Reporting in Nature Physics, Pradeep Bhat and colleagues explain a vexing phenomenon in fluid dynamics — the 'beads-on-a-string' structures that form in viscoelastic fluids (P. P. Bhat et al. Nature Phys. doi:10.1038/NPHYS1682; 2010).
It's easy to observe this effect: take a blob of saliva from the top of your tongue, place it between your thumb and index finger, then slowly pull your digits apart. With practice, you'll form a thread of fluid that initially thins and drains, but that eventually forms a string of different-sized spheres (pictured). Newtonian fluids, such as water, don't do this — instead, the threads quickly break.
Saliva differs from water in containing naturally occurring polymeric molecules that make it viscoelastic. This property was thought to cause the beads-on-a-string effect, yet computer models of viscoelastic liquids couldn't reproduce the phenomenon.
Bhat et al. report a new computer model that factors in inertia. They find that inertia causes beads to form even on threads of low-viscosity Newtonian fluids. But in viscoelastic fluids the beads last longer, grow bigger and become more spherical. The authors' simulations also reveal that enhanced radial flow occurs at certain regions of threads, causing additional, smaller beads to form in viscoelastic fluids. They conclude that the beads-on-a-string effect results from the interplay between capillary, viscous, elastic and inertial forces.
The model offers fresh ways to explore the behaviour of materials deformed beyond their equilibrium. This is of relevance to commercial processes such as electrospinning, in which electric charges are used to draw fibres from liquids.