pesticides and additives in terms of spray retention by leaf surfaces. This study attempts to link fundamental studies of the physics behind droplet impact with agronomic studies of spray application efficacy thank to its multiscale approach, from the droplet to the plant scale. It focused mainly on the most problematic targets for spray applications, i.e. small plants with predominantly vertical superhydrophobic leaves and contributes to answer the question of extension services about the reasons why black-grass (Alopecurus myosuroides Huds.) is so difficult-to-treat and Copyright. Aux termes de la loi belge du 30 juin 1994, sur le droit d’auteur et les droits voisins, seul l’auteur a le droit de reproduire partiellement ou complètement cet ouvrage de quelque façon et forme que ce soit ou d’en autoriser la reproduction partielle ou complète de quelque manière et sous quelque forme que ce soit. Toute photocopie ou reproduction sous autre forme est donc faite en violation de la dite loi et de des modifications ultérieures. why herbicide resistance emerged in some populations. The multiscale approach chosen focuses primarily on the behaviour at impact of spray droplets on superhydrophobic target surfaces (natural and artificial). Secondly, the emphasis is put on the plant architecture in order to take into account the variability of leaf orientations resulting in a broad range of droplet incidence angle. Finally, numerical simulations were conducted using a virtual spray-plant interception model in order to study the variability of deposits that results from the application technique, the droplet impact behaviour and the plant architecture. For instance, one can understand why using anti-drift nozzles when treating small grass weed may still result in low treatment efficacy and environmental contamination using the proposed approach. The first part focuses on the development of an experimental methodology based on high-speed imaging and image analysis relating accurately the behaviour of droplets at impact, specific to each formulation/surface combination for an agricultural spray, with their diameter and speed before impact. The originality of this methodology lies in the use of an artificial superhydrophobic surface, whose level of wettability is similar to a wheat leaf, and a moving agricultural nozzle to reflect the actual conditions of application. On a superhydrophobic surface, the Cassie-Baxter wetting regime is dominant, so different types of impacts are possible depending on the energy of the droplet, from direct adhesion at low impact energies, rebound at intermediate energies and up to fragmentation for high energies. This reference surface has proved to be a powerful tool for discriminating the effects of various spray additives thanks to its homogeneity allowing a high reproducibility of trials and to its very high level of hydrophobicity. The experimental methodology has been used to quantify the effect of a surfactant in relation with the spray application technology. Surfactants can alter the fate of the droplet impacts by the transition from the Cassie-Baxter wetting regime, favoring the rebound, to the Wenzel wetting regime promoting droplet adhesion. It has also been shown that various types of impact can coexist at similar impact energy levels and that the transitions between these impact outcomes can be described using probability density functions due to the intrinsic variability in droplet impact outcomes. The second part of the thesis deals with the use of a logistic fitting of experimental impact probability histograms based on the Weber number of droplets. This histogram of observed droplet impact type frequencies is the core of the retention model. In a phase diragram of the droplet velocity as a function of its diameter, the histogram is built by discretizing the domain of droplet impact occurrence into classes on the basis of the droplet Weber number that reflects the ratio between its kinetic and surface energies. Limits between classes are set to follow a geometrical increase of their Weber number, what results in convenient evenly spaced classes in a log-log scale graph droplet velocity versus diameter. For an easy comparison between formulations and since the dynamic surface tension at the time frame of the impact of a droplet is not a readily available data, the static surface tension of the carrier media is used to calculate the droplets Weber number. In each energy class, the probability for each type of impact is calculated as the relative volume of each impact outcome in relation to the total volume impacting on the surface. The impact behaviour is compared at various tilt angles on a superhydrophobic slide and excised black-grass leaves. The main effects of the tilt of the surface are the reduction of the available surface area for droplet interception and a shift of the thresholds between types of impact towards lower energies classes since the normal component of the speed is reduced. In the third part of the thesis, numerical simulations are performed in order to study the variability of deposits resulting from the application technique and from the droplet impact behaviour using the same plant model. A virtual nozzle produces droplets based on the Pearson system for random numbers. The jet is constructed from droplets drawn from the droplet spectra of agricultural nozzles of various spray qualities whose droplet size and speed were experimentally measured by an imaging technique called particle tracking velocimetry and sizing (PTVS). The droplet trajectories are computed to mimic the variability associated with the application method in the algorithm. The intersections between the droplet trajectories, assumed linear immediately near the target surface, and the three-dimensional representation of a scanned plant surface, consisting of a triangular mesh, is performed using a mathematical algorithm of intersection between straight lines and triangles in space. For each intercepted droplet, themodel assigns an impact behaviour according the impact probability histogram taking into account the angle of incidence of the droplet. For a given plant architecture, increasing the average size of the droplets resulted in a decrease in the retention level, irrespective of the wettability scenario studied. The percentage retention in all the simulations varied from 7% to 97% of a theoretical scenario of full adhesion of droplets. The increase in droplet sizes led to an increase of the coefficient of variation of the simulations, reflecting the variability of deposits. Moreover, the level of variability is substantially similar irrespective of the formulation studied. The variability of deposits also increased with the reduction of volume per hectare applied. This effect is enhanced for the nozzles producing larger droplets because of the reduced number of droplets for an identical applied volume. The mean level of retention decreased linearly with the reduction in plant size and the variability increased according to the square root of total leaf surface area because the number of droplets intercepted decreased. Consequently, environmental benefit from the a