Mesh Screen Allow Farmers to Dramatically Reduce the Amount of Pesticides They Spray

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Mesh Screen Allow Farmers to Dramatically Reduce the Amount of Pesticides They Spray

New approach makes sprayed droplets hit and stick to their targets. Using a simple mesh screen may allow farmers to dramatically reduce the amount of pesticides they spray.

By David L. Chandler

When spraying paint or coatings onto a surface, or fertilizers or pesticides onto crops, the size of the droplets makes a big difference. Bigger drops will drift less in the wind, allowing them to strike their intended targets more accurately, but smaller droplets are more likely to stick when they land instead of bouncing off.

Now, a team of MIT researchers has found a way to balance those properties and get the best of both — sprays that don’t drift too far but provide tiny droplets to stick to the surface. The team accomplished this in a surprisingly simple way, by placing a fine mesh in between the spray and the intended target to break up droplets into ones that are only one-thousandth as big.

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Photos illustrate how the tiny droplets produced by a mesh barrier prevent plants from being pummeled by the larger droplets from either rainfall or the spraying of pesticides, herbicides and fertilizers. The smaller droplets in the image at right have little effect on the plant, while the droplets at left batter its leaves heavily. Courtesy of the Varanasi Research Group. Via MIT

The findings are reported today in the journal Physical Review Fluids, in a paper by MIT associate professor of mechanical engineering Kripa Varanasi, former postdoc Dan Soto, graduate student Henri-Louis Girard, and three others at MIT and at CNRS in Paris.

Earlier work by Varanasi and his team had focused on ways to get the droplets to stick more effectively to the surfaces they strike rather than bouncing away. The new study focuses on the other end of the problem — how to get the droplets to reach the surface in the first place. Varanasi explains that typically less that 5 percent of sprayed liquids actually stick to their intended targets; of the 95 percent or more that gets wasted, about half is lost to drift and never even gets there, and the other half bounces away.

Atomizers — devices that can spray liquids in the form of droplets so small that they remain suspended in air rather than settling out — are crucial parts of many industrial processes, including painting and coating, spraying fuel into engines or water into cooling towers, and printing with fine droplets of ink. The new advance developed by this team was to make the initial spray in the form of larger drops, which are much less affected by breezes and more likely to reach their targets, and then to create the much finer droplets just before they reach the surface, by placing a mesh screen in between.

Though the process could apply to many different spraying applications, “the big motivation is agriculture,” Varanasi says. The runoff of pesticides that miss their target and fall on the ground can be a significant cause of pollution and a waste of the expensive chemicals. What’s more, the impact of finer droplets is less likely to damage or weaken certain plants.

Farmers already cover some kinds of crops with fabric meshes, to protect against birds and insects devouring the plants, so the process is already familiar and widely used. Many kinds of mesh materials would work, the researchers say — what matters is the size of the openings in the mesh and the material’s thickness, parameters the team has precisely quantified through a series of lab experiments and mathematical analysis. For their experiments, the researchers primarily used a commonly available and inexpensive fine stainless steel mesh.

The researchers propose that, after deploying the mesh over the crop, either directly supported by the plant stalks or supported on a framework, a farmer could simply use a conventional sprayer that produces larger drops, which would stay on course even in breezy conditions. Then, as the drops reach the plants, they would be broken up by the mesh into fine droplets, each about a tenth of a millimeter across, which would greatly increase their chances of sticking.

Read full article: MIT

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