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Thursday, 15 June, 2000, 11:23 GMT 12:23 UK
Splosh but no splash
BBC Nature
French researchers have found a neat way to make falling drops of liquid stick to a surface rather than splash right off.

Uncontrolled splashing is a waste. As much as 50% of a pesticide or herbicide applied to a crop can bounce straight off the leaves.

It can also be dangerous if the toxic substances in spray paints and other coatings jump back into the air and are breathed in.

But now researchers at Rhodia Recherches in Lyon have shown how the splashing of a droplet can be drastically reduced or even stopped altogether if some long, flexible, polymers are included in the liquid.

Flow properties

When a normal water droplet hits a waxy (hydrophobic) surface like a leaf, it will at first spread out and flatten. But because the surface repels water, the drop will immediately retract to try to minimise its contact with the leaf.

The retraction can be so fast that part of the drop actually has sufficient speed to jump straight back into the air.

But Vance Bergeron and colleagues found that they could control this "bounce back" by including just a little polyethyleneoxide (PEO) in their drops. Like all polymers, PEO consists of long chains of repeating units and its flow properties are quite different from those of water.

Polymers tend to be stretch-resistant, preferring to "curl up" - and the researchers think it is this characteristic that comes into play when PEO is added to the water.

Commercial applications

They think it produces a drag on the flowing water that is known as elongational or extensional viscosity.

The French experiments show that this makes the liquid sufficiently sluggish to stop the droplet splashing back on impact with a surface.

Adding trace amounts of a polymer to sprayed liquids such as agrochemicals, inks and even hairsprays, deodorants and polishes, may have huge cost savings.

Wastage could be drastically reduced, the researchers report in the journal Nature, as well as avoiding the haphazard contamination of the environment.

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