Spray retention

Surfactants also play a major role in reducting droplet sliding and increasing spray retention. When a drop impinges on an inclined surface (such as a leaf surface), it starts to slide as a result of gravity. During this process,... 

This immediately indicates the necessity of using spray mixtures of sufficiently low surface tension so that the displacement of the droplet owing to impingment can result in a sizeable interfacial area and proximal contact so that these short range intramolecular forces can come into play in spray retention. 

Total spray retention also depends very much on crop or plant canopy architecture and is not as easy to model. If flie canopy is even and homogenous then retention relationships can be derived and used in predictions, but in general it is difficult to estimate canopy retention with existing models [22,23,24]. The best model for retention [25] can... 

Nevertheless, a reduction in spray retention brought about by an increase in the dynamic surface tension may be offset by increasing the elasticity of the spray droplet (Hart, and Young, 1987 Wirth cl al., 1991). Therefore, an approach that involves the use of polymers to both increase the viscosity and avoid the use of surface-active agents is conceivable. 

Anderson NH, Hall DJ and Seaman D Spray retention effects of surfactants and plant species, Aspects of Applied Biology, 14 233-243 (1987). 

C. G. L. Furmidge, Studies at phase interfaces. 1. The shdrng of liquid drops on solid surfaces and a theory for spray retention, /. Colloid Sci., 17, 309-324... 

For a series of anionic surfactants with the same ionic head group, the life time of a micelle decreases with decreasing alkyl chain length of the hydrophobic component. Branching of the alkyl chain could also play a notable role in the life time of a micelle. Consequently, dynamic surface tension measurements need to be performed when selecting a surfactant as an adjuvant as this may have an important influence on spray retention. 

To obtain a realistic picture of spray retention, measurements should be made on field grown plants and the results obtained may be correlated to those obtained on glass house plants. In this case, it is possible to use glass house plants in selecting surfactants, if an allowance is made for the difference between the two sets of results. 

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 effect of surfactants on spray retention efficiency is influenced by droplet size and velocity, leaf angle, and surfactant type and concentration. Examination of spray retention and surface spreading on a range of outdoor-grown crop plants was examined by Anderson et Retention was determined by dynamic surface tension, whereas droplet spreading was related to equilibrium values. They concluded that the basis of selection could be determined by cost, lack of phytotoxicity, and ability to solubilize or aid penetration of the a.i. this would not prejudice the retention properties of the formulation. 

---