New approach makes sprayed droplets hit stick their targets

New approach makes sprayed droplets hit and stick to their targets

7:35 AM, 6th October 2018
MIT mesh droplets
Illustration 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 fertilizers. The smaller droplets at right have little effect on the plant, while the droplets at left batter its leaves heavily.

Using a simple mesh screen may allow farmers to dramatically reduce the amount of pesticides they spray. 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.

The findings are reported 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. 

Other 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 explained 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 said. Farmers already cover some kinds of crops with fabric meshes, to protect against birds and insects, 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.

The researchers propose that, after deploying the mesh over the crop, 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. The same principle could be applied to other uses, Girard pointed out, such as the spraying of water into cooling towers such as those used for electric power plants and many industrial or chemical plants. Using a mesh below the spray heads in such towers “Can create finer droplets, which evaporate faster and provide better cooling,” he said.

For painting and for applying other kinds of coatings, the finer the droplets are, the better they cover and adhere, Girard said. While most existing atomization methods rely on high pressure to force liquid through a narrow opening, which requires energy to create the pressure, this method is purely passive and mechanical, Girard says.

“Here, we let the mesh do the atomization essentially for free.” James Bird, an assistant professor of mechanical engineering at Boston University, who was not involved in this research, said the work “demonstrates a clever, and seemingly practical, method to aerosolize and disperse droplets. Yet, what impressed me most in this study is the elegance by which the authors dissect and recombine the complex dynamics to develop a fundamental understanding that is more than the sum of its parts.”

The team included Antoine Le Helloco, and Thomas Binder at MIT and David Quere at CNRS in Paris. The work was supported by the MIT-France program.

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