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David Bradley ISSUE #18
September 2001

Water waste!

A bouncing drop and a spread out splash   
On the right we have a bouncing drop, on the left a spread out splash.
While environmental activists often make blanket statements about the wholesale banning of pesticides and herbicides, there is now evidence that much of their use could be wasted. According to Vance Bergeron and David Quéré of the Laboratorie de Physique Statistique at the Ecole Normale Supérieure, in Paris, France, less than half of a pesticide dose sprayed on to crops by farmers actually sticks to the leaves because of their waxy outer layer. This, the team says, leads to inefficient use and can be potentially hazardous as much of the sprayed pesticide simply runs off and contaminates the soil rather than protecting the crops from bugs, while droplets of herbicide ineffectually bounce off the weeds too.

The team is now working with French chemical company Rhodia to develop a sugar-based additive that prevents herbicide droplets from rebounding when sprayed on to plants. The sugar additive, a polysaccharide formally named Cyamopsis tetragonoloba taub, and commonly referred to as Guar, provides a unique elongational viscosity to the droplets making them deform and spread out on impact with the leaf surface rather than retaining their bouncy droplet shape.