Self-assembly

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.
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.

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.

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. Full TextAbstract
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.
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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