A courser spray can be achieved by increasing the viscosity of the spray mix. Increased viscosity results in an upward shift of the droplet spectrum so that there are fewer of the fine drops that are subject to drift. Many spray additives can be added to a tank mix to increase its viscosity.
There are many "drift retardant" chemicals commercially available to pesticide applicators. These products are normally some type of long chain polymer or gum that increases the viscosity of the spray mixture. Unfortunately, the information related to performance of these products is limited. Results of a study conducted by Bouse et al. (1988) indicated that the effect of polymer concentration on droplet size is dependent on polymer type. For example, polyvinyl and polyacrylamide polymers were found to be more effective than linear alkyl epoxide or polymide copolymers in increasing volume median diameter and reducing the percentage of spray volume composed of small droplets subject to spray drift. Researchers at Ohio State have tested five drift retardant chemicals to determine their effects on droplet size, spray pattern, and spray drift reduction (Ozkan et al., 1992). They determined the reduction in spray volume contained in small droplets for spray mixtures containing water with drift retardants.
In comparison to spraying water only, all drift retardants tested reduced volume of portion of small droplets in the spray but at varying magnitudes. For example, the reduction of spray volume contained in droplets smaller than 100 microns ranged from 30% with the least effective product, to 68% with the most effective product.
Some studies have found that some of these polymers tend to be sheared by passing through a sprayer pump, as would occur in normal bypass, hydraulic mixing in common agricultural sprayers (Zhu et al., 1997). This means that the drift retardant would lose its ability to increase droplet size - its ability to reduce drift - as the spray tank became empty. Gums are not sheared as easily as the long chain polymers, and some types of polymers (poly-ethylene oxide) are sheared in fewer passes through a pump than other types of polymers (polyacrylamides).
Although drift retardant chemicals are effective in reducing the number of drift-prone droplets, in most cases, it is more effective to select the proper size and type of nozzles and operate sprayers at low pressure to produce the desired drop size rather than attempt to increase droplet size with a drift retardant chemicals