"Any kindergartener is familiar with this beading phenomenon, which you can demonstrate by stretching a glob of saliva between your thumb and forefinger," said Professor Osman Basaran of Purdue University.

Before the strand of spittle breaks, a string of beads is formed.

"The question is, why does this beading take place only in some fluids containing polymers but not others?" Professor Basaran said.

Purdue postdoctoral researcher Pradeep Bhat and his colleagues have determined the mechanism behind the beading and created a computational model to simulate the phenomenon.

Knowing the answer to this question might enable researchers to design systems that precisely control bead formation, leading to improvements in various technologies such as inkjet printing.

The information also might be used in a system that precisely dispenses the correct dose of medications for individual patients based on simple blood tests.

Saliva and other complex 'viscoelastic' fluids like shaving cream 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 the beads because they lack polymers.

Conventional wisdom has held that all fluids containing polymers should form the beads, but researchers have shown that assumption to be wrong and have demonstrated why.

The researchers tested saliva and a material contained in a strip on the leading edge of disposable razors.

"You moisten the razor strip with water, which causes it to swell, press it against a finger and pull it," said Professor Basaran.

"Unlike saliva, you see strands of liquids formed but no beads."

A key 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.

"It turns out that the inertia has to be large enough and the relaxation time has to be small enough to form beads," Bhat said.

The researchers discovered bead formation depends on two ratios -- the viscous force compared to inertial force and the relaxation time compared to the capillary time.

Because smearing 'satellite' beads form around droplets produced by an inkjet printer, learning how to control bead formation might be used to improve printing.

Findings also may help to improve an industrial process called electrospinning, used to make a variety of products, and spray coating used in painting.

"The idea is that, if you are operating an inkjet printer, for example, you would be able to control these ratios to prevent the bead formation," Basaran said.

Findings may help to perfect a new type of drug-dispensing technology being developed for 'personalized medicine.'

The technique involves using an inkjet-printing nozzle to deposit drops of medication onto an edible substrate, such as paper or a sugar pill.

The approach might be used by patients with disorders that require precise doses of medication depending on daily blood measurements.

"Patients might be able to do this even at home," said Professor Basaran

"The patient will perform a routine sort of blood analysis, similar to blood-glucose monitoring, and then use this device to 'print' the exact quantity of drug based on the blood measurement, which would be done every day."