Formation of two-dimensional diamond-like colloidal crystals using layer-by-layer electrostatic self-assembly

We report here that a two-dimensional (2D) diamond-like structure of micron-sized colloidal particles can be obtained by layer-by-layer self-assembly. Positively and negatively charged silica particles, 1 μm in diameter, were used in the experiments. On a positively charged, flat glass substrate, the first layer of negatively charged particles was prepared to form a non-close-packed 2D crystal. Then the second and third layers were fabricated using electrostatic adsorption. The positions of adsorbed particles were controllable by tuning the zeta-potential of the particles and the salt concentration of the medium. The FDTD calculations show that the 2D diamond structures of particles with higher refractive index (titania) have an absorption band in the wavelength range corresponding to the photonic band gap of the 3D bulk crystal. We expect these findings to be useful for the fabrication of novel photonic materials.

Clustering of charged colloidal particles in the microgravity environment of space

We conducted a charge-charge clustering experiment of positively and negatively charged colloidal particles in aqueous media under a microgravity environment at the International Space Station. A special setup was used to mix the colloid particles in microgravity and then these structures were immobilized in gel cured using ultraviolet (UV) light. The samples returned to the ground were observed by optical microscopy. The space sample of polystyrene particles with a specific gravity ρ (=1.05) close to the medium had an average association number of ~50% larger than the ground control and better structural symmetry. The effect of electrostatic interactions on the clustering was also confirmed for titania particles (ρ ~ 3), whose association structures were only possible in the microgravity environment without any sedimentation they generally suffer on the ground. This study suggests that even slight sedimentation and convection on the ground significantly affect the structure formation of colloids. Knowledge from this study will help us to develop a model which will be used to design photonic materials and better drugs.

Surface Plasmon Resonance of Two-Dimensional Gold Colloidal Crystals Formed on Gold Plates

The free electrons inside precious metals such as Au vibrate when the surface of the metal is irradiated with an electromagnetic wave of an appropriate frequency. This oscillation is referred to as surface plasmon resonance (SPR), and the resonance frequency varies with permittivity of the medium around the metal. SPR sensors are widely applied in the fields of bioscience and pharmaceutical sciences, including biosensing for drug discovery, biomarker screening, virus detection, and testing for food safety. Here, we fabricated a metal-insulator-metal (MIM) SPR sensor by constructing two-dimensional (2D) regular array of Au colloidal particles (2D colloidal crystals) on an insulator layer over a thin Au film coated on a glass substrate surface. The 2D crystals were fabricated by electrostatically adsorbing negatively charged three-dimensional crystals onto a positively charged thin insulator formed on Au film. The plasmon peaks/dips from the MIM structure were measured in aqueous solutions of ethylene glycol (EG) at various concentrations. Multiple plasmon peaks/dips were observed due to the localized SPR (LSPR) of the Au particles and the Fano resonance between the Au particles and thin film. The plasmon peaks/dips shifted to higher wavelengths on increasing EG concentrations due to an increase in the refractive index of the media. The observed peak/dip shift was approximately twice that of LSPR from an isolated Au particle. We expect the present MIM substrate will be useful as a highly sensitive sensor in the pharmaceutical field.

Crystallization of charged gold particles mediated by nonadsorbing like-charged polyelectrolyte

We report that the aqueous dispersions of negatively charged submicron-sized colloidal Au particles formed non-close-packed colloidal crystals by the addition of a like-charged linear polyelectrolyte, sodium polyacrylate (NaPAA). Au particles often form irregular aggregates in dispersions because of a strong van der Waals force acting between them. To prevent aggregation, we introduced negative electric charges on particle surfaces. By the addition of NaPAA, colloidal crystals were formed on the bottom of a sample cell because of the supply of Au particles by sedimentation and 2D diffusion even under very dilute conditions. Interparticle potential calculations demonstrated that the addition of NaPAA caused depletion attraction between the particles as well as a significant reduction in the interparticle repulsion because of the electrostatic screening effect. However, the electrostatic repulsion was strong enough to prevent the direct contact of particles in the excluded region between Au particles. Large-area crystals could be obtained by tilting the sample cell. By drying the sample, the Au particles came into contact and the non-space-filling crystals changed into closest packed crystals. These closest packed crystals exhibited a significant enhancement of Raman scattering intensity because of high hot-spot density.

Two-Dimensional Nonclose-Packed Colloidal Crystals by the Electrostatic Adsorption of Three-Dimensional Charged Colloidal Crystals

We demonstrate that nonclose-packed two-dimensional (2D) colloidal crystals fixed on flat solid surfaces can be obtained by the electrostatic adsorption of three-dimensional (3D) charged colloidal crystals onto oppositely charged substrates. 3D colloidal crystals of negatively charged polystyrene (diameter d = 500 nm) and silica (d = 510 and 550 nm) particles were formed in their aqueous dispersions. Then, a single layer of the 3D crystals (the particle volume fraction = ∼0.07-0.3) was adsorbed onto a glass surface, which was earlier modified with 3-aminopropyltriethoxysilane (APTES), a cationic silane coupling reagent. Under salt-free conditions, the lowermost layer of the 3D crystals, which was oriented parallel to the substrate, was adsorbed onto the substrate surface, forming 2D crystals. Centimeter-sized, large 2D silica crystals were produced by combining a unidirectional 3D crystallization of the silica colloid under a base concentration gradient and a unidirectional adsorption under an acidic concentration gradient, which allowed tuning of the charge number on the APTES-modified substrate. The interparticle separations of the resulted 2D crystals did not vary greatly (within 5%) over a large area (length: 2 cm); however, the separations were smaller than the initial value because of gravitational sedimentation. We also produced 2D crystals of gold particles (d = 250 nm), which we expect to be applicable as plasmonic materials. The present study will provide a facile strategy to produce nonclose-packed 2D colloidal crystals of various types of particles, including large and high-density particles.

Mechanism of diffusiophoresis with chemical reaction on a colloidal particle

A mechanism for diffusiophoresis of a charged colloidal particle undergoing surface chemical reaction is proposed. A theoretical model is constructed to describe the dynamics of the particle and the surrounding solution of a weak electrolyte. Theoretical analysis and numerical simulations of the model reveal that phoretic motion of the particle emerges in response to a concentration gradient of electrolyte. The concentration gradient breaks the spherical symmetry of the surface charge distribution, which gives rise to a net force on the particle and leads to directional motion of the particle.

Particle Adsorption on Hydrogel Surfaces in Aqueous Media due to van der Waals Attraction

Particle adhesion onto hydrogels has recently attracted considerable attention because of the potential biomedical applications of the resultant materials. A variety of interactions have been taken advantage of for adsorption, including electrostatic forces, hydrophobic interactions and hydrogen bonding. In this study, we report significant adsorption of submicron-sized silica particles onto hydrogel surfaces in water, purely by van der Waals (vdW) attraction. The vdW forces enabled strong adhesions between dielectric materials in air. However, because the Hamaker constant decreases in water typically by a factor of approximately 1/100, it is not clear whether vdW attraction is the major driving force in aqueous settings. We investigated the adsorption of silica particles (diameter = 25-600 nm) on poly(acrylamide) and poly(dimethylacrylamide) gels using optical microscopy, under conditions where chemical and electrostatic adsorption is negligible. The quantity of adsorbed particles decreased on decreasing the Hamaker constant by varying the refractive indices of the particles and medium (ethyleneglycol/water), indicating that the adsorption is because of the vdW forces. The adsorption isotherm was discussed based on the adhesive contact model in consideration of the deformation of the gel surface. The present findings will advance the elucidation and development of adsorption in various types of soft materials.

Numerical study of cluster formation in binary charged colloids

Cluster formation of oppositely charged colloidal particles is studied numerically. A simple Brownian dynamics method with a screened-Coulomb (Yukawa) potential is employed for numerical simulations. An equilibrium phase which consists of clusters and unassociated particles is obtained. It is shown that the equilibrium association number of clusters and their shapes are determined by charge numbers and charge ratio of the binary particles. The phase diagram of cluster formation for various charge numbers and their ratios is obtained. A simple relation between the association number and the charge ratio is found. It is demonstrated that in the case of high charge ratio the cluster takes a multilayer structure which is highly symmetric. It is also pointed out that the cluster-particle interaction changes dynamically in the cluster formation process, which is involved in the selection of final cluster structure.

Controlled Clustering in Binary Charged Colloids by Adsorption of Ionic Surfactants

We report on the controlled clustering of oppositely charged colloidal particles by the adsorption of ionic surfactants, which tunes charge numbers Z of particles. In particular, we studied the heteroclustering of submicron-sized polystyrene (PS) and silica particles, both of which are negatively charged, in the presence of cetylpyridinium chloride (CPC), a cationic surfactant. The surfactant concentration Csurf was selected below the critical micelle concentration. As CPC molecules were adsorbed, Z values of the PS and silica particles decreased, inverting to positive when Csurf exceeded the isoelectric point Ciep. Hydrophobic PS particles exhibited much lower Ciep than hydrophilic silica particles. At Csurf valuess between their Ciep values, the particles were oppositely charged, and clustering was enabled. To explain the clustering behavior, we investigated adsorption isotherms of the CPC and screened-Coulomb-type pair potential. Expected applications of the present findings are the control of colloidal associations and construction of various particle types into heterogeneous colloidal clusters.

Dynamics of polyelectrolyte solutions under a constant gradient of base concentration

Phase-separation dynamics of weakly charged polyacid solutions under a constant gradient of base concentration is studied both theoretically and numerically. The time-evolution equation of polymer volume fraction is derived by assuming that the chemical equilibrium of the dissociation reaction is locally established. Numerical simulations of the system in contact with two reservoirs in which the base concentrations differ are performed. The numerical results show that the polymer volume fraction can be transported by the concentration gradient of the base, which leads to the dynamic behavior of mesophase domain structures.

Mesoscopic simulation of phase behaviors and structures in an amphiphile-solvent system

We have performed a three-dimensional simulation of mesoscopic structures in a mixture of AB amphiphilic molecule and C solvent by employing the density-functional theory under the conditions that (i) the size of the AB is much larger than C and (ii) the affinity between A and B is much larger than the affinity between B and C. First, we have calculated the free energy of five periodic structures, i.e., the lamellar phase, hexagonally packed cylinders, body-centered-cubic spheres, face-centered-cubic spheres, and gyroid phase for different sets of the concentration of AB (ϕ[over ¯]_{AB}) and the χ parameter (χ_{AC}). By comparing the free energies for these structures, the χ_{AC}-ϕ[over ¯]_{AB} phase diagram has been obtained. In addition to these periodic structures, it has been shown that nonperiodic structures such as spherical and rodlike micelles can be obtained although they might be metastable phase.

Exclusion of impurity particles in charged colloidal crystals

Uniformly shaped, charged colloidal particles dispersed in water form ordered "crystal" structures when the interaction between the particles is sufficiently strong. Herein, we report the behavior of "impurity" particles, whose sizes and/or charge numbers are different from those of the bulk, on addition to the charged colloidal crystals. These impurities were excluded from the crystals during the homogeneous crystallization, crystal grain growth, and unidirectional crystallization processes. Such systems will be useful as models for studying the refinement of materials and crystal defects.

Recrystallization and zone melting of charged colloids by thermally induced crystallization

We examined the application of recrystallization and zone-melting crystallization methods, which have been used widely to fabricate large, high-purity crystals of atomic and molecular systems, to charged colloidal crystals. Our samples were aqueous dispersions of colloidal silica (with particle diameters of d = 108 or 121 nm and particle volume fractions of ϕ = 0.035-0.05) containing the weak base pyridine. The samples crystallized upon heating because of increases in the particle charge numbers, and they melted reversibly on cooling. During the recrystallization experiments, the polycrystalline colloids were partially melted in a Peltier cooling device and then were crystallized by stopping the cooling and allowing the system to return to ambient temperature. The zone-melting crystallization was carried out by melting a narrow zone (millimeter-sized in width) of the polycrystalline colloid samples and then moving the sample slowly over a cooling device to recrystallize the molten region. Using both methods, we fabricated a few centimeter-sized crystals, starting from millimeter-sized original polycrystals when the crystallization rates were sufficiently slow (33 μm/s). Furthermore, the optical quality of the colloidal crystals, such as the half-band widths of the diffraction peaks, was significantly improved. These methods were also useful for refining. Small amounts of impurity particles (fluorescent polystyrene particles, d = 333 nm, ϕ = 5 × 10(-5)), added to the colloidal crystals, were excluded from the crystals when the crystallization rates were sufficiently slow (∼0.1 μm/s). We expect that the present findings will be useful for fabricating large, high-purity colloidal crystals.

Exclusion of impurity particles during grain growth in charged colloidal crystals

We examine the spatial distribution of fluorescent-labeled charged polystyrene (PS) particles (particle volume fraction ϕ = 0.0001 and 0.001, diameter d = 183 and 333 nm) added to colloidal crystals of charged silica particles (ϕ = ϕ(s) = 0.035-0.05, d = 118 nm). At ϕ(s) = 0.05, the PS particles were almost randomly distributed in the volume-filling polycrystal structures before the grain growth process. Time-resolved confocal laser scanning microscopy observations reveal that the PS particles are swept to the grain boundaries of the colloidal silica crystals owing to grain boundary migration. PS particles with d = 2420 nm are not excluded from the silica crystals. We also examine influences of the impurities on the grain growth laws, such as the power law growth, size distribution, and existence of a time-independent distribution function of the scaled grain size.

Controlling profiles of polymer dots by switching between evaporation and condensation

We found that the profiles of the dots formed from the drying droplets of polymer solution can be modified by switching between the evaporation and condensation processes. When a polymer dot is exposed to solvent vapor during a certain time and is dried again, the dot profile changes from ringlike to flat. To obtain a flat dot, there exists an optimal exposure time. We conjecture that the change of the dot profile is due to the refluidization of the polymer film. Our results imply a new possibility for controlling the dot profile in inkjet printing technologies.

Final shape of a drying thin film

Drying processes of polymer solutions on a solid substrate enclosed by bank are studied in the slow limit of the solvent evaporation. A simple model is proposed to examine the final shape of the film after drying. Analytical expressions of the final shape in terms of the initial parameters are obtained. It is shown that the craterlike and basinlike shapes appear as final shapes of the film depending on the initial parameters. The "shape diagrams" which show parameter dependence of the final shape are presented in the absence/presence of diffusion. The final shape of the film in the geometry without bank is also discussed.

Effects of elasticity on drying processes of polymer solutions

Drying processes of polymer solutions on a solid substrate are studied, based on the two-fluid model for polymer solutions or gels. A simplified model which can describe a skin formation process is proposed to examine the diffusion process of polymers and the evaporation process of solvent. The authors show that when a skin (gel phase) is formed, the collective diffusion coefficient is greatly enhanced whereas the evaporation of solvent is considerably reduced. These two results originate from the bulk elasticity of polymer gels.

Simple model of skin formation caused by solvent evaporation in polymer solutions

A simple model is proposed for the skin formation in the evaporation process of a polymer solution at a free surface. In this model the skin is regarded as a gel phase formed near the free surface, and the dynamics is described by a diffusion equation for the polymer concentration with moving boundaries. The equation is solved both analytically and numerically. It is shown that the skin phase appears when the evaporation rate is high or when the initial polymer concentration is high. An analytical expression is given for the criterion for the skin phase to be formed.

Theoretical modeling of photo-induced wave propagation in liquid-crystalline Langmuir monolayers

A phenomenological model of wave propagation in photo-excited liquid-crystalline Langmuir monolayers is constructed. The spontaneous splay deformation of the liquid-crystalline order and the anisotropy of photo-excitation of molecules are taken into account in this model. Numerical simulations of the model well reproduce qualitative features of the wave propagation phenomenon observed in recent experiments. A linear stability analysis of the model equations reveals that an interplay between the spontaneous splay deformation and the anisotropy of the photo-excitation can lead to the wave propagation.

Generation, propagation, and switching of orientational waves in photoexcited liquid-crystalline monolayers

Photoinduced orientational waves in illuminated liquid-crystalline monolayers is one of the most remarkable far-from-equilibrium phenomena that systems of soft condensed matter exhibit. We model this behavior from a phenomenological point of view, taking the anisotropic photoexcitation of molecules into account. Numerical simulations as well as theoretical analyses of the model reveal that the intricate interplay between the spontaneous splay deformation of the liquid-crystalline order and the anisotropy of the photoexcitation can lead to the generation and propagation of orientational waves. The model can explain all the salient features of the phenomenon-in particular, the anomalous reversal of the propagation direction upon 90 degrees rotation of the polarization direction of illumination, which evaded theoretical explanation for nearly a decade.

Traveling waves in phase-separating reactive mixtures

A model of phase separation of chemically reactive ternary mixtures is constructed. In this model, spatially periodic structures that coherently propagate at a constant speed emerge through a Hopf bifurcation at a finite wave number. It is shown by computer simulations that both lamellar and hexagonal structures undergo a coherent propagating motion in two dimensions, and there are two types of traveling hexagons depending on the relative direction between the traveling velocity and the lattice vectors of the hexagonal structure. Amplitude equations for the traveling waves are derived, and the stability of the traveling and standing waves is discussed.

Time-modulated oscillatory structures in phase-separating reactive mixtures

We investigate the effect of temporal modulation on the spatiotemporal patterns produced by phase separation in chemically reactive ternary mixtures. The temporal modulation is introduced by making one of the reaction rates periodic in time. Our main concern is the stability of traveling waves, which appear above a Hopf-type bifurcation at a finite wave number and are stable in the absence of external modulation. It is shown by computer simulations in two dimensions that when the external modulation is present, various types of coherent standing waves emerge near the special point in the parameter space, where the Turing-type bifurcation line and the Hopf-type bifurcation line meet each other. We carry out a theoretical analysis to understand the phase diagram of the synchronized oscillatory structures obtained numerically.

Self-propulsion of cellular structures in chemically reacting mixtures

An alternative model of phase-separating reactive mixtures is proposed. In this model both phase separation and chemical reactions simultaneously take place and a traveling coherent structure can be formed through a Hopf bifurcation at a finite wave number. Numerical simulations show that, depending on the parameters, either lamellar or hexagonal structures travel at constant speeds in two-dimensional systems.

Hierarchical model in multiphase flow

A hierarchical model for two-phase flow is constructed. The model has two layered systems one corresponding to the macroscopic hydrodynamics described by the Navier-Stokes equation and the other to the interfacial dynamics described by the Cahn-Hilliard-type equation. Numerical simulations in some simple cases are carried out to examine the validity of the model. As an application of the model simulations of two colliding droplets under shear flow are presented.

Fatal epistaxis caused by rupture of an intratumoral aneurysm enclosed by a large prolactinoma--case report

A 72-year-old female presented with episodes of epistaxis. Neuroimaging demonstrated a large prolactinoma totally enclosing a large intracavernous aneurysm of the internal carotid artery. Adjacent bony structures were eroded and destroyed by tumor invasion and extension. Rupture of the intratumoral aneurysm caused fatal epistaxis rather than subarachnoid hemorrhage before surgery. Intratumoral aneurysm is rare and epistaxis caused by rupture of it is extremely rare. Lack of bony protection apparently have contributed to the aneurysmal growth and rupture.

Vestibular neuronitis evaluated by the combined galvanic test

Forty-two patients with unilateral vestibular neuronitis were examined by the combined galvanic test (CGT), consisting of the galvanic eye movement test (GEMT) and the galvanic body sway test (GBST). Results of the CGT were evaluated with respect to the time period from the onset of vestibular neuronitis. A favorable recovery was observed earlier with the GBST than the GEMT. We believe that this discrepancy may be due to the difference in compensation between the vestibulo-ocular reflex and the vestibulo-spinal reflex.

Compensatory process of vestibular neuronitis. From findings of combined galvanic test and caloric test

The compensatory process of vestibular neuronitis in 7 patients was followed up and evaluated using the Combined Galvanic Test (CGT) and other neuro-otological data. CGT is the simultaneous recording of both galvanic eye movement (GEM) and galvanic body sway (GBS), and has proved to be reliable in the diagnosis of retrolabyrinthine disorders. The results were as follows: The subjective symptoms remained in 3 cases from 1 month to 1 year. The recovery of the GBS response was seen earlier than that of both GEM and caloric responses. The recovery of GEM and caloric responses was observed at almost the same time. We observed the favorable recovery process of caloric response and GEM response in some cases which had recovery of GBS response within 4 weeks and suggest that the system of GBS response is different from that of GEM and caloric response. However, the systems of the latter two may be the same. We assume that the recovery of GBS response is earlier than that of GEM and caloric responses because the otolithic system recovers more easily than the semicircular canal system. The CGT and caloric test were reliable in examining the compensatory process of vestibular neuronitis.