The splash of a solid object into water–be it a coin or an
Olympic high diver–is capped off by a thin jet of fluid shooting
straight up from the surface. The detailed explanation of this
seemingly simple event has proved elusive. Now researchers
publishing in the 23 January Physical Review Letters think they
have a more complete explanation than their predecessors. Using
a combination of theory, simulation, and experiment, they
studied the collapse of the air cavity trailing the submerged
object, concluding that it ejects water like toothpaste squeezed
rapidly from its tube. A solid object plows a roughly
symmetrical tube of air into a fluid. The sideways pressure
exerted by the surrounding water quickly begins to collapse the
cavity, first by pinching off the cylinder at its midpoint. Twin
jets shoot straight up and down from the pinch-off point, called
a pinch singularity. Researchers had assumed that the
concentrated forces built up at the pinch point were sufficient
to drive jet formation, says Stephan Ge