Understanding Water Drop Back-Jet Physics

Scientists analyze what happens when water drops impact

Everyone knows that when a water drop falls on a stretch of water it generates a back-jet, a column of the liquid that shoots upwards at very high speed, and then falls back in. Researchers from the Foundation for Fundamental Research on Matter (FOM), the University of Twente, in the Netherlands, and the University of Seville, in Spain have spent a lot of time analyzing these jets, in order to understand what exactly generates the high speeds at which they are formed, despite the fact that the speed of the water drop itself is at times very low.

For their research, they used a high-fidelity camera that had the ability to film in slow-motion. Based on the readings they recorded with the device, the experts created a piece of computer software capable of simulating the exact reactions that water has when impacted by a liquid drop. They learned that, following the precise moment in which the drop impacts the water, a hole is generated in the surface of the liquid, which creates a pressure imbalance in the area.

This difference converts the surface tension of the water in an upwards force and makes the superficial layers of the water stretch to move in an close the gap that the drop left behind. But water is pushed inwards at very high speeds from all directions, so, when it meets with other H2O molecules coming from opposite directions, they collide very powerfully, and the result is excess water being ejected upwards, at a much higher speed than the drop falling into the water had itself.

The new research, published in the January 23rd edition of the journal Physical Review Letters, also details the researchers' theoretical model on the matter, which explains how each layer of the water accelerates to fill in the gap, creating the massive outburst of liquid. They also completed a high-detail computer model of the phenomenon, one that comprises more than 30,000 frames per second. This high-definition image allows them to play back the scenes at a very slow speed, and, based on that, elaborate a theory.

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