There are two main technologies applied in inkjet printers: the thermal technology and the piezoelectric technology.
Thermal inkjet technology (Bubble Jet)
The printhead of a thermal jet printer (Bubble jet) has a number of nozzles (up to 300 and more) an each of them “shoots” a microscopic drop of ink at very precise moments, producing tiny dots on the paper thus forming the text and graphics.
On the left you see a scheme of one such nozzle. Above the nozzle there is a chamber full of ink, at the top of which there is a miniature heater.
At the moment the nozzle has to print a dot on the paper, a burst of electric current is supplied to the heater and it gets instantly heated.
The ink that is in contact with the heavily heated surface evaporates and forms an air bubble that instantly pushes out a drop of ink through the nozzle opening.
Now you can see why you need to refill your print cartridges before the ink runs out. Besides printing, the role of ink is also to cool the heaters. If you try to print with a cartridge that does not have enough ink, the heaters will quickly overheat and eventually burn. In this way your print head will be irretrievably damaged.
The above technology is developed by Canon, but it is also used in most printers of Hewlett Packard, Lexmark, Xerox, and many others.
Piezoelectric inkjet technology
The principle of printing with this technology is not very different. The main difference is that the heater is replaced by a piezo crystal. It is known from physics that piezo crystals alter their shape and volume under the influence of electrical current.
When an electric current is supplied, the piezo element expands, thereby ejecting a drop of ink through the nozzle opening.
The Piezoelectric technology is mainly used by EPSON. These heads are usually permanent (that is, they are a part of the printer and the ink is contained in separate ink tanks).
The piezo heads use special inks, that turn into a jelly-like mass upon contact with air. That’s why you should take special care to keep these printers from drying out, as the replacement of an EPSON printer head usually costs as much as the entire printer!