An object is called chiral if its mirror image cannot be superimposed to its original image. Chiral patterns are ubiquitous in nature across various length scales. Chirality is important in understanding and controlling the behavior of living and non-living systems because the presence or absence of chirality in the material plays important roles in its interactions with molecules1, enzymes2, light3, and mechanical stress4. Processes that induce chirality have been extensively studied at the molecular and macroscopic scales, but they have been relatively unexplored at the mesoscale. Among many mesoscale starting materials, nature has many intriguing examples that exploit various functional surfaces composed of arrays of surface-attached fibers. Depending on their geometry and alignment, surface-attached fibers generate many interesting behaviors such as reversible adhesion of gecko5, water resistance of water strider legs6, and sensing of cilia7. For a better understanding of the pattern formation in fibrous surfaces, we have used surface-attached polymeric nanofiber arrays as a model system and studied evaporation-induced self-organization of nanofibers.
We investigated a mechanism for forming hierarchical chiral structures (Fig. 1a) from achiral starting materials during evaporative self-assembly of nanofiber arrays (Science 2009), and demonstrated how the interplay among elasticity, capillarity, and adhesion of fibrous surfaces could be tuned to produce assembly patterns with a diversity of shapes and sizes through coupling between continuum mechanics and materials chemistry (ACS Nano 2010) by collaboration with Prof. L. Mahadevan. The assembly process can be used for applications such as a capture-and-release system (Fig. 1b) and security features by structural color effects (U.S. Patent pending). Then, we studied a mechanism for long-range order formation because the self-assembly process tended to generate patterns with a short-range order due to random nucleation and propagation of patterns. We developed a force-mediated pattern formation mechanism that couples Moiré effects with evaporative self-assembly. By using two face-to-face nanofiber arrays and a liquid evaporating between them, we demonstrated long-range ordered Moiré patterns (Fig. 1c) with tunable periodicity and chirality (Physical Review Letters 2011) by harnessing the capillary forces acting on the fiber arrays. Our findings offer a new understanding of the pattern formation by self-organization of the nanofiber arrays and provide a simple approach and new opportunities for manufacturing complex, functional structures. Due to our contributions to the field of pattern formation from nanostructures, we wrote an invited review paper about self-organization of bio-inspired nanofibers (Nano Today 2012).
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2 Milton, R. C. D., Milton, S. C. F. & Kent, S. B. H. Total chemical synthesis of a D-enzyme – the enantiomers of HIV-1 protease show demonstration of reciprocal chiral substrate-specificity. Science 256, 1445-1448 (1992).
3 Hendry, E. et al. Ultrasensitive detection and characterization of biomolecules using superchiral fields. Nature Nanotech. 5, 783-787 (2010).
4 Mori, H., Hirai, Y., Ogata, S., Akita, S. & Nakayama, Y. Chirality dependence of mechanical properties of single-walled carbon nanotubes under axial tensile strain. Jpn. J. Appl. Phys. 2 44, L1307-L1309 (2005).
5 Autumn, K. et al. Adhesive force of a single gecko foot-hair. Nature 405, 681-685 (2000).
6 Gao, X. F. & Jiang, L. Water-repellent legs of water striders. Nature 432, 36 (2004).
7 Shah, A. S., Ben-Shahar, Y., Moninger, T. O., Kline, J. N. & Welsh, M. J. Motile cilia of human airway epithelia are chemosensory. Science 325, 1131-1134 (2009).