Figure 5. Convective assembly processes for the two-dimensional crystallization of colloids. (a) Vertical deposition process. (b) Horizontal deposition process. (c) Typical problems arising from convective assembly processes as reported by Dimitrov et al. Reprinted with permission from ref 198.

No abstract

Composite materials with periodic variations of density and/or sound velocities, so-called phononic crystals, can exhibit bandgaps where propagation of acoustic waves is forbidden. Phononic crystals are the elastic analogue of the well-established photonic crystals and show potential for manipulating the flow of elastic energy. So far, the experimental realization of phononic crystals has been restricted to macroscopic systems with sonic or ultrasonic bandgaps in the sub-MHz frequency range. In this work, using high-resolution Brillouin spectroscopy we report the first observation of a hypersonic bandgap in face-centred-cubic colloidal crystals formed by self-assembly of polystyrene nanoparticles with subsequent fluid infiltration. Depending on the particle size and the sound velocity in the infiltrated fluid, the frequency and the width of the gap can be tuned. Promising technological applications of hypersonic crystals, ranging from tunable filters and heat management to acousto-optical devices, are anticipated.

Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Here, binary colloidal monolayers featuring a close‐packed monolayer of large spheres (L) with a superlattice of small particles (S) are prepared in a single step using a Langmuir trough. Adjustment of the stoichiometry of the two particle types at the air–water interface leads to a high degree of control over the occupation of the interstitial sites in the close‐packed layer of large spheres by the small colloids. Thus, large areas of binary 2D crystals with LS2, LS6, and LS9 structures are fabricated in a controlled way. The process allows the formation of binary crystals over a wide range of particle size ratios from 0.19 to 0.40. The pH value of the subphase can be used to enhance the crystallization process by changing the contact angle of the particles at the interface. An interfacial polymerization of butyl cyanoacrylate is used to directly image the contact angle of the colloids at the interface. Transfer to solid substrates is achieved by a surface lowering technique. A variety of substrates with arbitrary topographies can thus be decorated with colloidal monolayers. Applied to a lithographic process, such monolayer architectures allow the generation of complex patterns, not accessible with conventional close‐packed monolayers.

The structural features and swelling properties of responsive hydrogel films based on poly(N-isopropylacrylamide) copolymers with a photo-cross-linkable benzophenone unit were investigated by surface plasmon resonance, optical waveguide mode spectroscopy, and atomic force microscopy. The temperature-dependent swelling behavior was studied with respect to the chemical composition of the hydrogel polymers containing either sodium methacrylate or methacrylic acid moieties. In the sodium methacrylate system, a refractive index gradient was found that was not present in the free acid gel. This refractive index gradient, perpendicular to the swollen hydrogel film surface, could be analyzed in detail by application of the reversed Wentzel-Kramers-Brillouin (WKB) approximation to the optical data. This novel approach to analyzing thin-film gradients with the WKB method presents a powerful tool for the characterization of inhomogeneous hydrogels, which would otherwise be very difficult to capture experimentally. In AFM images of the hydrogel layers, a macroscopic pore structure was observed that depended on the polymer composition as well as on the swelling history. This pore structure apparently prevents the often-observed skin barrier effect and leads to a quickly responding hydrogel.

A versatile and simple method is presented for the rapid fabrication of close‐packed colloidal 2D crystals with large domain sizes by floating and redeposition of colloidal monolayers at the air/water interface. A detailed analysis of the particle surface transformation and packing during the individual steps of the monolayer fabrication process has been conducted. It was found that the quality of the monolayer depends on parameters like colloidal particle distribution on the initial substrate, subphase pH, and addition of surfactants. The floating monolayers could be transferred and stacked onto many substrate types, regardless of surface polarity, roughness, or curvature.

We report on the observation of two hypersonic phononic gaps of different nature in three-dimensional colloidal films of nanospheres using Brillouin light scattering. One is a Bragg gap occurring at the edge of the first Brillouin zone along a high-symmetry crystal direction. The other is a hybridization gap in crystalline and amorphous films, originating from the interaction of the band of quadrupole particle eigenmodes with the acoustic effective-medium band, and its frequency position compares well with the computed lowest eigenfrequency. Structural disorder eliminates the Bragg gap, while the hybridization gap is robust.

The influence of various experimental parameters on the vertical deposition and structure formation of colloidal crystals on chemically patterned surfaces, with hydrophilic and hydrophobic areas, was investigated. The pattern dimensions range from about 4 to 400 microm, which is much larger than the individual particle size (255 nm), to control the microscopic crystal shape rather than influencing the crystal lattice geometry (as achieved in colloidal epitaxy). The deposition resolution and selectivity were tested by varying the particle concentration in the suspension, the substrate withdrawing speed, pattern size and orientation, and wetting contrast between the hydrophilic and hydrophobic regions. The evolution of colloidal crystal thickness with respect to the pattern dimensions and deposition parameters was further studied. Our results show that the pattern size has a rather strong influence on the deposited number of colloid layers and on the crystal quality. Better results are obtained when the lines of a stripe pattern are oriented parallel to the withdrawing direction rather than perpendicular. The deposition resolution (defined as the minimum feature size on which particles can be deposited) depends on the wetting contrast and increases with lower average hydrophobicity of the substrate.