Ultrasonic extraction or ultrasonication can enhance and accelerate the algae oil extraction processes. In an ultrasonic reactor, intense sonication of liquids generates ultrasonic waves that propagate into the liquid medium. During the low-pressure cycle, high-intensity small vacuum bubbles are created in the liquid due to the pressure imbalance. When these bubbles attain a certain critical dimension (cavity size), they collapse violently during a high-pres­sure cycle. As these bubbles collapse vigorously near the algae cell walls (i. e., implode) they create shock waves, and locally high-pressure and high-speed liquid jets. The resultant shear forces cause algae cell walls to mechanically rupture and release or help release their contents (algae lipids) into the sol­vent medium. This process of bubble formation and subsequent collapse is mechanistically called cavitation [32]. The advantages of the process include:

(a) Dry cake is not required for oil extraction.

(b) No caustic chemical is involved.

(c) The ultrasonication process can be used in conjunction with enzy­matic extraction.

(d) Environmental impact is minimal.

However, the process is not yet proven on a large scale and the energy cost needs to be lower in order to be competitive. Ultrasonication can be employed in a solvent extraction process, however, the process is usually classified as mechanical extraction, largely based on its mechanically induced cell wall rupture mechanism.