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.     

Tunable rheology of dense soft deformable colloids

In the last two decades, advances in synthetic, experimental and modeling/simulation methodologies have considerably enhanced our understanding of colloidal suspension rheology and put the field at the forefront of soft matter research. Recent accomplishments include the ability to tailor the flow of colloidal materials via controlled changes of particle microstructure and interactions. Whereas hard sphere suspensions have been the most widely studied colloidal system, there is no richer type of particles than soft colloids in this respect. Yet, despite the remarkable progress in the field, many outstanding challenges remain in our quest to link particle microstructure to macroscopic properties and eventually design appropriate soft composites. Addressing them will provide the route towards novel responsive systems with hierarchical structures and multiple functionalities. Here we discuss the key structural and rheological parameters which determine the tunable rheology of dense soft deformable colloids. We restrict our discussion to non-crystallizing suspensions of spherical particles without electrostatic or enthalpic interactions.     

土壤胶体迁移行为及其介导污染物迁移模拟与研究进展

本文综述了土壤多孔介质中胶体迁移的释放与沉积机制、影响胶体迁移的多种因素以及土壤中胶体与各种污染物的协同迁移作用,总结了模拟胶体迁移的数学模型以及计算机软件的应用。研究表明,胶体在土壤中的迁移主要受应变、附着、薄膜应变等迁移机制的影响,多孔介质的性质、流体的性质以及胶体自身的性质也会影响胶体的迁移。此外,胶体能有效吸附地下水多孔介质中的有机或无机污染物,并对其在地下环境中的迁移产生显著影响。目前已有许多学者通过数学模型来模拟胶体在土壤中的迁移过程,而计算机技术的进步也将促进更加先进的软件模型应用到胶体迁移的模拟中。     

Interaction between colloids with grafted diblock polyampholytes

The interaction between composite colloidal particles composed of a spherical core and grafted AB-diblock polyampholytes (diblock copolymers with oppositely charged blocks) are investigated by using a coarse-grained model solved with Monte Carlo simulations. The B block is end-grafted onto the core of the colloid and its linear charge density is varied, whereas the linear charge density of the A block is fixed. The brush structure of a single colloid, the mean force between two colloids, and the structure of solutions of such colloids have been determined for different linear charge densities of the B blocks and block lengths. Many features of the present system are controlled by the charge of the B blocks. In the limit of uncharged B blocks, (i) the grafted chains are stretched and form an extended polyelectrolyte brush, (ii) a strong repulsive force is operating between two colloids, (iii) and the solution is thermodynamic stable and displays strong spatial correlation among the colloids. In the limit where the charges of the two types of blocks exactly compensate each other, (i) the chains are collapsed and form a polyelectrolyte complex surrounding the cores, (ii) an attractive force appears between two colloids, and (iii) strong colloid clustering appears in the solution. These features become more pronounced as the length of the polymer blocks is increased, and a phase instability occurs at sufficiently long chains. A comparison with properties for other related colloidal particles is also provided.     

Colloidal crystals from surface-tension-assisted self-assembly: a novel matrix for single-molecule experiments

In this work, we develop a new method of creating colloidal crystals with cavities for the entrapment and long-term observation of single biomolecules. Colloidal crystals are first fabricated using surface-tension-assisted self-assembly. Surface tension helps to reduce the interparticle distance between dispensed colloids. Subsequently, the colloids are used as a matrix in which single fluorescently tagged molecules can be tracked using fluorescence microscopy. This method has a high efficiency of self-assembly for small volumes (4 microL) of colloidal suspensions (polystyrene colloids with diameters of 1000, 500, 200, and 100 nm) at low concentration (1% w/w). The spatial hindrance effect on the diffusion of molecules and their entrapment is discussed on the basis of fluorescence correlation spectroscopy results from the diffusion of molecules with different hydrodynamic radii in the cavities of colloidal crystals formed from micrometer- to nanometer-sized polystyrene spheres. Single horseradish peroxidase molecules turning over fluorescent products are tracked over a few seconds. This shows that colloidal crystals can be used to test the function of single molecules of enzymes and protein under controlled spatial confinement.     

混合正、负电性银胶体系的表面增强拉曼散射活性研究

Both negatively and positively charged silver colloids were prepared in aqueous solution for surface-enhanced Raman scattering (SERS) by the reduction of silver nitrate with sodium citrate and hydrogen peroxide under basic condition, respectively. By means of transmission electron microscopy (TEM), the observation of morphologies of negatively charged silver colloid (NSC) and positively charged silver colloid (PSC) has been presented, and the aggregation behaviors of NSC and PSC as well as the mixture of NSC and PSC induced by the addition of fuchsine basic molecules examined. SERS from fuchsine basic molecules in above mentioned colloidal systems were recorded respectively and compared with each other. The results show that the mixture of NSC and PSC could form the aggregation morphology different from that of single NSC and PSC and bring about the favorable effect on SERS behavior for the adsorbed fuchsine basic.     

The polydisperse cell model: nonlinear screening and charge renormalization in colloidal mixtures

We propose a model for the calculation of renormalized charges and osmotic properties of mixtures of highly charged colloidal particles. The model is a generalization of the cell model and the notion of charge renormalization as introduced by Alexander et al. [J. Chem. Phys. 80, 5776 (1984)]. The total solution is partitioned into as many different cells as components in the mixture. The radii of these cells are determined self-consistently for a given set of parameters from the solution of the nonlinear Poisson-Boltzmann equation with appropriate boundary conditions. This generalizes Alexanders's model where the (unique) Wigner-Seitz cell radius is solely fixed by the colloid packing fraction. We illustrate the technique by considering a binary mixture of the colloids with the same sign of charge. The present model can be used to calculate thermodynamic properties of highly charged colloidal mixtures at the level of linear theories, while taking the effect of nonlinear screening into account.     

Coarse-graining intermolecular interactions in dispersions of highly charged colloids

Effective pair potentials between charged colloids, obtained from Monte Carlo simulations of two single colloids in a closed cell at the primitive model level, are shown to reproduce accurately the structure of aqueous salt-free colloidal dispersions, as determined from full primitive model simulations by Linse et al. (Linse, P.; Lobaskin, V. Electrostatic Attraction and Phase Separation in Solutions of Like-Charged Colloidal Particles. Phys. Rev. Lett.1999, 83, 4208). Excellent agreement is obtained even when ion-ion correlations are important and is in principle not limited to spherical particles, providing a potential route to coarse-grained colloidal interactions in more complex systems.     

The foam analogy: from phases to elasticity

By mapping the interactions of colloidal particles onto the problem of minimizing areas, the physics of foams can be used to understand the phase diagrams of both charged and fuzzy colloids. We extend this analogy to study the elastic properties of such colloidal crystals and consider the face-centered cubic, body-centered cubic and A15 lattices. We discuss two types of soft interparticle potentials corresponding to charged and fuzzy colloids, respectively, and we analyze the dependence of the elastic constants on density as well as on the parameters of the potential. We show that the bulk moduli of the three lattices are generally quite similar, and that the shear moduli of the two non-close-packed lattices are considerably smaller than in the face-centered cubic lattice. We find that in charged colloids, the elastic constants are the largest at a finite screening length, and we discuss a shear instability of the A15 lattice.     

Engineering Substrate Topography at the Micro‐ and Nanoscale to Control Cell Function

The interaction of mammalian cells with nanoscale topography has proven to be an important signaling modality in controlling cell function. Naturally occurring nanotopographic structures within the extracellular matrix present surrounding cells with mechanotransductive cues that influence local migration, cell polarization, and other functions. Synthetically nanofabricated topography can also influence cell morphology, alignment, adhesion, migration, proliferation, and cytoskeleton organization. We review the use of in vitro synthetic cell–nanotopography interactions to control cell behavior and influence complex cellular processes, including stem‐cell differentiation and tissue organization. Future challenges and opportunities in cell–nanotopography engineering are also discussed, including the elucidation of mechanisms and applications in tissue engineering.     

Light Scattering Probes of Viscoelastic Fluids and Solids

In this article, we discuss recent advances in static and dynamic light scattering applied to soft materials. Special emphasis is given to light scattering methods that allow access to turbid and solid‐like systems, such as colloidal suspensions, emulsions, glasses, or gels. Based on a combination of single‐ and multispeckle detection schemes, it is now possible to cover an extended range of relaxation times from a few nanoseconds to minutes or hours and length scales below 1 nm up to several microns. The corresponding elastic properties of viscoelastic fluids or solid materials range roughly from below 1 Pa to several 100 kPa. Different applications are discussed such as light scattering from suspensions of highly charged colloidal particles, colloid and protein gels, as well as dense surfactant solutions.     

Rheological aspects of colloidal gels in thermoresponsive microgel suspensions: formation,structure, and linear and nonlinear viscoelasticity

This review focuses on the rheological aspects of colloidal gels that are a three-dimensional sparse network made of aggregated attractive particles formed in the aqueous suspensions of microgels composed of thermoresponsive polymers. Heating changes the dominant interparticle interactions from repulsive to attractive because of the hydrophilic-to-hydrophobic transition. Under appropriate conditions, the hydrophobic microgel suspensions form colloidal gels behave as a yield fluid. The elastic and yielding features of the colloidal gels are considerably different from those of the repulsive glass which is formed by the dense packing of the hydrophilic microgels at low temperatures. The thermoresponsive microgel suspensions undergoing colloidal gelation have attracted much attention from not only the academic interests but also the potentials as a functional suspension because they show interesting and marked changes in viscoelasticity when subjected to temperature variation. We discuss the criteria and dynamics of colloidal gelation, the structure, and linear and nonlinear viscoelasticity of the colloid gels with an emphasis on the results of the experimental studies.     

Blocking and ripening of colloids in porous media and their implications for bacterial transport

A model accounting for the dynamics of colloid deposition in porous media was developed and applied to systems containing similarly charged particles and collectors. Colloid breakthrough and intracolumn retention data confirmed that blocking reduced overall colloidal adhesion to soil. The surface coverage at which blocking occurred varied for the type of colloid, as shown by changes in the clean-bed collision efficiency, _0, and the excluded area parameter, β. Excluded area parameters were relatively high due to unfavorable interactions between particles and collectors, and ranged from 11.5 for one bacterium (Pseudomonas putida KT2442) to 13.7 and 24.1 for carboxylated latex microspheres with differing degrees of charged groups on their surfaces. Differences in β values for the three colloids were correlated with electrophoretic mobility, with the most negatively charged colloid (carboxylated latex; CL microspheres) having the highest β. No correlation between hydrophobicity and ₀ or β was found. Besides using colloidal particles capable of blocking, the addition of chemical additives to the soil has been suggested as a means for reducing attachment of colloids to porous media. Dextran addition caused an order-of-magnitude reduction in the overall (for carboxylated-modified latex; CMLs). This reduction was not attributed to blocking, but to the sorption of dextran to the soil which lowered ₀. The filtration-based numerical model used to fit the ₀ and β parameters was used to demonstrate that blocking could result in significantly enhanced bacterial transport in field situations.     

Selective Adhesion and Directional Migration of Endothelial Cells Guided by Cys‐Ala‐Gly Peptide Density Gradient on Antifouling Polymer Brushes

Selective adhesion and directional migration of endothelial cells (ECs) on biomaterials is critical to realize the rapid endothelialization. In this study, a Cys‐Ala‐Gly (CAG) peptide density gradient is generated on homogeneous cell‐resisting poly(2‐hydroxyethyl methacrylate‐co‐glycidyl methacrylate) brushes by immersing the brushes in a complementary gradient solution of CAG and competitive mercapto‐terminated methoxyl poly(ethylene glycol). The adhesion and spreading of smooth muscle cells (SMCs) is impaired effectively on the gradient surface. About six folds of adherent ECs over SMCs are achieved at the position (10 mm) of highest CAG density on the gradient surface in a co‐culture condition. Due to the gradient cues, ECs migrate fastest with the best directionality of 86.7% at the middle of the gradient, leading to the maximum net displacement as well.     

Novel three-dimensional Boyden chamber system for studying transendothelial transport

The rapid development in combinatorial chemistry of millions of novel potential drug candidates requires in vitro devices for reliable testing of their transendothelial transport and the uptake in specific cells. To date, this is often achieved in vitro by the use of regular planar Boyden chambers, which are not reflecting the three dimensionality of the blood vessel. This technical note describes the fabrication and biological validation of a novel three-dimensional Boyden chamber system for studying transendothelial transport. The key element of this new system is a porous thin-walled microchannel produced by a SMART (substrate modification and replication by thermoforming) process comprising a combination of microthermoforming and ion track technology. The membrane-like microstructure offers the opportunity to grow endothelial cells on the inner side of the channel resembling a more natural curved organization of vessels. After establishment of a confluent HUVECs layer in the porous microchannel this novel Boyden chamber was successfully applied to study the transendothelial transport of a polycationic cell penetrating peptoid through the 3D- or curved endothelial cell layer. Thus, this system will enable the investigation of such synthetic compounds as drug delivery systems with regard to their bioavailability and functionality under organotypic conditions.     

Electroosmotically enhanced mass transfer through polyacrylamide gels

We present an internal pumping strategy to enhance solute fluxes in polymer gels. The method is based on electroosmotic flow driven by an electric field applied across a gel that has been doped with charged colloidal inclusions. This work is motivated by the need to enhance the transport in gel-based biosensor devices whose response dynamics are often mass transfer limited. In this case, polyacrylamide gel slabs were doped with immobilized, charged silica colloids, and the flux of a fluorescent tracer was measured as a function of applied field strength, the volume fraction and size of the colloidal silica inclusions, and the bulk electrolyte composition. Significant flux enhancements were achieved with applied electric currents on the order of a few mA. Control experiments indicated that the flux enhancement was not due to any distortion of the gel diffusional properties in response to the presence of the inclusions. At a constant inclusion volume fraction, the electroosmotic solute flux enhancement was strongest for the smallest particle sizes that provide the highest total surface area, consistent with the electroosmotic mechanism whereby fluid flow is generated along the solid/liquid interface.     

Linking Semiconductor Nanocrystals into Gel Networks through All‐Inorganic Bridges

For colloidal semiconductor nanocrystals (NCs), replacement of insulating organic capping ligands with chemically diverse inorganic clusters enables the development of functional solids in which adjacent NCs are strongly coupled. Yet controlled assembly methods are lacking to direct the arrangement of charged, inorganic cluster‐capped NCs into open networks. Herein, we introduce coordination bonds between the clusters capping the NCs thus linking the NCs into highly open gel networks. As linking cations (Pt²⁺) are added to dilute (under 1 vol %) chalcogenidometallate‐capped CdSe NC dispersions, the NCs first form clusters, then gels with viscoelastic properties. The phase behavior of the gels for variable [Pt²⁺] suggests they may represent nanoscale analogues of bridged particle gels, which have been observed to form in certain polymer colloidal suspensions.     

A universal approach to fabricate ordered colloidal crystals arrays based on electrostatic self-assembly

We present a novel and simple method to fabricate two-dimensional (2D) poly(styrene sulfate) (PSS, negatively charged) colloidal crystals on a positively charged substrate. Our strategy contains two separate steps: one is the three-dimensional (3D) assembly of PSS particles in ethanol, and the other is electrostatic adsorption in water. First, 3D assembly in ethanol phase eliminates electrostatic attractions between colloids and the substrate. As a result, high-quality colloidal crystals are easily generated, for electrostatic attractions are unfavorable for the movement of colloidal particles during convective self-assembly. Subsequently, top layers of colloidal spheres are washed away in the water phase, whereas well-packed PSS colloids that are in contact with the substrate are tightly linked due to electrostatic interactions, resulting in the formation of ordered arrays of 2D colloidal spheres. Cycling these processes leads to the layer-by-layer assembly of 3D colloidal crystals with controllable layers. In addition, this strategy can be extended to the fabrication of patterned 2D colloidal crystals on patterned polyelectrolyte surfaces, not only on planar substrates but also on nonplanar substrates. This straightforward method may open up new possibilities for practical use of colloidal crystals of excellent quality, various patterns, and controllable fashions.