As any crop protection trader, agribusiness salesman or farmer knows, the spraying of agricultural products is inefficient. Even if the wind and temperature is perfect much of the spray will not adhere to the plant and will drip off leaves and stems onto the ground.
Researchers at MiT who have been studying the effectiveness of sprays on plants estimate that, “When farmers spray their fields with pesticides or other treatments, only 2 percent of the spray sticks to the plants.” This low retention rate not only wastes large amounts of agricultural products at great expense to the farmer, but also has a negative impact on wildlife as run off enters local ecosystems.
To date, agricultural chemists have focused on improving crop protection products’ ability to stay on leaves with the use of surfactants and similar chemical products. By adjusting the chemical make-up of a pesticide they are able to reduce surface tension, so that less of the spray bounces off, reducing waste.
However, droplets of spray are in contact with a plant for only fractions of a second, giving the chemical properties of a surfactant little to time to take effect. This started a research team from MiT looking for a completely new alternative to the problem. As team leader and associate professor of mechanical engineering at MiT Kripa Varanasi said, the team began ‘playing around with charge interactions’.
This is the idea that droplets of a product could be given positive and negative charges so that they would have a better chance of ‘sticking together’ on a leaf.
Whilst this idea may sound a little unusual, tests conducted in the laboratory have shown that the new system, “could allow farmers to get the same effects by using only 1/10 as much of the pesticide or other spray.”
The MiT website describes the breakthrough in more detail as follows;
“The new approach uses two different kinds of additives. The spray is divided into two portions, each receiving a different polymer substance. One gives the solution a negative electric charge; the other causes a positive charge. When two of the oppositely-charged droplets meet on a leaf surface, they form a hydrophilic (water attracting) ‘defect’ that sticks to the surface and increases the retention of further droplets.
Leaves of many plants have a natural tendency to be hydrophobic (water repelling), which is why they often cause droplets to bounce away. But creating these tiny hydrophilic bumps on the leaf surface strongly counteracts that tendency, the team found.”
The results were published in the scientific journal Nature Communications, and included these images of the procedural theory and practice, shown here;
“(a) Schematic of experimental set-up for simultaneous spraying of opposite polyelectrolytes. (b) Expected behaviour for the impact of a droplet with one polyelectrolyte polarity on a droplet with an oppositely charged polyelectrolyte. The coalesced drop sticks to the surface.”
“(c,d) Snapshots of simultaneous spraying on a superhydrophobic surface. Sprays with very low droplet density were used to enhance visualization and slow down the process. In the first row, the two sprayers are spraying water and the surface remains dry. Almost all droplets bounce off. Some small droplets are deposited but they are cleared as soon as another droplet impacts them. In the second row, opposite polyelectrolytes are sprayed. Individual droplets hitting the surface still bounce off. After 120 ms of spraying, the first event of a droplet of one polyelectrolyte hitting a droplet containing the opposite polyelectrolyte occurs. The coalesced drop sticks to the surface. Subsequent drops that hit this droplet also coalesce on it. Similar events happen all over the surface. Many droplets can be seen on the surface after 3 s of spraying. Scale bar, 1 cm.”
To watch a downloadable video of the spray in action, click here.
To watch the MiT video explanation of the discovery on YouTube, click here.
Given this evidence, it seems that the research team have discovered a novel and exciting new approach to combat the challenge of pesticide spray application. Naturally, further study is needed to examine how (and if) the process will function in the real world. So for that reason, one of the team, graduate student Mahak Damak, has already travelled to India to see for himself how products are applied on small farms. Whilst the team is also, “experimenting with different sprayer designs that could simplify the process further, potentially eliminating the need for two separate tanks”.
Even if this does not prove possible, it should be a simple task to use two tanks each filled with the same crop protection product, but with each tank having mixed into it the different polymer additives. Given that much of the third world uses hand-held spraying systems to treat their crops the process seems easily applicable for the majority of the world’s agriculture. The new system “should be easy to implement, and it doesn’t require extra equipment,” Damak adds.
Furthermore, the polymers used in the experiments are relatively low-cost and have no negative environmental impact, as the report itself states, “To study the effect of precipitation on drop impacts, we used two polyelectrolyte molecules. Linear polyethyleneimine was the positively charged polyelectrolyte, with NH2+ groups in solution, while polyacrylic acid was the negatively charged polyelectrolyte with COO− groups in solution.”
But the researchers then ran the experiments again using different polymer additives, and found equally impressive results. As they state, “We also show similar retention properties using different polyelectrolyte molecules … Chitosan (positively charged) and Alginate (negatively charged). These polyelectrolytes are polysaccharides that are non-toxic, biocompatible and biodegradable, which makes them excellent candidates for plant treatment”
The team is also pondering what further applications the use of positively and negatively charged polymers could have in different sprays. For example, could the same technology be applied to anti-frost chemicals for the citrus orchards in Florida?
Could two positive (or negative) charges be used to increase ‘bounce rate’ for where a spray isn’t wanted, for example in applying fertiliser to crops when it is wanted in the soil and not on leaves?
Indeed, could there be a use for the process outside of the agricultural industry? Is there a need for a more adhesive spray for hydrophobic surfaces in the paint and coatings industry? Could the technology be applied to adhesives? Or household cleaning products?
So many questions remain unanswered that it seems that the idea of positively and negatively charged chemical sprays might stick around for a while yet.