Colloid Volume Fraction Research Articles

Microstructure and dynamics of Janus particles in a phase separating medium.

The evolution of interactions and dynamics of Janus colloidal particles suspended in quasi-binary liquid mixtures undergoing phase separation is presented. The experimental system consisted of silica-nickel Janus particles dispersed in mixtures of 3-methylpyridine, water, and heavy water. Colloidal microstructure and dynamics were probed by ultra-small-angle X-ray scattering and ultra-small-angle X-ray photon correlation spectroscopy, respectively. The observed static and dynamic behaviors are significantly different from those found for Stöber silica colloids in this mixture. The Janus particles manifest a slow aggregation below the coexistence temperature and become strongly attractive upon phase separation of the solvent mixture. In the two-phase region, particles tend to display surfactant-like behavior with silica and nickel surfaces likely preferring water and 3-methylpyridine rich phases, respectively. While the onset of diffusiophoretic motion is evident in the dynamics, it is gradually suppressed by particle clustering at the investigated colloid volume fractions.

Thermally reversible nanoparticle gels with tuneable porosity showing structural colour.

We present colloidal gels formed from dispersions of PEG- and PEG+DNA-coated silica nanoparticles showing structural colour. The PEG- and PEG+DNA-coated silica colloids are functionalized using exclusively covalent bonds in aqueous conditions. Both sets of colloids self-assemble into thermally-reversible colloidal gels with porosity that can be tuned by changing the colloid volume fraction, although the interaction potentials of the colloids in the two systems are different. Confocal microscopy and image analysis tools are used to characteraize the gels' microstructures. Optical reflection spectroscopy is employed to study the underlying gel nanostructure and to characterize the optical response of the gels. X-ray nanotomography is used to visualize the nanoscale phase separation between the colloid-rich gel branches and the colloid-free gel pores. These nanoparticle gels open new routes for creating structural colour where the gel structure is decoupled from the form factor of the individual colloids. This approach can be extended to create unexplored three dimensional macroscale materials with length scales spanning hundreds of nanometers, which has been difficult to achieve using other methods.

Microstructure and Elastic Properties of Colloidal Gel Foams.

Colloidal gel foams are composed of a continuous, attractive particle network that surrounds and interconnects dispersed bubbles. Here, we investigate their stability, morphology, and elasticity as a function of foaming intensity, surfactant concentration and hydrophobicity, pH, and colloid volume fraction. Upon optimizing these parameters, highly stable colloidal gel foams are created. Within this stability region, the specific interfacial area between the continuous (colloidal gel) and dispersed (bubble) phase can be varied over 2 orders of magnitude leading to a concomitant increase in storage modulus, which scales nearly linearly with specific interfacial area. Our observations provide design guidelines for attractive-particle stabilized foams that enable the programmable assembly of architected porous materials.

Dynamic assembly of ultrasoft colloidal networks enables cell invasion within restrictive fibrillar polymers

In regenerative medicine, natural protein-based polymers offer enhanced endogenous bioactivity and potential for seamless integration with tissue, yet form weak hydrogels that lack the physical robustness required for surgical manipulation, making them difficult to apply in practice. The use of higher concentrations of protein, exogenous cross-linkers, and blending synthetic polymers has all been applied to form more mechanically robust networks. Each relies on generating a smaller network mesh size, which increases the elastic modulus and robustness, but critically inhibits cell spreading and migration, hampering tissue regeneration. Here we report two unique observations; first, that colloidal suspensions, at sufficiently high volume fraction (ϕ), dynamically assemble into a fully percolated 3D network within high-concentration protein polymers. Second, cells appear capable of leveraging these unique domains for highly efficient cell migration throughout the composite construct. In contrast to porogens, the particles in our system remain embedded within the bulk polymer, creating a network of particle-filled tunnels. Whereas this would normally physically restrict cell motility, when the particulate network is created using ultralow cross-linked microgels, the colloidal suspension displays viscous behavior on the same timescale as cell spreading and migration and thus enables efficient cell infiltration of the construct through the colloidal-filled tunnels.

Shear viscosity in hard-sphere and adhesive colloidal suspensions with reverse non-equilibrium molecular dynamics.

We employ the reverse non-equilibrium molecular dynamics method (RNEMD) of Müller-Plathe [Phys. Rev. E, 1999, 59, 4894] to calculate the shear viscosity of colloidal suspensions within the stochastic rotation dynamics-molecular dynamics (SRD-MD) simulation method. We examine the influence of different coupling schemes in SRD-MD on the colloidal volume fraction ϕc dependent viscosity from the dilute limit up to ϕc = 0.3. Our results demonstrate that the RNEMD method is a robust and reliable method for calculating rheological properties of colloidal suspensions. To obtain quantitatively accurate results beyond the dilute regime, the hydrodynamic interactions between the effective fluid particles in the SRD and the MD colloidal particles must be carefully considered in the coupling scheme. We benchmark the method by comparing with the hard sphere suspension case, and then calculate relative viscosities for colloids with mutually attractive interactions. We show that the viscosity displays a sharp increase at the onset of aggregation of the colloidal particles with increasing volume fraction and attraction.

Optically transparent dense colloidal gels.

Traditionally it has been difficult to study the porous structure of dense colloidal gels and (macro) molecular transport through them simply because of the difference in refractive index between the colloid material and the continuous fluid phase surrounding it, rendering the samples opaque even at low colloidal volume fractions. Here, we demonstrate a novel colloidal gel that can be refractive index-matched in aqueous solutions owing to the low refractive index of fluorinated latex (FL)-particles (n = 1.37). Synthesizing them from heptafluorobutyl methacrylate using emulsion polymerization, we demonstrate that they can be functionalized with short DNA sequences via a dense brush-layer of polystyrene-b-poly(ethylene oxide) block-copolymers (PS-PEO). The block-copolymer, holding an azide group at the free PEO end, was grafted to the latex particle utilizing a swelling-deswelling method. Subsequently, DNA was covalently attached to the azide-end of the block copolymer via a strain-promoted alkyne-azide click reaction. For comparison, we present a structural study of single gels made of FL-particles only and composite gels made of a percolating FL-colloid gel coated with polystyrene (PS) colloids. Further we demonstrate that the diffusivity of tracer colloids dispersed deep inside a refractive index matched FL-colloidal gel can be measured as function of the local confinement using Dynamic Differential Microscopy (DDM).

High-Efficiency Crystalline-Si Solar Cells with Screen-Printed Front-Side Metallization: A Percolation Model to Explain the Current Path

Resistive losses corresponding to the front-side metallization limit the efficiency of Si solar cells. At the front-side contact, the Si emitter is covered by a glass layer that is less than 1 μm thick embedded with Ag colloids to volume fraction >20%. Bulk Ag fingers are arranged on top of the glass layer. A similar microstructure is found for both n-type and p-type cells showing high efficiency. The Ag colloids constitute current filaments with reduced resistance in the glass layer, thereby introducing a percolative current which is the basis of the proposed model. This model is new and differs from the classical percolation model in its direct reliance on the macroscopic resistance of these filaments, and in considering the matrix as semiconducting rather than insulating. For periodically arranged Ag colloids of fixed diameter, the percolative limit of 13% in two dimensions (2D) and 15% in three dimensions (3D) depends only on the volume fraction of colloids but not their size. The resistance of randomly arranged and sized Ag colloids confirms the analytical results. The model explains quantitatively, consistent with microstructural analyses, why low contact resistances are found in solar cells with high colloid density. The introduced percolation model is also relevant for other systems in which metallic precipitates are found in a semiconducting matrix.

Continuous Paranematic Ordering of Rigid and Semiflexible Amyloid-Fe3O4 Hybrid Fibrils in an External Magnetic Field.

External magnetic field is a powerful approach to induce orientational order in originally disordered suspensions of magneto-responsive anisotropic particles. By small angle neutron scattering and optical birefringence measurement technology, we investigated the effect of magnetic field on the spatial ordering of hybrid amyloid fibrils with different aspect ratios (length-to-diameter) and flexibilities decorated by spherical Fe3O4 nanoparticles. A continuous paranematic ordering from an initially isotropic suspension was observed upon increasing magnetic field strength, with spatial orientation increasing with colloidal volume fraction. At constant dimensionless concentration, stiff hybrid fibrils with varying aspect ratios and volume fractions, fall on the same master curve, with equivalent degrees of ordering at identical magnetic fields. However, the semiflexible hybrid fibrils with contour length close to persistence length exhibit a lower degree of alignment. This is consistent with Khokhlov-Semenov theoretical predictions. These findings sharpen the experimental toolbox to design colloidal systems with controllable degree of orientational ordering.

Back Cover: Advanced Engineering Materials 1∕2016

Advanced Engineering MaterialsVolume 18, Issue 1 p. 170-170 Back CoverFree Access Back Cover: Advanced Engineering Materials 1∕2016 First published: 04 January 2016 https://doi.org/10.1002/adem.201670004AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract 3D silicon carbide structures are patterned via direct ink writing (DIW) of borosiloxane-colloidal SiC ink blends. These hybrid inks enable printing over a wide range of colloidal volume fractions (0.38 <Φ< 0.54) to produce relative densities of ≈30–50%. Structural (bars, cylinders, turbines) and functional (periodic lattices, microwave feedhorns) components are printed and sintered with excellent shape stability. The SiC feedhorns transmit microwaves (8.5 GHz) with very low insertion losses (≈0.05 dB) relative to commercial standards, demonstrating their potential in microwave optics applications. More details can be found in the article by R. F. Shepherd and co-workers on page 39. Volume18, Issue1January 2016Pages 170-170 RelatedInformation

Pair mobility functions for rigid spheres in concentrated colloidal dispersions: Force, torque, translation, and rotation.

The formulation of detailed models for the dynamics of condensed soft matter including colloidal suspensions and other complex fluids requires accurate description of the physical forces between microstructural constituents. In dilute suspensions, pair-level interactions are sufficient to capture hydrodynamic, interparticle, and thermodynamic forces. In dense suspensions, many-body interactions must be considered. Prior analytical approaches to capturing such interactions such as mean-field approaches replace detailed interactions with averaged approximations. However, long-range coupling and effects of concentration on local structure, which may play an important role in, e.g., phase transitions, are smeared out in such approaches. An alternative to such approximations is the detailed modeling of hydrodynamic interactions utilizing precise couplings between moments of the hydrodynamic traction on a suspended particle and the motion of that or other suspended particles. For two isolated spheres, a set of these functions was calculated by Jeffrey and Onishi [J. Fluid Mech. 139, 261-290 (1984)] and Jeffrey [J. Phys. Fluids 4, 16-29 (1992)]. Along with pioneering work by Batchelor, these are the touchstone for low-Reynolds-number hydrodynamic interactions and have been applied directly in the solution of many important problems related to the dynamics of dilute colloidal dispersions [G. K. Batchelor and J. T. Green, J. Fluid Mech. 56, 375-400 (1972) and G. K. Batchelor, J. Fluid Mech. 74, 1-29 (1976)]. Toward extension of these functions to concentrated systems, here we present a new stochastic sampling technique to rapidly calculate an analogous set of mobility functions describing the hydrodynamic interactions between two hard spheres immersed in a suspension of arbitrary concentration, utilizing accelerated Stokesian dynamics simulations. These mobility functions provide precise, radially dependent couplings of hydrodynamic force and torque to particle translation and rotation, for arbitrary colloid volume fraction ϕ. The pair mobilities (describing entrainment of one particle by the disturbance flow created by another) decay slowly with separation distance: as 1/r, for volume fractions 0.05 ≤ ϕ ≤ 0.5. For the relative mobility, we find an initially rapid growth as a pair separates, followed by a slow, 1/r growth. Up to ϕ ≤ 0.4, the relative mobility does not reached the far-field value even beyond separations of many particle sizes. In the case of ϕ = 0.5, the far-field asymptote is reached but only at a separation of eight radii and after a slow 1/r growth. At these higher concentrations, the coefficients also reveal liquid-like structural effects on pair mobility at close separations. These results confirm that long-range many-body hydrodynamic interactions are an essential part of the dynamics of concentrated systems and that care must be taken when applying renormalization schemes.

Low‐Density Crystals in Charged Colloids: Comparison with Yukawa Theory

AbstractCharged colloids can behave as Yukawa systems, with similar phase behaviour. Using particle‐resolved studies, we consider a system with an unusually long Debye screening length which forms crystals at low colloid volume fraction φ ≈ 0.01. We quantitatively compare this system with the Yukawa model and find that its freezing point is compatible with the theoretical prediction but that the crystal polymorph is not always that expected. In particular we find body‐centred cubic crystals where face‐centred cubic crystals are expected. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A multiblob approach to colloidal hydrodynamics with inherent lubrication.

This work presents an intermediate resolution model of the hydrodynamics of colloidal particles based on a mixed Eulerian-Lagrangian formulation. The particle is constructed with a small set of overlapping Peskin's Immersed Boundary kernels (blobs) which are held together by springs to build up a particle impenetrable core. Here, we used 12 blobs placed in the vertexes of an icosahedron with an extra one in its center. Although the particle surface is not explicitly resolved, we show that the short-distance hydrodynamic responses (flow profiles, translational and rotational mobilities) agree with spherical colloids and provide consistent effective radii. A remarkable property of the present multiblob model is that it naturally provides zero relative mobility at some finite inter-particle distance. In terms of mutual friction, this divergent force accurately represents the "soft" lubrication regime of spherical colloids and permits to resolve the increase of the solution viscosity up to moderately dense systems with volume fraction up to about 0.50. This intermediate resolution model is able to recover highly non-trivial (many-body) hydrodynamics using small particles whose radii are similar to the grid size h (in the range [1.6 - 3.2] h). Considering that the cost of the embedding fluid phase scales such as the cube of the particle radius, this result brings about a significant computational speed-up. Our code Fluam works in Graphics Processor Units and uses Fast Fourier Transform for the Poisson solver, which further improves its efficiency.

Dynamic electrophoresis of charged colloids in an oscillating electric field.

The dynamics of charged colloids in an electrolyte solution is studied using direct numerical simulations via the smoothed profile method. We calculated the complex electrophoretic mobility μ(ω) of the charged colloids under an oscillating electric field of frequency ω. We show the existence of three dynamically distinct regimes, determined by the momentum diffusion and ionic diffusion time scales. The present results agree well with approximate theories based on the cell model in dilute suspensions; however, systematic deviations between the simulation results and theoretical predictions are observed as the volume fraction of colloids is increased, similar to the case of constant electric fields.

Unfolding of collapsed polymers in shear flow: effects of colloid banding structures in confining channels.

Using hydrodynamic simulations, we demonstrate that confined colloidal suspensions can greatly enhance the unfolding of collapsed single polymers in flow. When colloids come in direct contact with the polymers due to the flow, the collapsed chains become flattened or elongated on the surface of the colloids, increasing the probability of forming large chain protrusions that the flow can pull out to unfold the polymers. This phenomenon may be suppressed if the colloid size is commensurate with the confining channels, where the colloids form well-defined banding structures. Here, we analyze the colloid banding structures in detail and their relation to the chain unfolding. We find that for colloid volume fractions up to 30%, the confined colloids form simple cubic (sc), hexagonal (hex), or a mixture of sc + hex structures. By directly changing the heights of the confining channels, we show that the collapsed polymers unfold the most in the mixed sc + hex structures. The diffuse (not well-defined) bands in the mixed sc + hex structures provide the highest collision probability for the colloids and the polymers, thus enhancing unfolding the most. Without colloidal suspensions, we show that the confining channels alone do not have an observable effect on the unfolding of collapsed polymers. The well-defined colloid bands also suppress the unfolding of noncollapsed polymers. In fact, the average size for noncollapsed chains is even smaller in the well-defined bands than in a channel without any colloids. The appearance of well-defined bands in this case also indicates that lift forces experienced by the polymers in confinement are negligible compared to those exerted by the colloidal band structures. Our results may be important for understanding the dynamics of mixed colloid polymer solutions.

Dual aging behaviour in a clay-polymer dispersion.

Clay-polymer compounds have recently attracted increasing attention due to their intriguing physical properties in colloidal science and their rheological non-trivial behaviour in technological applications. Aqueous solutions of Laponite clay spontaneously age from a liquid up to an arrested state of different nature (gel or glass) depending on the colloidal volume fraction and ionic strength. We have investigated, through dynamic light scattering, how the aging dynamics of Laponite dispersions at fixed clay concentration (Cw = 2.0%) is modified by the addition of various amounts of poly(ethylene oxide) (PEO) (CPEO = (0.05 ÷ 0.50) %) at two different molecular weights (Mw = 100 kg mol(-1) and Mw = 200 kg mol(-1)). A surprising and intriguing phenomenon has been observed: the existence of a critical polymer concentration C that discriminates between two different aging dynamics. With respect to pure Laponite systems the aging will be assisted (faster) or hindered (slower) for PEO concentrations respectively lower (CPEO < C) or higher (CPEO > C) than the critical concentration. In this way a control on the aging dynamics of PEO-Laponite systems is obtained. A possible explanation based on the balance of competitive mechanisms related to the progressive saturation of the clay surface by polymers is proposed. This study shows how a real control on the aging speed of the PEO-Laponite system is at hand and renders possible a real control of the complex interparticle interaction potential.

Inter-particle correlations in a hard-sphere colloidal suspension with polymer additives investigated by Spin Echo Small Angle Neutron Scattering (SESANS)

Using a neutron scattering technique that measures a statistically-averaged density correlation function in real space rather than the conventional reciprocal-space structure factor, we have measured correlations between poly(methyl-methacrylate) (PMMA) colloidal particles of several sizes suspended in decalin. The new method, called Spin Echo Small Angle Neutron Scattering (SESANS) provides accurate information about particle composition, including the degree of solvent penetration into the polymer brush grafted on to the PMMA spheres to prevent aggregation. It confirms for particles, between 85 nm and 150 nm in radius that inter-particle correlations closely follow the Percus-Yevick hard-sphere model when the colloidal volume-fraction is between 30% and 50% provided the volume-fraction is used as a fitted parameter. No particle aggregation occurs in these systems. When small amounts of polystyrene are added as a depletant to a concentrated suspension of PMMA particles, short-range clustering of the particles occurs and there is an increase in the frequency of near-neighbor contacts. Within a small range of depletant concentration, near-neighbor correlations saturate and large aggregates with power law density correlations are formed. SESANS clearly separates the short- and long-range correlations and shows that, in this case, the power-law correlations are visible for inter-particle distances larger than roughly two particle diameters. In some cases, aggregate sizes are within our measurement window, which can extend out to 16 microns in favorable cases. We discuss the advantages of SESANS for measurements of the structure of concentrated colloidal systems and conclude that the method offers several important advantages.

Counterion condensation on spheres in the salt-free limit

A highly-charged spherical colloid in a salt-free environment exerts such a powerful attraction on its counterions that a certain fraction condenses onto the surface of a particle. The degree of condensation depends on the curvature of the surface. So, for instance, condensation is triggered on a highly-charged sphere only if the radius exceeds a certain critical radius R*. R* is expected to be a simple function of the volume fraction of particles. To test these predictions, we prepare spherical particles which contain a covalently-bound ionic liquid, which is engineered to dissociate efficiently in a low-dielectric medium. By varying the proportion of ionic liquid to monomer we synthesise nonpolar dispersions of highly-charged spheres which contain essentially no free co-ions. The only ions in the system are counterions generated by the dissociation of surface-bound groups. We study the electrophoretic mobility of this salt-free system as a function of the colloid volume fraction, the particle radius, and the bare charge density and find evidence for extensive counterion condensation. At low electric fields, we observe excellent agreement with Poisson-Boltzmann predictions for counterion condensation on spheres. At high electric fields however, where ion advection is dominant, the electrophoretic mobility is enhanced significantly which we attribute to hydrodynamic stripping of the condensed layer of counterions from the surface of the particle.

Bridging and caging in mixed suspensions of microsphere and adsorptive microgel.

Gelation and glass transition in a mixed suspension of polystyrene (PS) microsphere and poly(N-isopropylacrylamide) (PNIPAM) microgel were studied as a function of the total colloid volume fraction and mixing ratio of these two components. The PNIPAM microgel, which is adsorbable on the PS microsphere surface, can induce bridging or stabilizing effect between microspheres depending on whether the volume fraction of microgel (ΦMG) is smaller or larger than the saturated adsorption concentration (Φ*MG) for a given volume fraction of the microsphere (ΦMS). Φ*MG is in a linear relationship with ΦMS, and the value of ΦMG/Φ*MG can be taken as an approximate measure of surface coverage. A state diagram of gelation and glass transition is constructed with the short-ranged attractive interaction, resulting from the well-defined bridging bonding. Keeping ΦMG/Φ*MG = 0.20 and increasing ΦMS from 0.25 to 0.55, the mixed suspension transforms from a bridging gel into an attractive glass; moreover, while keeping ΦMS = 0.45 and increasing ΦMG/Φ*MG from 0.20 to 1.2, the mixed suspension changes from a bridging gel into an attractive glass, and then to a repulsive glass. The bridging effect and the cage effect can be distinguished by the yielding behaviors in rheological measurements. In the nonlinear dynamic rheological experiments, one-step yielding, corresponding to the disconnecting of bridge network, is observed in the bridging gel, and one-step yielding, corresponding to the breaking of cage, is observed in the repulsive glass. However, a two-step yielding behavior is found in the bridging-induced attractive glass, which is attributed to the bridging effect of microgels and the caging effect of the dense environment.

Aggregation in Colloidal Suspensions: Evaluation of the Role of Hydrodynamic Interactions by Means of Numerical Simulations

Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne-Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

Sterically Stabilized Colloids with Tunable Repulsions

When studying tunable electrostatic repulsions in aqueous suspensions of charged colloids, irreversible colloid aggregation or gelation may occur at high salt concentrations. For many commonly used synthetic colloids, such as polystyrene and silica particles, the reason for coagulation is the presence of unbalanced, strongly attractive, and short-ranged van der Waals (VDW) forces. Here, we present an aqueous polystyrene model colloid that is sterically stabilized against VDW attractions. We show that the synthesis procedure, based on a neutral initiator couple and a nonionic surfactant, introduces surface charges that can be further increased by the addition of charged comonomer methacrylic acid. Thus, the interactions between the polystyrene spheres can be conveniently tuned from hard-sphere-like to charge-stabilized with long-ranged electrostatic repulsions described by a Yukawa-type pair potential. The particle size, grafting density, core-shell structure, and surface charge are characterized by light and neutron scattering. Using X-ray and neutron scattering in combination with an accurate analytic integral equation scheme for the colloidal static structure factor, we deduce effective particle charges for colloid volume fractions ≥0.1 and salt concentrations in the range of 1.5 to 50 mM.