May The Force (of Mayonnaise) Be With You!

By Roland Piquepaille

A team of chemists at Rice University was working last year on two separate projects: trying to create strong fibers from carbon nanotubes, and testing emulsions of oil and water. And they discovered that a force known as "negative first normal stress difference" was present in both solutions. The next step was to go to a grocery store and buy a more common emulsion, namely a jar of mayonnaise. And bingo! This force was also at work. They tell us more in this news release, "Bizarre Attractive Force Found in Mayonnaise." Unfortunately, they don't know what to do from their findings. Still, it's fun science.

Here is the introduction.

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.

Now, let's look at the experiments.

Last year, Matteo Pasquali, assistant professor of chemical engineering, and colleague professors and graduate students at Rice were trying to create a strong fiber of pure carbon nanotubes. The researchers were attempting to adapt the methods DuPont had pioneered in the creation of Kevlar, and they ended up with a solution of nanotubes that behaved like a liquid crystalline polymer. The solution exhibited negative normal stress, which was a confirmation that the nanotubes had made a liquid crystal.

At about the same time, Pasquali and graduate students Alberto Montesi and Alejandro Peña were testing emulsions of oil and water. Pasquali, Montesi and Peña found that negative normal stress was present in their oil and water emulsions when the concentration of water droplets was in a specific range.

They were puzzled by these results.

"When I first saw the data, I thought we had made a mistake," said Pasquali. "My students and I joked that we must be the only lab that had ever had negative normal stresses on two systems. However, we double- and triple-checked, and the effect was still there."

This is how the idea of testing ayonnaise came from.

"I went to the store, bought a jar of mayonnaise and tested it," said Montesi. "I was curious to see if it showed the same negative normal stress, and it did."

But what can we expect from these findings?

The findings clearly open the door to future studies about the underlying phenomena behind negative normal stress.

Pasquali added that more study is needed before practical applications will become apparent.

The research work has been published on February 6, 2004 by Physical Review Letters under the name "Vorticity Alignment and Negative Normal Stresses in Sheared Attractive Emulsions." Here is a link to the abstract.

Source: Rice University News Release, March 29, 2004