Gareth H. McKinley/MIT

ScienceShot: Spitting for Science

Drop a glob of spit on your index finger, press your thumb on top of it, and slowly pull your thumb away. See those little beads that appear as the spit stretches between your digits? Scientists have finally figured out why they form. They reported online 6 June in Nature Physics that a computer model reveals that saliva and other fluids such as shaving cream contain long polymer chains that, when stretched, form beads. Water and other fluids don't contain polymers and don't form the beads. For the beads to form the fluid's inertia has to be large enough and its viscosity small enough, the researchers say. The information could be used to prevent those annoying beads of ink that smear all over documents from inkjet printers. Something to remember the next time you spit on your finger. And don't forget to wash your hands.

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