A high-performing photo tool

Initially, the research developed an ultra-high speed photo recording tool, combined with LED lighting used in what is known as a PDIA ombroscopy (particle/droplet image analysis). The advantage of this technique, in comparison with traditional (direct) exposure, consists of being able to visualise the contours of objects (in this case, drops) in order to study the deformations which occur upon contact with the surface. ‘The challenge consists of being able to follow, with sufficient precision, the phase in which drops make contact with a very hydrophobic surface, given that this contact lasts on average 5 milliseconds and the diameter of some drops is no bigger than 50 microns. In order to avoid problems with blurring due to the relatively high speed of the drops, exposure time has to be minimal, which reduces the quantity of light captured by the camera. The use of an LED pulsed light was therefore required’. 

Finally, thanks to the use of an powerful LED light source, the spray bench developed by the Precision Agriculture Unit (Department of BioSystems Engineering) at Gembloux enabled drops to be photographed at the rate of 10,000 to 20,000 images per second. Although such equipment is only exceptionally used at the moment in agronomic research, the real originality of Mathieu Massinon’s work consists of extracting relevant information from these images and interpreting it.

The second stage consists of photographing, in the laboratory, a range of combinations of parameters relating to spraying: the size of the drops, the spray pressure, the speed of impact (just prior to impact), the surface tension, etc. ‘For this phase, which is really the unique aspect of my work, I was able to simulate most of the spray nozzles available on the market and, depending on the modifications made, study in detail the behaviour of drops upon impact: from direct adhesion with low impact energy to fragmentation with high energy, and rebound phenomena with intermediate energy. To better reflect the conditions of real application, I used an artificial superhydrophobic surface with wettability which is very close to that of wheat leaves. Overall, the test bench proved to be a very highly performing tool. It enabled me to draw up impact maps based on all the parameters considered, to measure the scale of variability of types of impact and, from there, to measure the great variability in retention levels’.

The third stage consisted of extrapolating the data collated, which were previously limited to the scale of the drop and the leaf, to the scale of the entire plant. ‘Understanding and systematising the behaviour of drops on a micro-scale is one thing. Taking account of the architecture of an entire plant is another ... The retention of the product may indeed vary, for example, depending on the orientation of the leaves and the density of the canopy. I therefore created a model which is capable of anticipating the quantity of product retained on the scale of the plant. To do so, using 3D technology, I virtually reconstituted a barley plant at the ‘two leaf’ stage, and tested retention using various parameters, for example, the  drop size distribution, the size of the plant or the physical-chemical properties of the mixture (essentially the surface tension). After numerical simulations, I developed various spray scenarios. Even before tests on the field, these enable the least efficient combinations to be eliminated during the development of new products, whether conventional products or bio-pesticides.’ Saving time, money and energy, not to mention the positive environmental impact.