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.

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. Full TextAbstract

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.

Phillips KR, England GT, Sunny S, Shirman E, Shirman T, Vogel N, Aizenberg J. A colloidoscope of colloid-based porous materials and their uses. Chem. Soc. Rev. 2016;45 (2) :281-322. Full TextAbstract
Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.
Kaplan CN, Wu N, Mandre S, Aizenberg J, Mahadevan L. Dynamics of evaporative colloidal patterning. Physics of Fluids. 2015;27 (9) :092105. Full TextAbstract

Drying suspensions often leave behind complex patterns of particulates, as might be seen in the coffee stains on a table. Here, we consider the dynamics of periodic band or uniform solid film formation on a vertical plate suspended partially in a drying colloidal solution. Direct observations allow us to visualize the dynamics of band and film deposition, where both are made of multiple layers of close packed particles. We further see that there is a transition between banding and filming when the colloidal concentration is varied. A minimal theory of the liquid meniscus motion along the plate reveals the dynamics of the banding and its transition to the filming as a function of the ratio of deposition and evaporation rates. We also provide a complementary multiphase model of colloids dissolved in the liquid, which couples the inhomogeneous evaporation at the evolving meniscus to the fluid and particulate flows and the transition from a dilute suspension to a porous plug. This allows us to determine the concentration dependence of the bandwidth and the deposition rate. Together, our findings allow for the control of drying-induced patterning as a function of the colloidal concentration and evaporationrate.

Vogel N, Utech S, England GT, Shirman T, Phillips KR, Koay N, Burgess IB, Kolle M, Weitz DA, Aizenberg J. Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies. Proc. Nat. Acad. Sci. 2015;112 (35) :10845-10850. Publisher's VersionAbstract
Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal’s curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies—potentially as more efficient mimics of structural color as it occurs in nature.