The fourth mode is known as instant atomization. In this process the jet appears to 
break up instantly as it issues from the nozzle, due to the very great aerodynamic forces 
on the liquid. This is the controlling process in most practical spraying applications. Very 
little is known about the mechanism of this type of atomization, because of the very high 
speeds of the small particles involved. At this point it would be well to illustrate why it 
has been difficult to learn much of the mechanism of instant atomization. For example, in 
a hydraulic nozzle operating under typical conditions, we would have to photograph, 
preferably in motion picture form, a jet or droplet one-half mm. or less in diameter 
and travelling 27,500 mm. per/sec., roughly 60 to 70 miles per hour, in order to see what 
is going on. This is very difficult. This is equivalent to taking a photograph of a golf 
ball travelling 3,000 miles per hour, assuming equal size images on the film. Photo- 
graphic equipment developed recently may be capable of this job, but it is quite costly. 
Trajectories and Deposition. The trajectory and deposition of pesticide particles are 
controlled by inertial, gravitational, aerodynamic, thermal, and electrical forces. 
When a droplet is formed it is generally given a velocity in some direction. Because 
of its momentum it will tend to continue in this direction except for acceleration in other 
directions by various forces, in accordance with Newton's second law of motion. Gravi- 
tational attraction will exert a force downward: 
3 
D Po 
Fy = 6 
where D is the particle diameter and p is its density. 
9 
Because air is ever present aerodynamic drag will exert a force opposite the 
instantaneous velocity relative to the air on a spherical particle of 
a d 
where pa =density of air, v = velocity of particle, and Cg = drag coefficient, which in 
turn depends on the Reynolds number ~vPawhere H is the viscosity of air. For small 
DvP, . 24u 
particles generally aie is small enough that Cg= p.vD is accurate. Then 
Fa 
F, = -3muvD 
78 
