Self-assembly

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.

Phillips KR, Vogel N, Burgess IB, Perry CC, Aizenberg J. Directional Wetting in Anisotropic Inverse Opals. Langmuir. 2014;30 (25) :7615-7620.Abstract

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.

Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. 2014;26 (4) :1622-1628.Abstract

Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the possible structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method for introducing anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol−gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure. Further, we prevent the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy and infrared spectroscopy studies clearly identify increasing degrees of sol−gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.

Vasquez Y, Kolle M, Mishchenko L, Hatton BD, Aizenberg J. Three-Phase Co-Assembly: In-situ Incorporation of Nanoparticles into Tunable, Highly-Ordered, Porous Silica FIlms. ACS Photonics. 2014;1 (1) :53-60.Abstract

We present a reproducible, one-pot colloidal co-assembly approach that results in large-scale, highly ordered porous silica films with embedded, uniformly distributed, accessible gold nanoparticles. The unique coloration of these inverse opal films combines iridescence with plasmonic effects. The coupled optical properties are easily tunable either by changing the concentration of added nanoparticles to the solution before assembly or by localized growth of the embedded Au nanoparticles upon exposure to tetrachloroauric acid solution, after colloidal template removal. The presence of the selectively absorbing particles furthermore enhances the hue and saturation of the inverse opals’ color by suppressing incoherent diffuse scattering. The composition and optical properties of these films are demonstrated to be locally tunable using selective functionalization of the doped opals.

Burgess IB, Abedzadeh N, Kay TM, Shneidman AV, Cranshaw DJ, Loncar M, Aizenberg J. Tuning and Freezing Disorder in Photonic Crystals using Percolation Lithography. Scientific Reports. 2016;6 (1) :19542.Abstract
Although common in biological systems, synthetic self-assembly routes to complex 3D photonic structures with tailored degrees of disorder remain elusive. Here we show how liquids can be used to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In these optofluidic crystals, dynamically tunable disorder is superimposed onto the periodic optical structure through partial wetting or evaporation. In both cases, macroscopic symmetry breaking is driven by subtle sub-wavelength variations in the pore geometry. These variations direct site-selective infiltration of liquids through capillary interactions. Incorporating cross-linkable resins into our liquids, we developed methods to freeze in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These percolation lithography techniques produced permanent photonic structures with adjustable disorder. By coupling strong changes in optical properties to subtle differences in fluid behavior, optofluidic crystals may also prove useful in rapid analysis of liquids.
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