Monday, April 05, 2004
BIZARRE ATTRACTIVE FORCE FOUND IN MAYONNAISE
Rice Engineers Find Evidence of Little-understood Force in Everyday Emulsions
Scientists at Rice University have discovered that a little-understood tensile force, which was previously thought to be an oddity found only in the types of plastics used to make bulletproof vests, occurs in everyday emulsions like mayonnaise and salad dressing.
First identified about 25 years ago, the phenomenon known as 'negative first normal stress difference' refers to an attractive force that is created within fluids under certain conditions.
For example, imagine two glass plates that are stacked like a sandwich, with a thin layer of liquid between. If the bottom plate is held still and the top plate is moved quickly to one side, it sets the fluid in motion, and creates forces within the fluid that, in turn, act upon the glass.
In simple fluids like water, this sliding motion creates complementary forces that tend to push the plate along in the direction that it was sliding. In more complex fluids like polymers, tension develops, creating forces that tend to push the plates apart. Around 1980, it was discovered that liquid crystalline polymers "
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