Supersonic Air Flow
Throwing a stone onto the surface of motionless lake triggers a spectacular series of events, beyond what we can see. When the stone hits the lake, a thin sheet of liquid called the "crown splash" is thrown upwards along the stone's rim. Meanwhile, below the surface of the water, a large cavity forms in the stone's wake.
Because of the pressure of the surrounding water and the pull of gravity, the underwater cavity that is formed by the stone hitting the lake's surface immediately starts to collapse and elongate into an "hourglass" shape. The cavity then closes in a single point, ejecting a thin, almost needlelike, liquid jet. The air flow during these changes reaches supersonic speeds.
This research was performed by Stephan Gekle (University of Twente), Ivo R. Peters (University of Twente), José Manuel Gordillo (Universidad de Sevilla), Devaraj van der Meer (University of Twente), and Detlef Lohse1 (University of Twente).