Living systems are compelling evidence that multi scale, adaptive architectures can be built simply by letting their smallest parts self-organize as dictated by their intrinsic properties. 

Efforts to explain the mystery of self-assembly have ranged from “collective thinking” within continuously reconfiguring bird flocks to sophisticated algorithms describing how chains of local interactions can give rise to bacterial communities and cellular machinery. Yet we have little understanding of how to systematically encode such interactions directly into physical building blocks to track and control their assembly into arbitrary, dynamic structures, particularly at the nanoscale. 

Using periodic nanofiber arrays as both a model system and a starting point for fabricating optical, adhesive, and microfluidic materials, our group has demonstrated and developed a predictive theory for how to create elaborate helical, hierarchical structures simply by tweaking the mechanical, chemical, and geometric features of the fibers. Intriguingly, step-by-step examination of the process revealed an unexpected concept: the final structure is achieved through hierarchical stages of not only assembly but also disassembly.

Our continuing analysis of this latter phase suggests how, with a comprehensive understanding of the forces at play, building block features can be used to program selective, inducible top-down self-disassembly steps, giving the assembled structures a wide variety of dynamic, reversible, responsive behaviors. Yet another surprising self-assembly principle has come from our recent work on colloids: we have discovered that letting two different sets of building blocks co-assemble from the bottom up can enhance the long-range order of each by mutual correction of defects, and this too has led us to explore a new realm of possibilities for bottom-up creation of hierarchical architectures.

Schaffner M, England G, Kolle M, Aizenberg J, Vogel N. Combining Bottom-Up Self-Assembly with Top-Down Microfabrication to Create Hierarchical Inverse Opals with High Structural Order. Small. 2015;11 (34) :4334-4340. Full TextAbstract
Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom-up assembly of polystyrene particles in the presence of a silica sol–gel precursor material (tetraethylorthosilicate, TEOS), which creates crack-free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top-down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano- and microscale enables the fabrication of materials with complex optical properties resulting from light–matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.
Phillips KR, Vogel N, Burgess IB, Perry CC, Aizenberg J. Directional Wetting in Anisotropic Inverse Opals. Langmuir. 2014;30 (25) :7615-7620. Publisher's VersionAbstract

Porous materials display interesting transport phenomena due to restricted motion of fluids within the nano- to microscale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy.

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