Thermal inkjet
The method that boils the ink to make it fire itself out — one vapour bubble per droplet, tens of thousands of times a second.
A thin-film resistor in the nozzle chamber heats for a few microseconds until the ink touching it boils. The resulting vapour bubble pushes out a droplet of a picolitre or so, and when the bubble collapses capillary action pulls fresh ink in behind it. Nothing moves mechanically inside the nozzle, which is the whole point: hundreds of nozzles fit on one chip, and the print head becomes cheap enough to mount on the cartridge and throw away with it. That detail became the industry's business model.
Canon's Ichiro Endo hit on the effect in 1977 — by the company's own telling, after a soldering iron accidentally touched an ink-filled syringe — and HP's labs arrived at the same idea independently a couple of years later. Canon called it Bubble Jet, HP called it thermal inkjet. Epson went the other way with piezoelectric crystals that shove the droplet out without boiling anything, which allows ink chemistry that heat would ruin.