When farmers spray their crops with pesticides, much of the liquid bounces off the leaves and winds up on the ground where it easily runs off into water supplies. In fact, for the many plants in the world that have waxy, water-repelling leaves, a mere two percent of pesticides sprayed on leaves stays put.

A team of researchers at MIT has recently discovered a novel approach to taking the bounce out of the droplets. When they added non-toxic, biodegradable polyelectrolytes of opposite charges to water, they found it created a stickiness in the droplets that increased retention on the leaf ten-fold. This means that farmers could potentially slash pesticide use by a staggering 90 percent, while still getting the same amount of product onto the plants.

Previous attempts to remedy the bounce problem have involved adding surfactants or polymers, but these have produced modest and mixed results, respectively. This is the first time polyelectrolytes have been tried.

For the study, which was published recently in Nature Communications, the researchers first tested spraying different combinations of water with and without electrolytes onto a man-made model of a leaf and recorded the activity of the droplets with a high speed camera. The images revealed that when drops with opposite charges coalesced on the surface they stick and attract subsequent drops, growing in size.

A series of further experiments, using just two drops, either with similarly charged electrolytes or a premixed solution combining positive and negative, revealed that neither helped the droplets stay put. What worked was allowing two droplets that contained oppositely charged electrolytes to merge in-situ, on the leaf. In the field, farmers would have to use a sprayer with two nozzles or heads, something many already have on hand.

After establishing how the model worked, they scaled up to a sprayer application, using liquids with different concentrations of polyelectrolytes and different surfaces. They measured how much of the liquid was retained by weighing the leaves, and how uniformly it spread, by measuring surface coverage. The maximum improvement in retention they achieved was a ten-fold increase. Finally, they used a lotus leaf to confirm that the technique works on a natural leaf as well.

“This is a quantum change,” says Kripa Varanasi, a co-author of the paper. “We were inspired to reduce pesticide run-off, and getting to the heart of the physics often helps develop new products. We were pleasantly surprised that a simple model with just two drops could carry us to the spray level and work there, too.”

Next on the agenda, he says, is farm trials. Varanasi suspects this method may prove to have another advantage on farms. Because this approach creates bigger, heavier drops it may also help leaves retain the droplets lost to wind and not just those lost to bounce.

If it works in these upcoming trials, it could prove a very simple way to achieve a very big reduction in pesticides. —Catherine Elton | 9 September 2016

Source: Damak M et al. Enhancing droplet deposition through in-situ precipitation. Nature Communications. 2016

Header Image: Chris Campbell/Flickr