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Why saliva forms beads on a string

Researchers’ solution to longstanding mystery could enable better inkjet printing and drug-dispensing systems.

The so-called “beads-on-a-string” phenomenon can be demonstrated with a very simple experiment. Stretch a glob of saliva between your thumb and forefinger, and you should see a string of beads form, before the strand eventually breaks.

Researchers have now discovered precisely why this occurs — a finding that could be used to improve industrial processes and for administering drugs in “personalized medicine.”

Saliva and other complex “viscoelastic” fluids, such as shaving gels and shampoo, contain long chains of molecules called polymers. In the case of saliva, the polymers are proteins known as mucopolysaccharides. In comparison, liquids such as water and other so-called “Newtonian” fluids do not form beads when stretched, because they lack dissolved polymers.

MIT’s Gareth McKinley, professor of mechanical engineering, and researchers at Purdue University and Rice University found that in addition to the elasticity of a fluid a key contributing factor in the beading mechanism is fluid inertia, or the tendency of a fluid to keep moving unless acted upon by an external force. Other major elements are a fluid's viscosity; the time it takes a stretched polymer molecule to "relax," or snap back to its original shape when stretching is stopped; and the "capillary time," or how long it would take for the surface of the fluid strand to vibrate if plucked.

Gareth McKinley describes how a material can be "stringy."
Video: Melanie Gonick; still images/additional footage: Gareth McKinley/Colin McKinley/NASA

The researchers discovered that the bead formation depends on a delicate balance of two ratios: the viscous force compared to inertial force and the relaxation time compared to the capillary time. The researchers reported their findings in the June 6 issue of Nature Physics.

“What makes this result a breakthrough is that bead formation is more complicated than a lot of people had assumed. The insight that inertia matters is a surprise. Earlier attempts to explain this phenomenon without inertia are either wrong or assume inappropriate physics,” says Yuriko Renardy, professor of mathematics at Virginia Tech University, who was not involved in the research.

Understanding how these beads arise could enable researchers to design systems that precisely control the formation of the beads, leading to improvements in various technologies, such as inkjet printing. The information also might be used to develop drug-dispensing systems for patients with certain disorders that require precise doses of medication depending on daily blood measurements.

Material from a Purdue University press release was used in this story.

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