Green Fingers

Differences in surface tension between two layers of fluid can cause fascinating finger-like instabilities.  Here glycerol is spread in a thin film on a silicon wafer.  Then a wire coated in oleic acid, which has a lower surface tension than glycerol, was touched to the wafer.  As the oleic acid spreads across the film surface, Marangoni and capillary stresses cause variations in the film thickness, which results in the dendritic patterns seen here. (Photo credit: B. Fischer et al.)

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