London, Jan 25 (ANI): A pebble, if dropped in water, creates an air-filled cavity that ejects air at supersonic speeds, researchers the University of Twente in the Netherlands have found.

Stephan Gekle and his colleagues used high-speed photography, and spotted a cavity of air forming in an hourglass shape - with the top of the hourglass at the surface of the water and its base at the sinking object.

To measure the speed of air rushing out upwards, they marked the air with smoke before the splash, reports New Scientist.

Even though their camera took 15,000 frames per second, they still couldn’t measure the fastest speeds directly, so they simulated the behaviour they had observed.

They found that shortly before the cavity closes, the pressure of the air at the bottom of the hourglass becomes higher relative to the “neck”.

The difference pushes the air out at speeds faster than sound.

The study has been published in the Physical Review Letters. (ANI)