Charging a spray to control where it lands

1. An uncharged spray relies on the nozzle alone, so each droplet travels in whatever direction it happens to leave the nozzle and much of the liquid misses the target.
2. An electrostatic sprayer gives every droplet an electric charge as it forms, so the forces between charges then control where the spray goes.
3. All of the droplets are given the same sign of charge, which makes them like charges, so they repel one another.
4. That repulsion pushes the droplets apart, so they spread out into a fine, even cloud instead of joining together into large drops.
5. Small, well separated droplets coat a surface more evenly and cover a larger area for the same volume of liquid.
6. The target is given the opposite charge, either directly or by earthing it, so the droplets and the target are unlike charges and attract.
7. Because that attraction is a non-contact force, the droplets curve towards the target instead of travelling in straight lines.
8. Droplets are therefore pulled onto surfaces the nozzle is not pointing at, including the sides and the far face of the object.
9. Once part of a surface is coated, the charge already sitting there repels further droplets towards the bare areas, so the coating tends to even itself out.
10. The two forces have separate jobs: repulsion spreads the spray out, and attraction delivers it to the target.

Insecticide sprayers

1. A crop sprayer charges the insecticide droplets as they leave the nozzle, so they repel one another and spread across the full width of the spray boom.
2. The crop is connected to the ground through its stems and roots, so it behaves as though it carries the opposite charge to the droplets and attracts them.
3. Droplets are pulled onto the undersides of the leaves as well as the top surfaces, which matters because many pests feed underneath.
4. Less insecticide drifts away on the wind or falls onto the bare soil between the rows.
5. A smaller volume of insecticide gives the same protection, so the cost to the farmer falls.
6. Less chemical reaches the soil, hedgerows and nearby watercourses, so contamination of the environment is reduced.
7. The spray still has to be used with care, because charge cannot hold onto droplets that have already drifted well away from the charged crop.

Paint sprayers

1. In a spray booth the paint is broken into a mist of tiny droplets and each droplet is charged as it is formed.
2. The droplets all carry the same sign of charge, so they repel one another and spread into an even mist rather than arriving as blobs.
3. The metal object being painted, such as a car body or a bicycle frame, is earthed or given the opposite charge.
4. The droplets are attracted to every part of that metal surface, including edges, curves and the back of each tube, which a directed spray would miss.
5. Paint that would otherwise be lost as overspray is drawn onto the object instead of settling on the floor and walls of the booth.
6. The layer of paint is more even in thickness, so fewer coats are needed and the finish is smoother.
7. Wasting less paint saves material and reduces the volume of solvent released into the air of the workshop.

Photocopiers and laser printers

1. A laser printer starts by giving a rotating drum an even coating of charge across its whole surface.
2. A laser beam is then scanned across the drum and removes the charge from the areas that will be left blank on the page.
3. The charged pattern left on the drum is an image of the print, held in place because the drum surface is a poor conductor.
4. Toner powder is given the opposite charge to the drum, so it is attracted to the charged areas and stays away from the discharged ones.
5. The paper is then charged more strongly than the drum, so the toner is attracted off the drum and onto the sheet as it passes.
6. Heated rollers melt the toner so that it fuses into the paper fibres, and the image no longer depends on charge once it has cooled.
7. The same rule does all of the work: unlike charges pull the powder to the places that should be printed, and like charges keep it away from the rest.