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MIT researchers make pesticides stick to leaves

When farmers spray their fields with pesticides or other treatments, only 2 percent of the spray sticks to the plants. A significant portion of it typically bounces right off the plants, lands on the ground, and becomes part of the runoff that flows to streams and rivers — often causing serious pollution. But a team of MIT researchers aims to fix that.

By using a clever combination of two inexpensive additives to the spray, the researchers found they can drastically cut down on the amount of liquid that bounces off. The findings appear in the journal Nature Communications, in a paper by associate professor of mechanical engineering Kripa Varanasi, graduate student Maher Damak, research scientist Seyed Reza Mahmoudi, and former postdoc Md Nasim Hyder.

Previous attempts to reduce this droplet bounce rate have relied on additives such as surfactants, soaplike chemicals that reduce the surface tension of the droplets and cause them to spread more. But tests have shown that this provides only a small improvement; the speedy droplets bounce off while the surface tension is still changing, and the surfactants cause the spray to form smaller droplets that are more easily blown away.

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.

When the MIT team began studying the problem of pesticide runoff, which is a major agricultural problem worldwide, they soon realized that part of the reason for the limited success of earlier attempts to address the problem was that the droplet bouncing happens so quickly, in a matter of milliseconds. That means that most countermeasures, especially those based on chemical properties, just didn’t have time to make much of a difference. “So we thought, what else can you do? And we started playing around with charge interactions,” Varanasi says.

They found that the combination of the two different polymer additives “can pin the droplets” to the surface, “and this all happens during the time it’s spreading,” before the droplets starts a retraction that leads to their bouncing away, according to Varanasi.

Read more at MIT News

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