A team of researchers has uncovered a previously unknown phenomenon that could improve the way we design materials at the molecular level. By unlocking a transformation between two types of structural defects on the surface of liquid droplets, the research opens new possibilities for controlling molecular patterns with unprecedented precision. This discovery has broad applications across a range of technologies, including vaccine design, the creation of self-assembling structures, and the synthesis of complex nanoparticles. When guest molecules are positioned on liquid droplet surfaces, they typically spread out quickly due to diffusion, making it challenging to achieve precise control over their placement. However, the researchers discovered that droplets made from certain materials undergo a process known as "interfacial freezing," in which the droplet's surface forms a crystalline molecular monolayer while the bulk of the droplet remains liquid.

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.