Wetting, Spreading, and Retention

Foliar application of herbicides may involve volumes ranging from high (e.g., 1000 liter/ha) to ultralow (e.g., 1 liter/ha), and maximum capture by the leaf surface depends upon such factors as droplet size, impaction, wetting, spreading, and retention. The interactions at various interfaces involved in pesticide application and their effects on transfer and performance have been reviewed by Tadros.  

The first interface occurs between the spray solution and the atmosphere (air) and determines the droplet spectrum, rate of evaporation, drift, etc. The second interface occurs between the liquid droplets and the leaf surface. The droplets impinging on the surface are subject to a number of processes which determine their adhesion, retention, and further spread. The nature of the deposit formed is governed by the rate of droplet evaporation and the concentration gradient of the surfactant across the droplet. 

Droplet adhesion is determined by the in flight kinetic energy of the droplet and its surface energy on impaction with the leaf surface. Adherence 

The problem of droplet sliding in relation to spray retention has also been considered by Tadros. Spray retention increases as the volume applied increases, up to the point of incipient runoff. Above this point, the volume of spray retained begins to decline, and retention is determined by the movement of drops on the leaf surface. Factors influencing sliding include surface roughness, presence of hairs, leaf age, position on the leaf surface, environmental conditions, surfactant incorporation and concentration, droplet size and velocity, and wind speed. 

The final distribution of herbicide over the treatment area is determined by surface wetting, which can be assessed from the value of the contact angle, though its measurement is complicated by hysteresis. Surface wetting is influenced also by (1) the spread factor, which depends on the ratio between the diameter of the wetted area and the diameter of the drop, and (2) the spreading coefiicient(s), which reflects the tension when a solid-liquid and a liquid-air interface are replaced by a solid-air interface. 

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