Trapping Droplets In Unusual Shapes

Shake a bottle of oil-and-vinegar-based salad dressing and you’ll soon see spheres at the interface where the two immiscible liquids meet. That’s because a sphere is the most energetically favorable shape for droplets formed by mixing such liquids. The spherical shape minimizes the surface area and thus the interaction between the hydrophobic oil and the hydrophilic vinegar. Scientists at the University of Massachusetts, Amherst, have now come up with a way to distort the shapes of droplets and trap them in unusual forms (Science 2013, DOI: 10.1126/science.1242852). Thomas P. Russell, Mengmeng Cui, and Todd Emrick found that when they combined carboxylated polystyrene nanoparticles with amine-terminated polymers, they could create a nanoparticle surfactant. Typical of surfactants, these nanoparticles gather at the interface of two immiscible liquids, such as a drop of water in silicone oil. When the researchers distorted the shape of the drop, either by applying an electric field or via physical manipulation, they found that more nanoparticles crammed into the interface, locking the new shape in place. “The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage,” the researchers say.