Why saliva forms beads
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 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 beads when stretched, because they lack polymers.MIT’s Gareth McKinley, professor of mechanical engineering, and researchers at Purdue University and Rice University found that a key factor in the beading mechanism is fluid inertia, or the tendency of a fluid to keep moving unless acted upon...