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. 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 Since the cost of pesticides can be a significant
part of a farmer’s budget, reducing the amount that’s wasted could improve the overall
economics of the small-farming business, while also reducing soil and water pollution. Decreasing the amount of pesticide sprayed
can also reduce the exposure of farmers to the spray chemicals. Based on the laboratory tests, the team estimates
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. And the polymer additives themselves are natural
and biodegradable, so will not contribute to the runoff pollution. The new approach would require only minor
changes to the existing equipment that farmers use, to separate the pesticide into two streams
to which small amounts of each polymer could be added. The polymers themselves are extracted from
common, low-cost materials that could be produced locally.