To improve its image, traditional agriculture has focused upon reducing as far as possible the quantity of pesticides used in farming. To do so, there is a need to understand in detail - and mastering - what happens when micro-drops of the sprayed product meet the surface of the plant being sprayed. This is the subject addressed by Mathieu Massinon, author of a thesis(1) on the retention of phytosanitary products by plants known as ‘superhydrophobic’.    

For the uninitiated, spraying crops to protect them against parasites (insects, fungi, bacteria, weeds, etc.) is a fairly simple task. Farmers just need to fill the tractor’s sprayer with the right amount of the active ingredient (mixed with water according to the manufacturer’s instructions) and disperse the mixture across the field using the best possible spraying equipment. As long as they keep an eye on the weather (hot sun and strong wind should obviously be avoided if they want to minimise the chances of the product evaporating or being blown away), the use of the product by the farmer should be straightforward. 

But this vision is too simplistic. It ignores the complex physical-chemical mechanisms at play in an operation of this kind. Although the physics of drops has been studied for more than quarter of a century, there are still many things which need to be understood and improved if it is to be applied to the field of crop protection. This investigation is even more necessary in the current context: pesticides have received bad press with the general public and are subject to increasingly strict marketing regulations aiming to avoid collateral effects in terms of public health and the environment. 

This ‘complex mechanism’ refers to the retention of the phytosanitary product by the plant (and hence its final effectiveness), which varies not only in relation to the wind or the ambient temperature at the point of spraying. Although it is easy to understand that the liquid used reaches the plant in the form of millions of sprayed drops, people are generally unaware that the diameter of these drops, depending on the type of spray nozzle and pressure used, can be as small as twenty microns. Current advice given to farmers is to use bigger nozzles. These produce drops which are relatively large (up to one millimetre) and are less likely to be dispersed by the wind. It ignores, however, the fact that these ‘large’ drops contain more energy at the time they hit the plant. They are therefore likely to be subject to the phenomenon of fragmentation or splashing, which sees the initial drop subdivide into several smaller droplets, which run the risk of landing ... on the soil. When dealing with products which operate on the basis of contact between the plant and the product, this consists effectively of spreading an active ingredient on the soil which will then simply be washed away by the rain ...

(1) Massinon Mathieur, Multiscale approach of spray retention on superhydrophobic plant surfaces, Doctoral thesis, 2016.