Angle-Independent Color Change in Thermoresponsive Gel-Immobilized Colloidal Amorphous Film Attached to PET Substrate.

Gel-immobilized colloidal amorphous structures comprise short-range-ordered monodisperse submicrometer particles embedded into a soft polymer gel. They exhibit an angle-independent structural color that is tunable in response to external stimuli via a volume change in the gel, which has significant potential for the development of sensors that respond to stimuli via angle-independent color changes. In this study, the amorphous structure of a charged colloidal suspension in water was immobilized in a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) gel film and simultaneously attached to a polyethylene terephthalate (PET) substrate. The gel film exhibited a uniform angle-independent color that changed in response to changes in temperature (i.e., thermosensitivity). Attachment to the PET substrate suppressed changes in the gel film area and film distortion, despite significant volume changes in the gel. Consequently, the degree of thermosensitivity was enhanced. The PET-attached gel-immobilized colloidal amorphous film was easy to handle and had excellent flexibility, allowing it to wrap around the surfaces of curved objects. These features are advantageous for sensor applications.

Practical Preparation of Elastomer-Immobilized Nonclose-Packed Colloidal Photonic Crystal Films with Various Uniform Colors.

Colloidal photonic crystals, which are three-dimensional periodic structures of monodisperse submicron-sized particles, are expected to be suitable for novel photonic applications and color materials. In particular, nonclose-packed colloidal photonic crystals immobilized in elastomers exhibit significant potential for use in tunable photonic applications and strain sensors that detect strain based on color change. This paper reports a practical method for preparing elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform Bragg reflection colors using one kind of gel-immobilized nonclose-packed colloidal photonic crystal film. The degree of swelling was controlled by the mixing ratio of the precursor solutions, which used a mixture of solutions with high and low affinities for the gel film as the swelling solvent. This facilitated color tuning over a wide range, enabling the facile preparation of elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform colors via subsequent photopolymerization. The present preparation method can contribute to the development of practical applications of elastomer-immobilized tunable colloidal photonic crystals and sensors.

Tuning of Optical Stopband Wavelength and Effective Bandwidth of Gel-Immobilized Colloidal Photonic Crystal Films.

We show that both the optical stopband wavelength and effective bandwidth of films of gel-immobilized loosely packed colloidal photonic crystals can be controlled over a wide range. When the gelation reagent of the charge-stabilized colloidal crystals was photopolymerized under ultraviolet light using different upper- and bottom-light intensities, it resulted in a gel-immobilized colloidal crystal film with a broadened Bragg reflection peak. Moreover, the width of the Bragg peak increased from 30 to 190 nm as the difference between the light intensities increased. Films with wider Bragg peaks exhibited a brighter reflection color because of the superposition of the shifted Bragg reflections. Furthermore, the Bragg wavelength could be varied over a wide range (500-650 nm) while maintaining the same broadened effective bandwidth by varying the swelling solvent concentration. These findings will expand the applicability of colloidal crystals for use in photonic devices and color pigments.

Amorphous photonic structure in a charged colloidal system showing angle-independent uniform color.

Structural colors derived from the scattering or reflection of light by submicron-sized ordered structures are interesting due to their non-fading and tunable colors. Herein we show a colored angle-independent amorphous phase of monodisperse colloids between the crystalline and random phases in a charged colloidal system. The amorphous photonic structures with low packing density display uniform color over large areas; this is easily altered by adjusting the particle volume fraction. The transmission spectra of amorphous phases display an intersection point at the Bragg wavelength, allowing design of the transmission color. We demonstrate that the aqueous suspended colloidal amorphous structures can be immobilized in hydrogel films, while preserving their spectral properties. The sensitivity of the gel to ethanol allows the color to be tuned by changing the ethanol concentration. This angle-independent uniform color with easy tunability has great potential for application in advanced color materials and sensors.

Preparation of monodisperse hybrid gel particles with various morphologies via flow rate and temperature control.

We report a facile method for preparing monodisperse hybrid smart gel particles with various morphologies by using microfluidic techniques and the swelling-shrinking phenomenon of thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel particles. We demonstrate that PNIPAM-polyacrylamide snowman-like, raspberry-like, and dumbbell-like hybrid gel particles can be prepared by controlling the flow rate and temperature.

Enhancement of Thermosensitivity of Gel-Immobilized Tunable Colloidal Photonic Crystals with Anisotropic Contraction.

A thermosensitive poly(N-isopropylacrylamide) (PNIPAM) circular gel film containing single-crystalline colloidal crystals was prepared, and the circular edge was pinched with washers to restrain the in-plane shrinkage of the gel film. Upon heating, the film shrunk in the thickness direction selectively, resulting in a change in the crystal structure of colloids embedded in the anisotropic shrunken gel. This led to the manipulation of the optical stop-band wavelength of the colloidal photonic crystals with a higher thermosensitivity and over a wider range of wavelengths covering a wide margin on either side of the visible-light region. The present method for enhancement of the sensitivity is very simple, and the principle should also be applicable to the gel-immobilized colloidal crystals that respond to stimuli other than temperature changes. The present findings contribute to the progress of practical applications of gel-immobilized colloidal photonic crystals as tunable photonic crystals and biological and chemical sensors.

Crystallization and reentrant melting of charged colloids in nonpolar solvents.

We explore the crystallization of charged colloidal particles in a nonpolar solvent mixture. We simultaneously charge the particles and add counterions to the solution with aerosol-OT (AOT) reverse micelles. At low AOT concentrations, the charged particles crystallize into body-centered-cubic (bcc) or face-centered-cubic (fcc) Wigner crystals; at high AOT concentrations, the increased screening drives a thus far unobserved reentrant melting transition. We observe an unexpected scaling of the data with particle size, and account for all behavior with a model that quantitatively predicts both the reentrant melting and the data collapse.

Surface treatment of flow channels in microfluidic devices fabricated by stereolithography.

A microfluidic device with three-dimensional flow channels was fabricated by stereolithography, and hydrophilic surface treatment of the flow channel was performed by coating the wall of the channel with a silica layer. After the treatment, the device produced monodisperse oil-in-water (O/W) emulsions. The silica layer on the channel surface was then coated with a fluorinated silane coupling agent to make it hydrophobic, thus enabling the treated device to produce monodisperse inverted water-in-oil (W/O) emulsions.

Widely tunable lasing in a colloidal crystal gel film permanently stabilized by an ionic liquid.

Widely tunable laser action by low threshold optical excitation can be realized by a colloidal crystal gel (CC-G) film permanently stabilized by a non-volatile ionic liquid. The CC-G film consists of a non-close-packed structure of microparticles immobilized in a polymer gel. Optical excitation with green light gives rise to a single and very narrow laser emission with red color. The single laser emission peak can be tuned in a wide wavelength range by applying mechanical stress.

New route to produce dry colloidal crystals without cracks.

We report a new route for fabricating opal films in which the particles are bounded by polymer gel. A loosely packed colloidal crystal immobilized in a hydrogel is homogeneously shrunk and dried and converted into a crack-free opal film. The obtained film shows a uniform diffraction color and excellent transmission spectral properties inherited from the starting state. The optical stop band wavelength is adjustable by varying the amount of gel-polymer.

Tuning the effective width of the optical stop band in colloidal photonic crystals.

The optical stop band in colloidal crystals is characterized by the central frequency and bandwidth. Although the former is known to be highly tunable by changing the lattice constant, the latter is basically determined by the refractive index contrast between the particles and the background medium that is intrinsic to the materials. In this study, we show that the effective bandwidth in gelled colloidal crystals can also be tuned by controlling the fabrication conditions. Single-domain gelled colloidal crystals were prepared by photopolymerization under various photoirradiation conditions. It was observed that the width of the stop band in the transmission or reflectance spectrum could be expanded by simply adjusting the irradiation time.

Gelation of colloidal crystals without degradation in their transmission quality and chemical tuning.

A single-domain colloidal crystal with high transmission quality, prepared by a shear-induced process, was fixed as a hydrogel film by photopolymerization. Upon gelation, the original optical quality was almost perfectly preserved. By replacing the solvent, the gelled crystal could be converted to smaller lattice constant crystals without significant degradation in its transmission characteristics. The conversion results in a stop-band wavelength coverage across the entire visible light range.

Critical concentration for colloidal crystallization determined with microliter centrifuged suspensions.

We report an elegant method using centrifugal sedimentation for determining the critical particle concentration for colloidal crystallization. A small amount of a dilute suspension of monodispersed particles stored in a flat capillary cell was centrifuged to temporarily generate a nonequilibrium gradient of the particle concentration including a crystalline-noncrystalline phase boundary in the cell. In the recovering process after the centrifugation, the particle concentration of the crystalline phase at the boundary was found to always have the equilibrium value, although the global concentration distribution evolved with time. The critical concentration was determined based on spatially resolved spectrometry. The present method requires only one batch of a suspension of the order of microliters and is applicable up to high concentration regions near the closest packing without the effect of the particle aggregation.

Equilibrium characteristic at ordered-disordered phase boundary in centrifuged nonequilibrium colloidal-crystal system.

We show that a crystalline-noncrystalline boundary that temporarily appeared in a sediment centrifuged from a relatively dilute colloidal suspension can be regarded to be at a phase equilibrium. On the basis of this, we can determine the critical particle concentration for colloidal crystallization using an extremely small amount of specimen by spatially resolved spectroscopy.