Tiny Droplets… Lead to Exotic Properties
Chameleon-like color changes are observed by confining liquid crystals within small drops.
The molecules in liquid crystals form intricate twisted, helical morphologies with left- or right-handedness, including exotic phases called “blue phases” in which arrays of defects are organized into striking patterns. Confinement of these defect structures within droplets enables a much finer degree of control over its color, morphology, and stability than previously thought possible.
The extraordinary sensitivity of this defect structure to confinement points to new strategies—not possible in bulk phases—for three-dimensional assembly of responsive, adaptable materials. These new strategies also could control optical properties for sensing or tunable photonic crystals for lasers and switchable colored displays. The optical-controlled lasers could advance energy-efficient computing, while these switchable displays could lead to electronic paper.
Biology uses stress, defects, molecular configuration (for example, handedness), and hierarchical design strategies to assemble and/or create a wide range of materials capable of serving different functions. Perhaps the best example is the complex functions carried out by cell membranes that are mesostructured and liquid crystalline.
The hierarchical organization in biological systems serves as an amplifier that allows highly localized, nanoscale molecular events to propagate into larger length scales. This propagation results in dynamic functional properties of biological systems that have not yet been fully realized in synthetic material designs. Liquid crystals with a particular handedness form helical morphologies, including exotic “blue phases” in which defects are organized into three-dimensional patterns.
Through a combination of simulations and experiments, scientists at the University of Chicago and the University of Wisconsin found that these “blue phases” are extraordinarily sensitive to confinement within droplets. Several of these new morphologies have properties that can be finely tuned. For instance, manipulating the strength of the anchoring interaction between the “blue phase” and the droplet surface, droplet size, or temperature can control the wavelength at which “blue phase” droplets reflect light—thereby providing a liquid-state analog of nanoparticles, where size dimensions are used to control optical properties.
Also, weak experimentally relevant anchoring conditions lead to unanticipated behavior, including enhanced stability of “blue phases” with respect to that observed in the bulk. This is a particularly interesting result because “blue phases” are generally stable only over a narrow range of temperatures. Finally, organizing the array of defects within “blue phases” at the droplet surface raises intriguing possibilities for controlled placement and assembly of functionalized nanoparticles at the droplet interface.
J. A. Martínez-González, Y. Zhou, M. Rahimi, E. Bukusoglu, N. L. Abbott, and J. J. de Pablo, “Blue-phase liquid crystal droplets.” Proceedings of the National Academy of Sciences USA 112, 13195 (2015). [DOI: 10.1073/pnas.1514251112].