Technology and Science

Electrostatic spraying was developed over 80 years ago.  Almost all industrial metal parts (including automobiles and airplanes) are painted using electrostatics.  The concept is quite simple:

  • The solid or liquid particles are charged using a high-voltage source (10kV to 100kV).

  • These charged particles are sprayed from a nozzle towards the metal object to be coated.

  • In free space, these charged particles exhibit a force of attraction to the object, which is grounded.

  • Further, these charged particles are repelled from each other, which makes the spray very evenly distributed.

  • If the force of attraction is greater than other forces (including gravity), these charged particles move towards and evenly coat the metal object.

  • If all parameters are optimum, all of the particles attach to the metal object, and none falls on the ground.  Deposition efficiency is defined as the ratio of the amount of paint that attaches to the object divided by the total amount of paint that leaves the nozzle.  100% deposition efficiency is ideal, which means that all of the paint is deposited on the object.  In reality, deposition efficiency is about 95% in industrial painting, which is still good!

 

Electrostatic spraying has resulted in much less expensive and more ecological painting of metal objects.

 

The physics of electrostatic spraying of grounded metal objects is well known:

In free space near a grounded, perfectly conducting object, a charged particle causes an opposite charge to form on the object's surface. This causes a Coulomb force of attraction of the charged particle towards the charged object.  The math is complex, but it can be simplified by replacing the grounded, perfectly conducting object with a phantom image charge located within the object and with an opposite charge.  This works well for industrial electrostatic painting of metal objects.

 

Unfortunately, agricultural plants are not like metal objects.  Agricultural plants are not perfect conductors and ironically are not perfectly grounded.  Further, agricultural pesticide spraying experiences additional forces that reduce deposition efficiency, including wind and turbulence.  Current electrostatic spraying technology suffers from poor deposition efficiency, because plants are dielectrics (not perfect conductors) and because plants' roots usually extend only about 0.2 meters into the ground, which means that plants are not perfectly grounded.  Further, the electrical properties of plants vary from one plant species to the next and even from one plant to the next within a field.  This means that the deposition efficiency is poor and inconsistent using current electrostatic spraying technology.

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