Meniscus lithography and Moiré patterns

Meniscus lithography and Moiré patterns. Bottom-up pattern formation at the micro- and nanoscale is being actively studied for a better understanding of self-organization and for such applications as data storage and sensors. One low-energy approach relies on the evaporation of liquids to produce patterns of nanoparticles across multiple length scales. In one variation, the local rate of evaporation can be controlled by placing a mask over the liquid. Additionally, capillary forces not only form menisci between the particles but can also organize the particles into patterned clusters. Random imperfections and instabilities, though, make achieving long-range order difficult. In a new approach, Joanna Aizenberg and colleagues at Harvard University have found a simple, scalable way for masked evaporation to produce complex patterns and long-range order. The researchers place two arrays of vertical nanopillars—one the substrate and one the mask—face to face with a liquid sandwiched between them. In an example of mutual feedback, the liquid forms menisci between the substrate and mask, and as the liquid evaporates, capillary forces bend the substrate’s pillars toward the mask’s. The technique yields a deterministic pattern completely controlled by the mask geometry. In particular, when the mask and substrate are identical and at a slight angle to each other, the misalignment produces a Moiré pattern that is preserved by the bent pillars, as shown in the figure, even after the two arrays are separated. The periodicity and chirality of the Moiré pattern are easily tunable by changing the angle between the surfaces. (S. H. Kang et al., Phys. Rev. Lett., in press.)