Dispersions Of Colloidal Particles Research Articles

X-ray photon correlation spectroscopy during homogenous shear flow

We report x-ray photon correlation spectroscopy measurements of advective and diffusive dynamics in a dispersion of colloidal particles subjected to homogeneous shear flow in a rotating-disk shear cell. Intensity autocorrelation functions from scattering data collected using homodyne detection respond to the variation in velocity across the scattering volume when the scattering vector has a component parallel to the flow direction. Theoretical expressions for the impact of homogenous shear flow on the correlation function provide a quantitative prediction of the dependence of correlation functions on the scattering vector and shear rate. Under most circumstances, the applied shear deformation dominates the decay of the intensity correlation function. When scattering data are collected perpendicular to the flow direction, it is possible to measure the diffusive dynamics of the particles free from effects of the superimposed shear flow; however, this approach only works below some upper shear rate limit, beyond which data are affected either by shear effects (caused by the finite width of the detector) or by particle transit through the scattering volume.

Microrheology of biomaterial hydrogelators

Microrheology uses the motion of dispersed colloidal probe particles to measure the viscosity or viscoelastic moduli of soft materials. The distinct advantages of microrheology include small sample volume requirements, access to a large range of time scales for the dynamic response and short acquisition times. These advantages make microrheology important for studies of biomaterial hydrogelators. Recent advances have enabled the precise characterization of hydrogelator sol–gel transitions, measurements of rare and scarce materials and high-throughput screening of hydrogel rheology over a large composition space. In this review, we focus on multiple particle tracking microrheology, including the considerations that define its operating regimes and its recent applications. Those interested in biomaterial rheology will find these methods as accessible as bulk rheological measurements and straightforward to implement in their own work.

Bespoke periodic topography in hard polymer films by infrared radiation-assisted evaporative lithography

Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, are created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. We show that the height of the surface features increases with an increase in several key parameters: the initial thickness of the film, the volume fraction of particles, and the pitch of the pattern. The results are interpreted by using models of geometry and particle transport. The patterned coatings can function as “paintable” microlens arrays, applicable to nearly any surface. Compared with existing methods for creating textured coatings, IRAEL is simpler, inexpensive, able to create a wide variety of bespoke surfaces, and applicable to nearly any substrate without prior preparation.

From craquelures to spiral crack patterns: influence of layer thickness on the crack patterns induced by desiccation

As a film of dispersed colloidal particles consolidates by desiccation on a rigid substrate, enormous stresses develop. Most of the time, these stresses exceed the strength of the material causing crack formation. Although undesirable in most industrial cases, crack patterns exhibit a useful method to characterize paintings. Indeed, crack patterns are the signature of the particles, solvent and substrate type, the drying conditions and the film thickness. Here we focus on the influence of this last parameter on the final crack morphologies. In particular, using a colloidal dispersion with transparency properties, we observe that craquelures, delamination and spiral crack morphologies can be obtained by changing only the film thickness. An extensive description of the formation dynamics and final geometry of each pattern is presented, highlighting the key geometric parameters that may easily be used by a broad audience, including the mechanical engineering, physics, chemistry, geology and art communities.

Lanthanide Phytanates: Liquid-Crystalline Phase Behavior, Colloidal Particle Dispersions, and Potential as Medical Imaging Agents

Lanthanide salts of phytanic acid, an isoprenoid-type amphiphile, have been synthesized and characterized. Elemental analysis and FTIR spectroscopy were used to confirm the formed product and showed that three phytanate anions are complexed with one lanthanide cation. The physicochemical properties of the lanthanide phytanates were investigated using DSC, XRD, SAXS, and cross-polarized optical microscopy. Several of the hydrated salts form a liquid-crystalline hexagonal columnar mesophase at room temperature, and samarium(III) phytanate forms this phase even in the absence of water. Select lanthanide phytanates were dispersed in water, and cryo-TEM images indicate that some structure has been retained in the dispersed phase. NMR relaxivity measurements were conducted on these systems. It has been shown that a particulate dispersion of gadolinium(III) phytanate displays proton relaxivity values comparable to those of a commercial contrast agent for magnetic resonance imaging and a colloidal dispersion of europium(III) phytanate exhibits the characteristics of a fluorescence imaging agent.

Simulating the formation of colloidal sulfur in gas-phase condensation

A method is proposed and realized for simulating the formation of colloidal sulfur during condensation from the gas phase. Calculations have been performed on the optimal parameters of the condensation, which increase the proportion of extracted sulfur from the gas phase and produce dispersed colloidal particles of size 0.5-5 μm, which produces dense structures of sulfur clusters.

Use of wide-angle X-ray diffraction to measure shape and size of dispersed colloidal particles

Laboratory X-ray diffraction is used to investigate the size and shape of dispersed plate-like and spherical colloidal particles. Analysis of the wide-angle diffraction data provides information about the size and shape of crystals from the width of the Bragg peaks according to the Debye–Scherrer formula. The measurements, data analysis, and evaluation are discussed. It is shown that X-ray diffraction with conventional laboratory equipment on dispersed particles is feasible as a tool to determine both particle size and shape. Data for two samples – gold colloids and nickel (II) hydroxide particles are presented. The advantages and limitations of the method are discussed. X-ray diffraction measurements that are made in combination with dynamic light scattering can be used to estimate the thickness of stabilizing layers of polymers.

Formation of lamellar structures from spherical particles

We report disorder to lamellar transition in a system of spherically symmetric particles where the interparticle potential consists of a short-ranged attraction and a longer-ranged repulsion. The system provides a simplified model for aqueous dispersions of colloidal particles and globular proteins that may exhibit stable/metastable clusters or microscopic phases. By using a non-mean-field density functional theory, we predict that under appropriate conditions, a lamellar phase with alternating condensed and dilute layers of particles is thermodynamically more stable than a uniform disordered phase at the same temperature and molecular number density. Formation of the lamellar structure may prohibit the macroscopic fluid-fluid phase transition. At a given condition, the width of the condensed lamellar layers increases with the overall particle density but the trend is opposite for the dilute lamellar layers. A minimal lamellar periodicity is obtained when the condensed and dilute layers have approximately the same thickness.

Functional colloidal particles stabilized by layered silicate with hydrophilic face and hydrophobic polymer brushes

AbstractIn this study, we describe a new strategy for producing narrowly dispersed functional colloidal particles stabilized by a nanocomposite with hydrophilic clay faces and hydrophobic polystyrene (PS) brushes on the edges. This method involves preparation of polymer brushes on the edges of clay layers and Pickering suspension polymerization of styrene in the presence of the nanocomposites. PS brushes on the edges of clay layers were prepared by atom transfer radical polymerization. X‐ray diffraction and thermogravimetric analysis results indicated that PS chains were grafted to the edges of clay platelets. Transmission electron microscope results showed that different morphologies of clay‐PS particles could be obtained in different solvents. In water, clay‐PS particles aggregated together, in which PS chains collapsed forming nanosized hydrophobic domains and hydrophilic clay faces stayed in aqueous phase. In toluene, clay‐PS particles formed face‐to‐face structure. Narrowly dispersed PS colloidal particles stabilized by clay‐PS were prepared by suspension polymerization. Because of the negatively charged clay particles on the surface, the zeta potential of the PS colloidal particles was negative. Positively charged poly(2‐vinyl pyridine) (P2VP) chains were adsorbed to the surface of PS colloidal particles in aqueous solution at a low pH value, and gold nanoparticles were prepared in P2VP brushes. Such colloidal particles may find important applications in a variety of fields including waterborne adhesives, paints, catalysis of chemical reactions, and protein separation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1535–1543, 2009

Like-charge interactions between colloidal particles are asymmetric with respect to sign.

Two-dimensional dispersions of colloidal particles with a range of surface chemistries and electrostatic potentials are characterized under a series of solution ionic strengths. A combination of optical imaging techniques are employed to monitor both the colloid structure and the electrostatic surface potential of individual particles in situ. We find that like-charge multiparticle interactions can be tuned from exclusively repulsive to long-range attractive by changing the particle surface composition. This behavior is strongly asymmetric with respect to the sign of the surface potential. Collective long-range attractive interactions are only observed among negatively charged particles.

Polymer-Dispersed Bicontinuous Cubic Glycolipid Nanoparticles

We found that certain amphiphilic polymers such as PEO-PPO-PEO triblock copolymer (PL) can directly disperse a cubic glycolipid, 1-O-phytanyl-beta-D-xyloside (beta-XP), into bicontinuous cubic nanoparticles in water medium. The use of synchrotron small-angle X-ray diffraction (SSAXD) permitted the identification of the exact structure of these dispersed particles in the colloidal state. Dynamic light scattering method was used to obtain particle size distributions. The dispersion quality and the dispersion time can be improved by co-dissolving the lipid and the polymer in a common solvent. The mean volume diameter of these dispersed colloidal particles depends on the mixing time and polymer concentration. About 5 wt % (0.18 mol %) of polymer to lipid weight was found to be sufficient to produce stable colloidal dispersions. At this polymer content and at 3 h of stirring time, the mean volume diameter of cubic colloidal particles was found to be 1.0 microm. Increase of dispersion time to 6 h reduced the colloidal particle size from 1.0 microm to 660 nm. At 3 h of mixing time, the increase of polymer content, from approximately 5 to approximately 10 wt %, reduced the particle mean diameter from 1.0 microm to 675 nm. Irrespective of these dispersion times and polymer contents, the dispersed colloidal particles exhibit predominately the Pn3m cubic phase structure, the same as that of a beta-XP-water binary mixture, although a weak coexistence of Im3m cubic phase is identified in these colloidal particles. This coexistence is found to have the characteristics of a Bonnet relation, which forms convincing evidence for the infinite periodic minimal surface descriptions (IPMS). Considering the biotechnological significance, the preparation of these colloidal dispersions was carried out in a phosphate-buffered saline (PBS) system. These cubic colloidal dispersions exhibited good stability and the cubic phase structure remained intact in the PBS system.

Self-assembly of nematic colloids.

Dispersions of colloidal particles in nematic liquid crystals show new classes of interparticle forces, which are anisotropic, long range, and several thousand times stronger than van der Waals forces in water-based colloids. These forces are responsible for a variety of new self-assembled colloidal microstructures, which cannot be observed in isotropic solvents, such as chains of microspheres and cellular soft solid materials. Basic principles of particle self-assembly in 2D nematic colloids are discussed, showing this is a novel paradigm in colloid science, which could lead to new approaches of colloidal self-assembly for photonic devices.

Gold-Labeled Block Copolymer Micelles Reveal Gold Aggregates at Multiple Subcellular Sites

There is increasing interest in the usefulness of block copolymer micelles as drug delivery vehicles. However, their subcellular distribution has not been explored extensively, mostly because of the lack of adequately labeled block copolymers. In a previous study, we showed that fluorescently labeled block copolymer micelles entered living cells and co-localized with cytoplasmic organelles selectively labeled with fluorescent dyes. The details of the observed co-localizations were, however, limited by the resolution of the fluorescence approach, which is ca. 500 nm. Using transmission electron microscopy (TEM), we established time- and concentration-dependent subcellular distributions of gold-labeled micelles within human embryonic kidney (HEK 293) cells and human lung carcinoma (A549) cells. Gold particles were incorporated into poly(4-vinylpyridine)-block-poly(ethylene oxide) (P4VP21-b-PEO45) micelles. Data from dynamic light scattering (DLS) and TEM analyses revealed that the sizes of the gold particles ranged from 4 to 8 nm. The cells survived up to 24 h in the presence of low gold-labeled micelle concentrations (0.73 microg/mL), but cell death occurred at higher concentrations (i.e., kidney cells are more susceptible than lung cells). Over 24 h periods of equivalent exposure, lung cells internalized significantly more gold-incorporated micelles than kidney cells. Although micelles were added to the cell culture media as dispersed colloidal particles, the presence of serum in these media caused aggregation. These aggregates occurred mainly close to the cell plasma membrane at early times (5-10 min); however, at later times (24 h) aggregated particles were seen inside endosomes and lysozomes. Thus, gold-incorporated (labeled) micelles can serve as a valuable extension of the fluorescence approach to visualizing the localization of micelles in subcellular compartments, improving the resolution by at least 20-fold.

Optical measurements of the thermal properties of nanofluids

The authors show that the thermal conductivity and diffusivity of colloidal particle dispersions can be rapidly obtained with high accuracy and reproducibility by exploiting a noninvasive, all-optical thermal lensing method. Applications of this technique to model suspensions of spherical monodisperse particles suggest that classical models for the effective properties of composite media hold up to rather high volume fractions, while no “anomalous” thermal conductivity effects are found.

Colloidal electroconvection in a thin horizontal cell. II. Bulk electroconvection of water during parallel-plate electrolysis

We recently have reported [J. Chem. Phys. 122, 164701 (2005)] a family of electroconvective patterns that arise when charge-stabilized colloidal dispersions are driven by constant (dc) vertical electric fields. Competition between gravity and electrokinetic forces acting on the individual spheres in this system leads to the formation of highly organized convective instabilities involving hundreds of spheres. Here, we report a distinct class of electroconvective patterns that emerge in confined aqueous dispersions at higher biases. These qualitatively resemble the honeycomb and labyrinthine patterns formed during thermally driven Rayleigh-Benard convection, but arise from a distinct mechanism. Unlike the localized colloidal electroconvective patterns observed at lower biases, moreover, these system-spanning patterns form even without dispersed colloidal particles. Rather, they appear to result from an underlying electroconvective instability during electrolysis in the parallel plate geometry. This contrasts with recent theoretical results suggesting that simple electrolytes are linearly stable against electroconvection.

Marangoni flows during drying of colloidal films

In this study, we consider the drying of a thin film that contains a stable dispersion of colloidal particles so that a coating of these particles is formed after the liquid is driven off by evaporation. For sufficiently thin films, we show that evaporative cooling can drive a Marangoni flow that results in surface deformation of the drying film. A thin-film approximation is used to describe the velocity and temperature fields, and the particle transport equation with convective terms retained is used to describe the concentration field. A coupled finite difference/spectral element scheme is implemented numerically to solve the particle transport equation, where high accuracy is required to resolve sharp gradients within the film and to ensure particle conservation during drying. The model employed is capable of describing the evolution of film thickness and concentration field up to the time when maximum packing is nearly reached at some point in the domain. Three types of film structures are observed, all characterized by a final nonuniform thickness. In the first type, observed at low Peclet numbers, the maximum concentration is reached at the thinnest points in the film, which surround elevations with lower particle concentrations. This mode of instability suggests that dried coatings will have pronounced nonuniformities, resulting in the formation of craters or pinholes. In the second type, observed at high Peclet numbers, a closely packed skin of nonuniform thickness is formed, with low concentration fluid remaining beneath the elevations. In the final stages of drying one would expect capillary pressure to pull particles in the underlying fluid toward the skin, thus creating voids under a seemingly homogeneously applied coating. Finally, still at relatively large particle Peclet numbers and when the destabilizing Marangoni stresses are sufficiently strong, floating lumps of closely packed particles may form in the vicinity of film elevations.

Synthesis and acid functionalization of cerium oxide nanoparticles

Colloidal cerium oxide has been obtained by controlled oxidation of soluble Ce(III) salts in hydrothermal conditions. The homogeneous nucleation of CeO2 through thermolysis of this oxidizing solution allows the formation of well dispersed colloidal particles. Under optimal conditions, well crystallized particles with an average size of 8 nm are obtained. The surface is terminated by acetate groups which can be substituted by grafting phosphonic acids or phosphoric acids. Particularly, the grafting of 2-carboxyethylphosphonic acid or phosphonoacetic acid increases the acidic character of the surface as observed by impedance spectroscopy.

Single particle motion in colloidal dispersions: a simple model for active and nonlinear microrheology

The motion of a single Brownian probe particle subjected to a constant external body force and immersed in a dispersion of colloidal particles is studied with a view to providing a simple model for particle tracking microrheology experiments in the active and nonlinear regime. The non-equilibrium configuration of particles induced by the motion of the probe is calculated to first order in the volume fraction of colloidal particles over the entire range of Pe, accounting for hydrodynamic and excluded volume interactions between the probe and dispersion particles. Here, Pe is the dimensionless external force on the probe, or Péclet number, and is a characteristic measure of the degree to which the equilibrium microstructure of the dispersion is distorted. For small Pe, the microstructure (in a reference frame moving with the probe) is primarily dictated by Brownian diffusion and is approximately fore–aft symmetric about the direction of the external force. In the large Pe limit, advection is dominant except in a thin boundary layer in the compressive region of the flow where it is balanced by Brownian diffusion, leading to a highly non-equilibrium microstructure. The computed microstructure is employed to calculate the average translational velocity of the probe, from which the ‘microviscosity’ of the dispersion may be inferred via application of Stokes drag law. For small departures from equilibrium (Pe), the microviscosity ‘force-thins’ proportional to , the microstructural boundary layer coincides with the lubrication range of hydrodynamic interactions causing the microviscosity to enter a continuous ‘force-thickening’ regime. The qualitative picture of the microviscosity variation with Pe is in good agreement with theoretical and computational investigations on the ‘macroviscosity’ of sheared colloidal dispersions, and, after appropriate scaling, we are able to make a direct quantitative comparison. This suggests that active tracking microrheology is a valuable tool with which to explore the rich nonlinear rheology of complex fluids.

Stable Dispersions of Aromatic Hydrocarbon Colloidal Particles by Laser Ablation

Dispersions of colloidal particles of polycyclic aromatic hydrocarbons were obtained by nanosecond laser ablation. Aqueous suspensions of micron-sized crystals were irradiated with the second harmonic output of a Nd:YAG laser and converted into colloidal particles. Ionic and nonionic surfactants were used in the ablation process to stabilise the particles. Dispersions with good long term stability were obtained. A strong dependence of particle formation rate on laser fluence was found and the particle formation process is discussed on the basis of UV-Vis spectra and microscopic examination of the crystals.

Ellipsometric light scattering for the characterization of thin layers on dispersed colloidal particles

The thin interface layer on colloids in a liquid can be characterized by applying nulling ellipsometry to a scattering experiment. Close to 90 degrees scattering angle the ellipsometric quantities tan(Psi) and Delta of particles smaller than the wavelength behave like at the Brewster angle of reflection ellipsometry. The minimum of tan(Psi) and the slope of Delta depend on thickness and refractive index of the layer, while the location of the minimum is sensitive to the refractive index of the particle. As an example, the swelling of a thermosensitive layer on colloidal particles has been determined from measurements at two wavelengths.