If your social life is anything like mine, you have probably spent a few Saturday nights rubbing saliva between your thumb and forefinger and watching a beaded liquid string form as you slowly pulled your fingers apart.

saliva_BOAS.jpg
       Some polymer-containing fluids, like saliva, form 
         beads-on-a-chain structures when stretched.

Well if you haven't, try it now. Trust me, it's cool.

"At first it may look like a wire as you are separating your two fingers. But then, all of a sudden, little beads start forming on it," Dr. Osman Basaran, a professor at Purdue University's School of Chemical Engineering, said during a phone interview with me last week. "That's how I started thinking about this problem." 

Basaran and his colleagues provided a detailed explanation of how these structures form in this month's Nature Physics.

Their work has some very useful medical applications, particularly in managing drug dosage.

Several examples of these liquid beads-on-a-chain structures exist in nature, from fish slimes to silk threads. Until now, scientists did not have a detailed understanding of how these necklaces took shape.

However, scientists did know that these beaded string-forming liquids contained polymers.  Polymers consist of molecules strung up into long chains and they can impart visco-elastic properties to fluids. As NOVA's Peter Tyson explained in his article on the engineering behind the Twin Towers, visco-elastic materials can flow like fluids while having some capacity to remake their original shape after being stretched. It was generally assumed that this property was the underlying factor in the formation of these liquid necklaces. 

But Basaran's team found there are additional forces involved in this process, including capillarity and inertia. The group developed computer programs to accurately describe how a polymer-containing liquid assembles into a beaded string structure.

So what is actually happening when you pull your fingers apart after rubbing them with saliva?

Capillarity (the "thinning" force) causes this fluid string to thin out. If it were the only force involved, the thread would break immediately. But, the viscosity of the liquid and inertia (the force that resists changes in velocity) counteract this thinning.

Furthermore, Basaran and his colleagues found that inertia induces the formation of the first liquid bead, which appears in the middle of the thread. And the liquid's visco-elastic nature, thanks to the polymers, causes more beads to form throughout the string. 

Basaran and his colleagues have not only solved the mystery of how saliva can transform into beads-on-a-chain, but their work could be very useful in developing applications for personalized medicine. 

Basaran is currently collaborating with other researchers to develop a strategy for printing fluids containing drugs onto edible substrates, like sugar pills. The technology would be similar to how an inkjet printer ejects ink onto paper. 

The accuracy of this "medical printing" is crucial. This is where Basaran's work comes in.

"You have these complex fluids, and they can form beads-on-a-string during processing," he said. "The beads go all over place...You'll lose fluid and the stuff will be a God-awful mess."

By understanding the optimal conditions required for these beads to develop, scientists can prevent them from ever forming.

Basaran envisions a future where devices could measure out precise dosages for each patient, based on an individual's medical profile.

"A patient might measure something in his or her blood, like a diabetic [measures blood glucose]," he said. "Then a device would know exactly what dosage should be put on that edible sugar pill."

Why else is this important? Here at NOVA, we love doing weird science experiments at our desks. Give it a try. Stick your fingers in your mouth and build a saliva necklace. It's the phenomena of complex fluid dynamics right at your fingertips.

Intern Rebecca Cheung is a graduate student at the University of British Columbia's School of Journalism.

Image Courtesy Gareth H. McKinley/MIT.

User Comments:

I love this article, very interesting and great writing:)

Great article. It reminds me of the basketball player who couldn't control his saliva, but he sure can dribble :-P

awesome article Becs

im just curious if you can tell what shape an object that is dropped in water is from the ripples that come from it and what practical application would that info be good for

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