Archive Article: 1997/09/06
EVERY picture tells a story. The six opposite certainly reveal a surprising tale about the effect surfactants and adjuvants have on spray properties.
These pictures underpin recent work conducted at Silsoe Research Institute by Dr Clare Butler Ellis. She has been astounded by the dramatic effect on droplet size of some spray additives.
With many growers using tank mixes and lowering volume rates there is a danger of unwittingly altering the droplet size of your spray formulation, she points out.
Even manufacturers of todays agrochemical products have little knowledge about the way their inventions perform with certain nozzle types, adjuvants and surfactants. The priority for a formulation chemist is stability and shelf-life, Dr Butler Ellis explains.
But is the physical property so important? Yes, how a spray performs in terms of the droplet size, where the liquid sheet breaks up and the number of perforations formed from the nozzle does make all the difference between hitting a crop target, hitting the ground or losing half the spray to the wind.
Many growers add adjuvants to improve the mode of action of an agrochemical. Often an oil or surfactant is chosen to improve spreadability and chemical uptake. Yet both attempts can be completely thwarted, says Dr Butler Ellis, by additives altering the droplet size.
Work at Silsoe confirms that water soluble surfactants generally reduce the droplet size and oil-based adjuvants form an emulsion, increasing the droplet size.
The photographs opposite show these effects on water; the oil-based adjuvant, LI700, and the cationic surfactant, Ethokem, are sprayed through flat fan and twin fluid nozzles.
Note the point at which the liquid sheet changes into droplets when LI700 and Ethokem is added to water and sprayed through the flat fan nozzle.
With an emulsion the sheet breaks up nearer to the nozzle, creating bigger droplets, says Dr Butler Ellis. "So if your spray is a water soluble autumn grass weed killer, there is a danger of producing big droplets that hit the soil, not the weed."
What if your spray product is already an emulsifiable formulation? Could more LI700 make droplets bigger? "Probably not, because the effect of oil-based additives seems to be on-off," says Dr Butler Ellis. Unlike Ethokem, a cationic surfactant, which has an additive effect making droplets smaller.
In the photograph it can be seen that Ethokem holds the liquid sheet together for longer, making finer droplets. "Clearly an application designed for fine spray, such as reducing flow rate, could be in danger of increasing drift by the addition of such a surfactant," she claims.
Understanding the impact of these additives sprayed through a hydraulic nozzle is hard enough. But with twin fluid nozzles spray formation occurs inside the nozzle body, so you cannot see it.
Twin fluid nozzles are gaining popularity, largely because of their flexibility in altering flow rates.
Ethokem has a dramatic effect on the water sprayed from a twin fluid nozzle. It creates bubbles within bubbles increasing the droplet size, but not the mass, explains Dr Butler Ellis. "So the droplets fall more like feathers – than stones – arriving at their target at slower speeds and therefore, much more likely to be retained."
Clearly this combination of nozzle and surfactant has a lot to offer growers attempting to improve efficacy, without increasing drift.
"But the story is complex. Many factors interact. Only by getting a better grasp on what happens when adjuvants and surfactants are added to a spray, can we understand and predict what might happen with new formulations and combinations," she concludes.