One common chemtrail type is called the "Pulsed" or "Drippy-Edge" type because of it's intermittent oscillating appearance. This pulsed appearance is an artifact of an oscillating diaphragm pump.
Diaphragm pumps work similar to a piston engine. They push using a flexible diaphragm that oscillates back and forth. One-way valves keep the pumped substance moving in one direction, like the valves in a piston engine.
Diaphragm pumps are used for pumping fine-particle semi-solids (like toner used in laser printers) and liquids containing small solids. The aluminum and barium powders that whistleblowers reported being loaded onto jets is a perfect example of a substance that would require a diaphragm pump.
Below are images of diaphragm pumps and chemtrails exhibiting the the pulsed signature of diaphragm pumps.
Images
Bypass Air Injection
Images that show the "oscillating" pulsing artifact created when a diaphragm pump is used to pump small-particle aerosols.
Diaphragm Pumps
Fuel Line Injection
Below we see the somewhat smoother trails resulting from aerosol injection into fuel lines. Fuel line injection results in a trail being produced by the Auxiliary Power Unit (APU), usually in the rear of the fuselage.
There is often a hint of pulsing in the trail due to the use of oscillating diaphragm pumps to transfer small-particulate metals, but the effect is subdued by aerosol premixing that occurs in the fuel lines.