How to Make a Star-Shaped Droplet
Ordinarily, a fluid droplet minimizes its surface area by forming a sphere, but over the past decade, researchers have been studying oil droplets tens of micrometers in diameter that can form a variety of shapes in a soapy (surfactant) fluid. At the right temperature, an oil droplet’s surface layer can form a 2D crystalline shell—even as the oil inside remains liquid—and the droplet can become a 20-sided icosahedron or one of several other shapes. Catherine Quilliet of Grenoble Alps University in France and her colleagues have now demonstrated the reversible, temperature-controlled transformation of a hexagonal, “pillow”-shaped droplet into a six-pointed star and have explained the mechanism using simulations and analytical arguments [1].
The shell’s observed shapes result from its requirement to minimize surface energy by becoming more sphere-like, while also being subject to other constraints. These include the crystal structure’s strong resistance to stretching and the need to accommodate defects in the crystal, which are locations at the vertices where the 2D crystal pattern is broken at the molecular scale. Defects, which allow the shell’s curvature to become concentrated at small points, are necessary for an otherwise flat crystalline surface to wrap around and enclose a droplet without much stretching. The surface tension (or energy per area) is temperature dependent, which explains why the droplet deforms in response to temperature changes.
In previous experiments, the temperature evolution led to merging and displacement of defects, which caused the shell to switch from one geometric form to another. However, during the hexagon–star transformation, the defects remain stationary. The researchers found that the transformation was caused not by defects merging but by creases forming in the droplet surface.
Team member Eli Sloutskin of Bar-Ilan University in Israel says that the point of the research is to learn how a curved, 2D crystalline surface responds to mechanical stresses, since little is known about the elastic properties of such structures. These droplets may also be exploited for their optical, electronic, or self-assembly properties, which could inspire new material designs. Finally, similar mechanical constraints arise in biological membranes within cells and in the protein shells of viruses, so these droplets may also serve as biological model systems.
–David Ehrenstein
David Ehrenstein is a Senior Editor for Physics Magazine.
References
- C. Quilliet et al., “Lenticular hexagon-to-hexagram shape transformation: Nano-origami in liquid droplets,” Phys. Rev. Lett. 136, 084002 (2026).