Polygonal Droplets

Spheres are a special shape; they provide the smallest possible surface area necessary to contain a given volume. And since surface tension tries to minimize surface energy by reducing the surface area, drops and soap bubbles are, generally, spherical. There’s subtlety here, though: namely, what if reducing the surface area doesn’t minimize the surface energy?

That’s the issue at the heart of this study. It looks at microscale oil droplets, like the ones above, that are floating in water and stabilized by surfactants. We’d expect droplets like these to be round, and above a critical temperature, they are. But as the temperature drops, the surfactant molecules along the droplet’s interface crystallize. The drop itself is still liquid, but interface is not.

This changes the rules of the game. There’s no way for the surfactant molecules to form a sphere when solidified; they simply can’t fit together that way. So instead defects form along the interface and the drop becomes faceted. As the temperature drops further, the energy relationship between the water, oil, and surfactants continues shifting, causing the droplet to change shape – even to increase its surface area – all to minimize the overall energy. The effect is reversible, too. Raise the temperature back up above the critical point, and the interface “thaws” so that the drop becomes round again. (Image and research credit: S. Guttman et al.; via Forbes; submitted by Kam-Yung Soh)

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