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Evaporation and spread of surfactant amended droplets on leaves
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In order for pesticide applications to be effective,they need to cover the treated area uniformly with a desired dose.The area of coverage is determined in part by the process of evaporation and spread ofthe droplets on a leaf surface. This process varies with the type of leaf surface (hairy vs. waxy, for example) and the physical properties of the pesticide.

Nonionic surfactants are often recommended for use in formulations and tank mixtures to increase the effectiveness of foliar-applied pesticide spray treatments. Nonionic surfactants break the surface tension so pesticide droplets spread more evenly and adhere longer on leaf surfaces. Spray mixtures containing surfactants can provide better spray coverage, longer-lasting residue, better active ingredients uptake, and improved penetration of the active ingredient into leaf tissue.This can result in better efficacy, less pesticide use, and less overall cost.

Close-up videos (below) recorded in Dr. Heping Zhu's lab clearly demonstrate the differences in the spread and evaporation process of a single droplet with and without surfactant on hairy and waxy leaf surfaces. 

droplet on waxy leafDroplets containing a surfactant deposited on a waxy geranium leaf surface have a much larger area of coverage than droplets without the surfactant.

droplet on hairy leafSimilarly, on a hairy geranium leaf surface, droplets containing a surfactant penetrate and spread along paths between hairs while the same size droplet without the surfactant remains on top of the hairs and does not readily spread out.

There are disadvantages to using surfactants, however, as over use of surfactant can cause an adverse effect on plants. When used in excess, surfactants can enhance the potential for crop injury (phytotoxicity) on the desirable plants.

While the addition of nonionic surfactants to pesticide mixtures can increase pesticide efficiency and reduce the corresponding cost of pesticide use, to achieve optimum performance and lessen the potential for unwanted phytotoxicity, always read and follow the label instructions on surfactant selection and use.

The files below demonstrate how droplets spread and evaporate on hairy and waxy leaf surfaces for foliar agro-chemical spray applications with and without surfactants.
Left-click the link to view the video.  Right-click the link and choose 'Save target as...' to save the video to your computer.

Waxy leaf no surfactant top view
Waxy leaf no surfactant side view
Waxy leaf with surfactant top view
Waxy leaf with surfactant side view
Hairy leaf no surfactant top view
Hairy leaf no surfactant side view
Hairy leaf with surfactant top view
Hairy leaf with surfactant side view

For more information see:

Evaporation and deposition coverage area of droplets containing insecticides and spray additives on hydrophilic, hydrophobic and crabapple leaf surfaces.

Evaporation and Coverage Area of Pesticide Droplets on Hairy and Waxy Leaves.

Evaporation Rate and Development of Wetted Area of Water Droplets with and without Surfactant at Different Locations on Waxy Leaf Surfaces.

Adjuvant effects on evaporation rates and wetted area of droplets on waxy leaves.

Droplet evaporation and spread on waxy and hairy leaves associated with type and concentration of adjuvants.

ARS contact:
Heping Zhu