Deposition and Dissipation of Droplets Applied Aerially at Low Spray Volumes Using Penetrator

Marion Farris and Marc C. Hirrel


 
ABSTRACT

Evaporation simply defined means to change a liquid (or solid) into a vapor. From experience, everyone knows that water placed in an open container decreases in volume upon standing. When this occurs we say that evaporation has taken place and can be explained in terms of the motion of molecules. At any given temperature above absolute zero the molecules of a liquid move - some slowly, some at intermediate rates, some very fast. A rapidly moving molecule near the surface of the liquid may possess sufficient kinetic energy to overcome the attraction of its neighbors and escape, i.e., evaporate, to the space above the liquid. This activity continues until a dynamic: equilibrium exists where the vapor and liquid below it are in balance with each other, i.e., evaporation and condensation cease to occur.

Several factors affect evaporation. Some of those factors are temperature, relative humidity, size of droplet or body, purity of the material, etc. Temperature, as it changes, causes an increase or decrease in molecular activity and thus, affects rate of evaporation. Relative humidity, as it decreases or increases, affects evaporation; and mixtures of compounds do have an affect on evaporation. The vapor pressure of a solution containing a nonvolatile substance or a substance of low-volatility (oils or solids) is lower than that of the pure liquid. This can be changed to the opposite by mixing highly volatile materials such as alcohols, esters, etc., with water. This can be explained because the molecular activity is changed (slowed when adding an oil or speeded up when adding an alcohol or other material).

What we are concerned with is a droplet of material (usually water and pesticide) reaching the target to which it is directed. As an aerial applicator travels across a field at a high rate of speed the material he is spraying (the droplets) has numerous forces working an it. Size of the droplet, temperature, relative humidity and other factors come into play. With this knowledge, some adjuvants can be useful tools in the control of evaporation. The material we have experimented with is PENETRATOR, an emulsified highly refined mineral oil with no obvious phytotaxicity.

Deposition of low volume sprays into a cotton canopy can be improved by adding PENETRATOR. Even at a spray volume of 9.34l/ha (1 GPA) with PENETRATOR at 0.59l/ha (B fl. oz/A) gave an increase of about 64% over water alone. However, this low rate may not protect the crop below 61 cm (2 ft.) above the soil, if the spray volume was increased to 28.074l/ha (3 GPA). PENETRATOR rates as low as 0.30l/ha (4 fl. oz/A) in a spray volume of 9.34l/ha (1 GPA) increased area of coverage by approximately 37%, but was not consistent enough to risk losing insect control. Dissipation of droplets from a altitude of 6.1 m (20 ft.) showed a straight line relationship with water alone. The rate of deposition was greater at lower temperatures and higher relative humidities. When PENETRATOR was added at 0.59 l/ha (B fl. oz/A) in a spray volume of 18.7l/ha (2 GPA), the rate of droplet dissipation was less than for water alone. Correlation coefficients of height by droplet density or height by area covered for PENETRATOR was 0.59 and 0.44, respectively. Compared to water alone at nearly the same conditions, the correlation coefficients were 0.89 and 0.63, respectively. Thus, PENETRATOR may aid in protecting the droplets from evaporation or other physical effects. Such a characteristic would make height above the canopy less critical during aerial application and improve uniformity of coverage.



Reprinted from Proceedings: 1989 Beltwide Cotton Research Conferences pp. 305 - 308
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998