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A Shear Mystery


Whipping cream will keep its shape unless disturbed. Everyone has had problems with a ketchup bottle at one time or another. After struggling and only getting a few drops, a flood suddenly gushes out and buries your food. With perfect timing, the ketchup changes from a thick paste to a runny liquid. If you find yourself wondering 'why?' you're in good company. Physicists are puzzled, too.

Ketchup is one of many complex fluids - including whipped cream, blood, film emulsions, nail polish and some plastics - that share a property called 'shear thinning.' Normally thick like honey, they can become thin and flow like water when stirred or shaken. The phenomenon is common enough, yet scientists aren't sure why it happens.

The ketchup-like behavior of pure fluids at their critical point is still only theoretical. Even simulations using supercomputers can't prove the theory. Think of that the next time you whack the bottom of a ketchup bottle. Even supercomputers can't predict the outcome!

Fact Credit:
NASA Marshall Space Flight Center
Science @ NASA

Further Reading
The Properties of Gases and Liquids
by Bruce E. Poling, J. M. Prausnitz, John P. O'Connell, John Prausnitz


Related Web Links
CVX-2
by Glenn Research Center

Complex Fluids
by Spallation Neutron Source





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