HOLES are the last thing you would expect to find in a fluid. But take a mixture of ordinary cornstarch and water and shake it, and it forms cylindrical holes that last for as long as shaking continues.
鈥淚t鈥檚 completely counterintuitive,鈥� says physicist Robert Deegan of the University of Texas at Austin, a member of the team that stumbled on the effect.
Normally, any holes in the surface of a liquid quickly fill up under hydrostatic pressure. But not so in the cornstarch mixture, which, unlike most liquids, exhibits a curious property known as shear thickening: the more you stir, the thicker it gets.
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Deegan and his colleagues placed a 5-millimetre layer of cornstarch mixture in a tray which they vibrated vertically at about 120 hertz at accelerations of around 15 g. At first, the surface of the fluid formed regular patterns expected in vibrated fluids. But when the team blew open holes in the surface with a straw, the holes remained open for as long as the fluid was shaken (Physical Review Letters, vol 92, p 184501).
The researchers were able to create similar holes in a suspension of glass microspheres in aqueous sodium polytungstate, another fluid which exhibits shear thickening. But they were unable to reproduce the effect in normal fluids such as water. 鈥淭his strongly suggests that the shear thickening effect is responsible,鈥� says Deegan.
However, no one completely understands shear thickening in cornstarch, says Michael Cates, a physicist at the University of Edinburgh in the UK. 鈥淚 think it鈥檒l be a while before they can explain this.鈥�
And as if the holes are not difficult enough to explain, the cornstarch mixture behaves even more bizarrely at higher accelerations. The surface morphs into worm-like protrusions that seem to stand up and wave, and even crawl across the surface. (See video at ). 鈥淚t looks like 鈥楾he Thing鈥� crawling out of its hole,鈥� Deegan says.