Cornell researchers have developed a two-phase liquid crystal system that can rapidly change - and hold - its shape, transforming from a transparent thin liquid film to an opaque emulsion, and then back again, all with a brief jolt of a high-frequency electric field.
The approach could be leveraged to create fast, self-tinting "smart" windows or for making emulsions that control chemical reactions in material synthesis.
The research published July 16 in Nature. The lead author is former postdoctoral researcher Sangchul Roh.
Researchers have long sought thin liquid films that have optical properties. But those materials are difficult to create and operate, because liquid structures require constant stabilization.
"The main problem is that liquids relax by themselves," said Nicholas Abbott, a Tisch University Professor in the Robert F. Smith School of Chemical and Biomolecular Engineering in Cornell Engineering and the paper's senior author. "In this system, we've got liquids, yet we can trap them in desired structures and then, on demand, release the system from that constraint and let it relax back to its initial shape. It's an unusual level of control."
The system is built on the interactions of two immiscible substances: a liquid crystal oil, which the researchers concocted, and an isotropic oil that shared just the right combination of properties, including viscosity, interfacial tension and surface energy. When confined between two electrodes, the isotropic oil automatically coats the electrodes and the liquid crystal oil forms a slab between these two films. This initial layered - or ground - state is transparent, with optical properties that are similar to glass.
By applying an electric field, dispersed oil droplets are generated in the liquid crystal, and the system transforms into a metastable - i.e., long-lived - emulsion. Typically, if two simple oils form an emulsion, they'll separate back into their layered state once the electric field is removed. But in this case, the emulsion is stabilized because topological defects form in the liquid crystal. These nanoscopic structures create an energy barrier that traps the dispersed droplets in place and prevents them from coalescing.
However, the real surprise for the research team, which included doctoral student and co-author Youlim Ha, was that the liquid crystal also contains a mechanism that allows the researchers to quickly break the emulsion when the electric field is applied again. Transient structures called solitons propagate across the surface of the droplets and pull them all together, like a rubber band, and the system reverts to its initial layered state.
"The emulsion forms in less than a second, and we can turn it off in a couple of seconds. So it's very rapid transformations in the morphologies of this two-phase liquid system," Abbott said. "There's no real precedent for that."
Because the ground state is transparent and the emulsified state is milky, an obvious application of the technology would be a smart window that can turn opaque in seconds without needing a constant electric field to be applied, according to Abbott. There is also the potential to use the emulsions to control chemical reactions in synthesizing material and perhaps even organizing emulsion droplets to generate different material colors.
"If you can make periodic arrays of materials on the micrometer length scale, they have interesting optical properties. You get interference effects, just like the colors in a bird's wing. There's no dye; it's just that white light interferes at certain wavelengths. So you can actively control the colors of materials," Abbott said. "You can think of camouflages and making signs that are colored. Or smart labels in a shop. If you want to change the price dynamically, depending on supply and demand, then the system we have would let you do that."
The research was supported by the U.S. Department of Energy and the Cornell Ignite program.
The researchers made use of the Cornell NMR Facility, which is supported, in part, by the National Science Foundation.
