The same is true for electrostatic forces. The action of a great number of particles 
under the influence of primary forces andinteractionof primary forces should be studied, 
therefore. 
In addition, the aerodynamic flow pattern, produced by existing pesticide applica- 
tion equipment, as well as proposed equipment, should be thoroughly studied. This would 
include aircraft, airblast, and hydraulic spray equipment. The penetration of pesticide 
sprays into plant foliage should be correlated with various aerodynamic patterns and 
with the initial energy possessed by the spray. The characteristics of variously shaped 
force fields and their resulting gradients need thorough investigation to take best ad- 
vantage of the forces we have available. 
We have seen that the forces on a particle depend upon its size. It is also generally 
believed that the biological effectiveness of a pesticide often depends upon the size of 
particles into which it is divided. This is usually based on calculations of the theoretical 
coverage obtained from dividing a certain volume into certain particles of various sizes. 
The following illustrates this by showing how 10 gallons of liquid would cover a 
perfectly flat surface with droplets having a contact angle of 90 : 
Percent of one acre 
D Di ~ an distance 
“Wie recheriaa ane Soiree are in mm, poveree at Contact 
angle - 90 
10 2,213,500 0.00683 266.0 
20 267,000 0.0194 133.0 
30 79,100 0.0354 88.7 
40 335350 0.0547 65.5 
50 17,080 0.0765 BSI4 
75 5,600 0.134 35.5 
100 235 0.217 26.6 
150 632 0.398 Lite’: 
200 267 0.610 13.31 
300 Ubu ols 8.87 
400 SiS}53) 1.74 6.65 
500 Viel 2.24 bers 
750 5.6 4.24 3655 
1000 Zols 6.83 2.66 
For plant surfaces these figures would have to be divided by the acres of plant 
surface per acre of land which may vary from 5 to 20 and multiplied by a spread factor 
corresponding to actual angle of contact. 
From these figures it is not difficult to appreciate the reasoning in favor of low 
gallonage--fine droplet spraying. Theoretically, the reduction in coverage resulting 
from a 50 percent decrease in volume can be offset by reducing the droplet size by 
only 50 percent. Consequently, it has always been assumed that pest control generally 
improves as we divide our pesticides finer and finer, although experimental evidence 
to support this exists for only a few pest-pesticide combinations. It is also reasonable 
to assume that variable deposits are wasteful. 
However, we are certain that, quite often, the finer the droplets the more difficult 
it is to control their placement. If all applications were made with large spray volumes 
and with a coarse spray with a low surface tension, we would have less trouble with 
deposition. But because of dusting, low-volume spraying, and perhaps sprays of higher 
surface tension, deposition is difficult. 
4One micron equals approximately 1/25000 inch, 25 microns equals approximately 1/1000 inch, 
81 
