Modes of ejecting ink

Thermal Bubble: Ink is drawn by vacuum from the tank section of the cartridge to the minuscule heating chamber(s) of the printhead. An electric current heats a resistor. The heat forms an air bubble and as the bubble expands an ink droplet is ejected. The electric current is temporarily removed, the bubble shrinks, a vacuum is formed, more ink is drawn into the chamber and the cycle repeats at a mind boggling pace.

Figure 1 shows the thermal bubble process. 

Figure 2 shows a magnification of the printhead elements (after removing the printhead plate). 

Figure 3 shows an actual heating chamber.

Figure 4 shows why it is important to never allow a cartridge to run completely dry. The ink acts as a coolant.

When it is missing, the resistor overheats 

and acquires an encrustation that interferes with the formation of the bubble - therefore no ink is ejected. For the user that heating chamber becomes a chamber of horror as the printout looks horrible.

Figures 2,3, and 4 courtesy of Dr. W. Reick of TB Accessories Brazil

Piezoelectric: One of the ink chamber walls is a ceramic transducer that swells and shrinks as electric pulses are applied to it. Each pulse means a swelling and each swelling means the ejection of an ink droplet. Figure 5 shows the Piezoelectric ink ejection process. Refer to the relevant entry in the section ‘Devilish Dictionary’ for more information on the piezo-electric phenomenon.