Acoustic nonlinearity as a mechanism for liquid drop explosions in drop-chain fountains generated by a focused ultrasound beam

Ultrasonic atomization has been used in air humidifiers and is also involved in therapeutic applications of intense ultrasound such as boiling histotripsy. An as-yet unexplained phenomenon occurs when a focused ultrasound beam in water creates an acoustic fountain in the form of a drop chain, which explodes in less than a millisecond. In the present paper, we seek to develop a nonlinear theory to explain this phenomenon. We hypothesize that standing wave harmonics are generated inside the water drops due to acoustic nonlinearities, which, along with localized heat deposition in the drop center, may generate a superheated vapor bubble that causes the explosion.