Viscoelastic Fingers

This series of photos shows two plates with a thin layer of polymer-laced, viscoelastic liquid.  As the two plates are separated, complex instabilities form.  The lower section of each photograph shows the fluid on the plate, with finger-like Saffman-Taylor instabilities forming as air rushes in between the gap in the plates. As the separation increases, the polymers in the liquid stretch under the increased strain, inducing elastic stresses in the fluid that cause the formation of secondary structures. (Photo credit: R. Welsh, J. Bico, and G. McKinley)

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