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11 May 2004 (All day)
Swiping a finger through a cake batter leaves nary a trace, much to the relief of surreptitious batter-tasters everywhere. But vibrate the batter, and it might be a different matter. Researchers have recently discovered that it's possible to poke a hole in certain liquids when they're shaking. The creepy phenomenon could open a window into the behavior of granular substances as diverse as snow, quicksand, and breakfast cereal.
Cornstarch and water make a famously bizarre mixture. Throw a pasty ball of it against the wall, and it shatters--but the pieces on the floor will ooze into a puddle. The strange behavior happens because the fine grains of cornstarch suspended in the water slide past each other easily at slow speeds, but they jam up when slammed around--analogous to how cornflakes clog up the opening of the cereal box when poured too quickly.
Now cornstarch has added another trick to its repertoire. In the 7 May issue of Physical Review Letters, Robert Deegan and colleagues at the University of Texas, Austin, describe how they can poke holes in liquid mixtures of cornstarch and water. They vibrated dishes of the mixture at various frequencies and poked holes in the surface with puffs of air. At frequencies above 120 Hz, the holes would stay open indefinitely. If the dish vibrated more forcefully, something even stranger happened: Fingers of fluid rose up to centimeters from the rims of the holes, stayed upright for a few seconds, and then fell, making another hole. This happened repeatedly until the surface was covered with holes and weird fingers of fluid.
Deegan and his colleagues aren't sure what causes the holes to grow fingers and spread. They suspect that the reason the holes don't collapse as they would in a still fluid is because the vibrating cornstarch particles are moving too quickly; the forces as they slide past each other make them jam when the hole tries to collapse. The group got the same result with a mixture of glass microspheres in water, and they say it would probably work with any granular fluid, if vibrated at the right frequency.
Physicist Tomas Bohr of the Technical University of Denmark in Lyngby is impressed by the team's technique. Moreover, he says, the findings could have interesting real-world implications. "There are so many strange mixtures of clays and magma. ... Who knows what these systems are doing."