Viscoelastic fluids are a type of non-Newtonian fluid in which the stress-strain relationship is time-dependent. They are often capable of generating normal stresses within the fluid that resist deformation, and this can lead to interesting behaviors like the bead-on-a-string instability shown above. In this phenomenon, a uniform filament of fluid develops into a series of large drops connected by thin filaments. Most fluids would simply break into droplets, but the normal stresses generated by the viscoelastic fluid prevent break-up. For this particular photo, the stresses are generated by clumps of surfactant molecules within the wormlike micellar fluid. Similar effects are observed in polymer-laced fluids. (Photo credit: M. Sostarecz and A. Belmonte)

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