Starbursts and flowers: when spreading droplets break bad
Karen Daniels, NCSU
A droplet of pure water placed on a clean glass surface will spread axisymmetrically: there is nothing to break the symmetry. For more interesting fluids or more interesting surfaces, new patterns of spreading are possible. I will highlight two systems in which dramatic instabilities arise through the interaction of surface tension, elasticity, and the interface between them. The first example -- starburst fractures -- arises when droplets are placed on very soft substrates, such that elastic forces are in direct competition with capillary forces. The second example is the surprising case of a liquid metals, where fingering instabilities are unexpected due to typically large interfacial tensions. However, electrochemical oxidation can lower the interfacial tension of gallium-based liquid metal alloys, thereby inducing drastic shape changes including the formation of fractals.