Reconfigurable self-assembly of mesoscale optical components at a liquid-liquid interface.

Sindy K.Y. Tang , Ratmir Derda , Aaron D. Mazzeo , and George M. Whitesides * IO N A method that uses magnetic fi elds to template the self-assembly and reconfi guration of 2D optical components with engineered surface wettabilities at a liquid–liquid interface is described. The optical components are mesoscale tiles of diffraction gratings each fabricated to contain a magnetic strip. Application of a magnetic fi eld to the tiles, suspended at the interface between two immiscible liquids, assembles them into an array of gratings. The orientations of the tiles and the resulting optical effects are reconfi gurable by a change in the magnetic fi eld. It is possible to preserve the assembled patterns, if desired, by transferring them onto solid substrates. This procedure can be useful for generating coatings or fi lms with interesting optical effects and for visualizing magnetic fi elds. Self-assembly methods allow the distribution and alignment of small, preformed objects into desired patterns. [ 1–4 ] It is a potential alternative to conventional top-down techniques (e.g., photolithography) and serial processing steps (e.g., pick-andplace robotics) for scalable and low-cost fabrication of a wide range of structures with length scales from nanometers (e.g., self-assembly of diblock copolymers, [ 5–8 ] templated self-assembly using immunoglobulins, [ 9 ] DNA, and viruses [ 10–14 ] ) to centimeters. [ 1 , 15 ] Self-assembly can generate materials with unique and useful optical properties: for example, 3D-assembly of colloids can form photonic crystals, [ 16,17 ] suspensions of silver nanoparticles [ 18 ] and gold-coated silicon hexagons [ 19 ] provide the refl ective element for liquid mirrors, and selective wetting of polymers on long hydrophobic strips forms optical waveguides. [ 20 ] The organization of mesoscale (millimeter to centimeter) optical modules is less explored than that of nanoscale structures. Mesoscale structures offer the potential to generate regular arrays of functional optical elements over large areas. Some functions (e.g., diffraction, lensing) cannot be obtained easily (or at all) from the direct assembly of nanostructures. The self-assembly of mesoscale plates into various arrays and porous structures using lateral capillary interactions between menisci at the edges of the plates has been previously demonstrated. [ 21–31 ] Most of these assemblies were static: once assembled, it was diffi cult to reconfi gure the assembled pattern. There are few strategies that allow real-time reconfi guration of self-assembled mesoscale structures without requiring a change in the design and fabrication of individual components. Mao et. al. showed the reconfi guration of assembled structures by