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!

About the Author

NASA Marshall Space Flight Center

NASA Marshall Space Flight CenterThe George C. Marshall Space Flight Center, located in Huntsville, Alabama, is the U.S. government's civilian rocketry and spacecraft propulsion research center. As the largest NASA center, MSFC's first mission was developing the Saturn launch vehicles for the Apollo program.