Osmotic compression of mixtures of polymers and particles

Aqueous dispersions of nanometric ceria particles have been concentrated through osmotic stress. Mixed dispersions of ceria with small adsorbing macromolecules of poly (vinylpyrrolidone) have been prepared by the same method. The osmotic pressure of pure ceria dispersions results from electrostatic repulsions between particles. The osmotic pressure of dispersions containing a non-saturating amount of PVP is the same as that of pure dispersions, and the colloidal stability is depressed with respect to the pure dispersions. The osmotic pressure of dispersions containing an excess of PVP is increased by the free macromolecules, and the colloidal stability is enhanced. The organization of particles in these dispersions has been examined by small-angle x-ray scattering and cryotransmission electron microscopy. In pure ceria dispersions and in saturated dispersions, a liquid-like short-range order was found; when the concentration is increased, this short-range order follows a three-dimensional swelling law. In dispersions containing a non-saturating amount of PVP, the structure shows an alternance of clusters and voids, and the separations of clusters follow an unusual one-dimensional swelling law.     

High elongation of polyelectrolyte chains in the osmotic limit of spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy

We investigate the conformation of long polyelectrolyte chains attached to colloidal latex particles by cryogenic transmission electron microscopy (cryo-TEM). The dense grafting of the polyelectrolyte chains ("polyelectrolyte brush") leads to a confinement of the counterions and a concomitantly high osmotic pressure within the polyelectrolyte layer attached to the core particles. Cryo-TEM has provided first model-independent direct proof for the strong stretching of the polyelectrolyte chains by direct visualization. If salt is added, cryo-TEM clearly shows how chains collapse because of the strong screening of the electrostatic interaction. Moreover, the analysis of interacting particles by cryo-TEM shows that the polyelectrolyte chains retract at close contact. Hence, we demonstrate how cryo-TEM can be used to analyze directly the spatial structure of polyelectrolyte brushes in situ.     

Mechanomorphological Guidance of Colloidal Gel Regulates Cell Morphogenesis

Microstructural morphology of the extracellular matrix guides the organization of cells in 3D. However, current biomaterials-based matrices cannot provide distinct spatial cues through their microstructural morphology due to design constraints. To address this, colloidal gels are developed as 3D matrices with distinct microstructure by aggregating ionic polyurethane colloids via electrostatic screening. Due to the defined orientation of interconnected particles, positively charged colloids form extended strands resulting in a dense microstructure whereas negatively charged colloids form compact aggregates with localized large voids. Chondrogenesis of human mesenchymal stem cells (MSCs) and endothelial morphogenesis of human endothelial cells (ECs) are examined in these colloidal gels. MSCs show enhanced chondrogenic response in dense colloidal gel due to their spatial organization achieved by balancing the cell–cell and cell–matrix interactions compared to porous gels where cells are mainly clustered. ECs tend to form relatively elongated cellular networks in dense colloidal gel compared to porous gels. Additionally, the role of matrix stiffness and viscoelasticity in the morphogenesis of MSCs and ECs are analyzed with respect to microstructural morphology. Overall, these results demonstrate that colloidal gels can provide spatial cues through their microstructural morphology and in correlation with matrix mechanics for cell morphogenesis.     

Preferred orientation in filtercakes of kaolinite

The orientational order parameter of dense colloidal dispersions of plate-like particles as a function of volume fraction is measured using neutron diffraction. This non-invasive experimental approach directly provides the full particle orientation distribution from which the order parameters can be calculated. The orientation parameters are shown to be linked to the solids fractions of the cakes and the macroscopic permeability of the samples. However, this study suggests that, although orientation can be relevant for a given system, other factors can have a stronger influence, for example, the degree of dispersion or colloidal stability of the clay and may be the principle factor that controls permeability. In addition, we report enhanced ordering of these materials under the influence of an external cross-flow field.     

Spontaneous formation of mesostructures in colloidal monolayers trapped at the air-water interface: a simple explanation

The spontaneous formation of loosely bound ordered aggregates, foam, voids, chains, striations, and loops (see Figure 1a), called mesostructures hereafter, has been observed in colloidal monolayers trapped at the air-water interface. The distance between particles in these mesostructures is of the order of the particle radius (micrometers), implying that the colloidal interaction potential has a minimum at such distances, which could induce the phase separation of colloidal monolayers in dense and dilute regions. This is at odds with the accepted theory (Derjaguin-Landau-Verwey-Overbeek (DLVO)) of colloidal interactions, which predicts a secondary minimum at distances of nanometers between pairs of interacting particles. Moreover, the introduction of capillary, hydrophobic, and dipolar interactions between particles in an extended DLVO theory is not able to explain the spontaneous formation of mesostructures either. Recently, a great deal of effort has focused on understanding the mechanism behind the phenomenon of long-range attraction between colloidal particles confined in interfaces. In particular, this attraction has been employed to explain the spontaneous formation of mesostructures. Here, we show that the appearance of our mesostructures is due to the contamination of colloidal monolayers by silicone oil (poly(dimethylsiloxane)), which arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. The difference in the interfacial tension of water and silicone oil accounts for the formation of the experimentally observed mesostructures.     

颗粒模板法制备大孔Al2O3材料

采用颗粒模板法制备了大孔氧化铝(Al2O3)材料. 扫描电子显微镜(SEM)结果显示, 大孔Al2O3结构中的大孔呈“囊泡状”且孔道的贯通性较差. Zeta电位测量表明, 共沉积条件下聚苯乙烯(PS)和Al2O3两种胶体颗粒带有相反的电荷, 在静电引力作用下先发生了吸附, 再沉积在一起. 吸附在PS微球表面的Al2O3纳米颗粒形成的吸附层是导致大孔呈“囊泡状”和孔道不贯通的主要原因. 采用聚二烯丙基二甲基氯化铵(PD)溶液对PS胶体微球带电性质进行了改性, PS微球的Zeta电位由−44.36 mV变成了+37.41 mV, 进而消除了沉积过程中二元颗粒间的吸附现象. 扫描电子显微镜显示, 大孔样品中“囊泡状”大孔消失, 同时孔道贯通性得到改善.     

Patterned assembly of colloidal particles by confined dewetting lithography

We report the assembly of colloidal particles into confined arrangements and patterns on various cleaned and chemically modified solid substrates using a method which we term "confined dewetting lithography" or CDL for short. The experimental setup for CDL is a simple deposition cell where an aqueous suspension of colloidal particles (e.g., polystyrene spheres) is placed between a floating deposition template (i.e., metal microgrid) and the solid substrate. The voids of the deposition template serve as an array of micrometer-sized reservoirs where several hydrodynamic processes are confined. These processes include water evaporation, meniscus formation, convective flow, rupturing, dewetting, and capillary-bridge formation. We discuss the optimal conditions where the CDL has a high efficiency to deposit intricate patterns of colloidal particles using polystyrene spheres (PS; 4.5, 2.0, 1.7, 0.11, 0.064 microm diameter) and square and hexagonal deposition templates as model systems. We find that the optimization conditions of the CDL method, when using submicrometer, sulfate-functionalized PS particles, are primarily dependent on minimizing attractive particle-substrate interactions. The CDL methodology described herein presents a relatively simple and rapid method to assemble virtually any geometric pattern, including more complex patterns assembled using PS particles with different diameters, from aqueous suspensions by choosing suitable conditions and materials.     

TEM-induced structural evolution in amorphous Fe oxide nanoparticles

Exposure to the high energy electron beam of a TEM changes the morphology of amorphous Fe oxide nanoparticles from solid spheres to hollow shells. Amorphous Fe oxide nanoparticles prepared via high-temperature methods using hexadecylamine and trioctylphosphine oxide surfactants were compared to crystalline gamma-Fe2O3 particles of similar size. Both sets of particles are fully characterized via SQUID magnetometry, X-ray powder diffraction, BET surface analysis, EPR spectroscopy, high-resolution transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Time-resolved TEM images reveal that the amorphous Fe oxide particles evolve from solid spheres into hollow shells in <2 min, whereas crystalline gamma-Fe2O3 are unaffected by the electron beam. The resulting nanocrystalline Fe oxide shells bear striking resemblance to core-shell nanocrystals, but are a result of a morphology change attributed to restructuring of particle voids and defects induced by quasi-melting in the TEM. These results thus imply that caution is necessary when using TEM to analyze nanoparticle core-shell and heterostructured nanoparticles.     

Dynamics and collapse of two-dimensional colloidal lattices

One interesting aspect of colloidal particles is the formation of colloidal crystals at the 2D and 3D levels. Here we report the dynamics and collapse of colloidal lattices at liquid-liquid interfaces using Pickering emulsions as an experimental template. The colloidal particles oscillate around their equilibrium positions. The short-time diffusion constant (<10 s) of single particles increases with increasing lattice spacing; the oil-phase viscosity has an effect on diffusion only at large interparticle distances. Strikingly, we observe that the equilibrium structure can be disturbed when increasing the output laser intensity in a confocal laser scanning microscope, which leads to the collapse of colloidal lattices in the presence of small laser powers.     

Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.     

Osmotic compression and expansion of highly ordered clay dispersions

Aqueous dispersions of nanometric clay platelets (Laponite) have been dewatered through different techniques: centrifugation, mechanical compression, and osmotic stress (dialysis against a polymer solution). The positional and orientational correlations of the particles have been determined through small-angle neutron scattering. Uniaxial compression experiments produce concentrated dispersions (volume fraction > 0.03) in which the platelets have strong orientational and positional correlations. The orientational correlations cause the platelets to align with their normal along a common axis, which is the axis of compression. The positional correlations cause the platelets to be regularly spaced along this direction, with a spacing that matches the average volume per particle in the dispersion. The swelling law (volume fraction versus separation distance) is one-dimensional, as in a layered system. Changes in the applied osmotic pressure cause the water content of the dispersion to either rise or decrease, with time scales that are controlled by interparticle friction forces and by hydrodynamic drag. At long times, the dispersions approach osmotic equilibrium, which can be defined as the common limit of swelling and deswelling processes. The variation of the equilibrium water content with the applied osmotic pressure has been determined over 1 decade in volume fractions (0.03 < phi < 0.3) and 3 decades in pressures. This equation of state matches the predictions made from the knowledge of the forces and thermal agitation for all components in the dispersion (particles, ions, and water).     

Ternary inverse opal system for convenient and reversible photonic bandgap tuning

A ternary system, consisting of air, an air-core/dense-silica-shell core-shell particle, and liquids has been used to fabricate an inverse opal structure with low fill factor, high refractive index contrast, and reversible tuning capabilities of the bandgap spectral position. The original close-packed opal structure is a ternary self-assembled photonic crystal from monodisperse and spherical polystyrene-core/silica-shell colloidal particles with air as the void material. Calcination removed the polystyrene and converted the core-shell particles to hollow spheres with a dense shell. In a final step, liquid is infiltrated only in the voids between the hollow spheres, but does not penetrate in the shell. This allows facile and reversible tuning of the bandgap properties in an inverse opal structure.     

Gas-solid coexistence in highly charged colloidal suspensions

Aqueous suspensions of highly charged polystyrene particles with different volume fractions have been investigated for structural ordering and phase behavior using static light scattering (SLS) and confocal laser scanning microscope (CLSM). Under deionized conditions, suspensions of high-charge-density colloidal particles remained disordered whereas suspensions of relatively low charge density showed crystallization by exhibiting iridescence for the visible light. Though for the unaided eye crystallized suspensions appeared homogeneous, SLS measurements and CLSM observations have revealed their inhomogeneous nature in the form of the coexistence of voids with dense ordered regions. CLSM investigations on disordered suspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered (amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transition and are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty, P. S.; Tata, B. V. R. J. Colloid Interface Sci. 2003, 264, 101]. On the basis of our experimental and simulation results, we argue that the reported reentrant disordered state [Yamanaka et al. Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al. Langmuir 2003, 19, 3236] in charged colloids observed at high charge densities is a gas-solid coexistence state.     

Formation and dynamics of capillary-flow-induced colloidal rings

We have directly observed the ring formation of colloidal particles of 1 μm diameter at the contact lines of air, water, and oil using a laser scanning confocal microscope. Colloidal rings form and grow through the transport of particles induced by capillary flow due to water evaporation. In addition, we observe the sudden "jump in" of particles into the ring and the "depletion" of particles in the ring. Particle-tracking experiment shows that the particles within the ring exhibit 1D-like motion along the circular ring geometry, and the pair correlation function of the ring configuration suggests an equilibrium interparticle distance of approximately 2.8 μm. It is also found that the structure and formation speed of the colloidal rings can be controlled by accelerating water evaporation by the addition of methanol as a cosolvent.     

Brownian dynamics simulation of adsorbed layers of interacting particles subjected to large extensional deformation

We present Brownian dynamics simulations of the compression and expansion of monolayers adsorbed at a planar interface. The surface-active species are modelled as monodisperse spherical particles that can form particle-particle elastic bonds. The objective is to model the large compression and expansion of viscoelastic protein films investigated in Langmuir trough experiments. We determine the stress-strain response of the system and the associated microstructural changes induced by the large deformation of the interface as a function of particle adsorption energy, and bond breakability and stiffness. We also study the effect of the velocity of compression and the type of compression (uniaxial or homogeneous) on the mechanism of collapse of the adsorbed films. Furthermore, we present simulations on complex mixed systems containing both bond-forming particles (modelling protein) and nonbond-forming particles (modelling surfactant). We find that the preferential desorption of one type of particle or the other, upon compression, is sensitive to the extent of bond breakability of the bond-forming species.     

Opal and inverse opal photonic crystals: Fabrication and characterization

Three-dimensional photonic crystals made of close-packed polymethylmethacrylate (PMMA) spheres or air spheres in silica, titania and ceria matrices have been fabricated and characterized using SEM, XRD, Raman spectroscopy and UV–Vis transmittance measurements. The PMMA colloidal crystals (opals) were grown by self-assembly from aqueous suspensions of monodisperse PMMA spheres with diameters between 280 and 415 nm. SEM confirmed the PMMA spheres crystallized uniformly in a face-centred cubic (fcc) array, and UV–Vis measurements show that the colloidal crystals possess pseudo photonic band gaps in the visible and near-IR regions. Inverse opals were prepared by depositing silica (SiO₂), titania (TiO₂) or ceria (CeO₂) in the voids of the PMMA colloidal crystals using sol-gel procedures, then calcining the resulting structure at 550 °C to remove the polymer template. The resulting macroporous materials showed fcc ordering of air spheres separated by thin frameworks of amorphous silica, nanocrystalline titania or nanocrystalline ceria particles, respectively. Optical measurements confirmed the photonic nature of the inverse opal arrays. UV–Vis data collected for the opals and inverse opals obeyed a modified Bragg’s law expression that considers both diffraction and refraction of light by the photonic crystal architectures. The versatility of the colloidal crystal template approach for the fabrication of macroporous oxide structures is demonstrated.     

Study on the stability of modified colloidal silica with polymer in aqueous environment

The influence of polyvinyl alcohol (PVA) and polyacrylamide (PAM) on the stability of acidic colloidal silica in aqueous environment was examined. Experiments were carried out on the original colloidal silica (CS), PVA modification and PAM modification. PVA- and PAM-modified CS could remain stable for 20 and 13 days in the oven under 50 °C; however, nonmodified CS is only stable for 9 days. The thermal stability of CS was significantly improved through PVA modification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that the modified particles reveal better dispersibility. The action mechanism between polymer and CS was studied by particle size distribution, zeta potential, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR). The results show that the modified silica particles acquire more negative charge and the interactions among silica particles and PVA are stronger than among silica particles and PAM. Such differences appeared to be due to the loss of water molecules between C–OH of PVA and Si–OH bonds, which form a dense monomolecular adsorbed layer on the surface of the silica particles, thus increasing space steric repulsion. However, for the situation of PAM, the interaction was weakened. The results provided important basis to optimize stability of acidic CS in the aqueous environment.     

Predicting the phase diagram of two-dimensional colloidal systems with long-range interactions

The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometer-range binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of kBT. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.     

On the Theory of Structural Transformations in Magnetic Fluids

Results of studying a new scenario of condensation phase transition in the ensemble of ferrofluid particles are reported. Phase transition in this ensemble starts with the formation of chain clusters. Chains whose length exceeds a certain critical value dependent on temperature and magnetic field undergo collapse and are transformed into very dense globules. The globule evolution leads to the separation of ferrofluid into two phases with different particle density. Initial (chain collapse) and final (equilibrium) stages of phase transition are investigated. Calculations of the critical length of a chain corresponding to its collapse, as well as the characteristics of dense phase at the final separation stage are in good agreement with the results of known experiments.     

Spectroscopic studies of fractal aggregates of silver nanospheres undergoing local restructuring

We present an experimental spectroscopic study of large random colloidal aggregates of silver nanoparticles undergoing local restructuring. We argue that such well-known phenomena as strong fluctuation of local electromagnetic fields, appearance of "hot spots" and enhancement of nonlinear optical responses depend on the local structure on the scales of several nanosphere diameters, rather than the large-scale fractal geometry of the sample.