Beads of Saliva

You can stretch a glob of saliva between your thumb and forefinger. Before the strand of spittle breaks, a string of beads is formed.

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.

Key factors involved in the beading mechanism are :

• fluid inertia, or the tendency for a fluid to keep moving unless acted upon by an external force
• viscosity, or the time it takes a stretched polymer to 'relax' or snap back to its original shape when the stretching ceases
• capillary time, or how long it would take for the surface of the fluid strand to vibrate if plucked

In order to form beads the fluid inertia has to be large enough and the relaxation time has to be small enough. Bead formation depends on two ratios:

• the viscous force compared to the inertial force
• the relaxation time compared to the capillary time

Learning how to control bead formation could be used to improve printing because smearing 'satellite' beads form around droplets produced by an inkjet printer. Understanding bead formation could also help improve industrial processes such as electrospinning which is used to make a variety of products, and spray coating used in painting.

Reference:
Pradeep P. Bhat, Santosh Appathurai, Michael T. Harris, Matteo Pasquali, Gareth H. McKinley, Osman A. Basaran. Formation of beads-on-a-string structures during break-up of viscoelastic filaments. Nature Physics, 2010; DOI: 10.1038/nphys1682

Activities

1. Design an experiment to measure the viscosity of shampoo.
2. Suggest ways that the viscosity of shampoo could be changed.
3. Design an experiment to test one of the hypotheses above.
4. Design an experiment to measure the fluid of inertia of a range of different fluids.