Binary Colloidal Suspensions Research Articles

Ultrafast Quenching of Binary Colloidal Suspensions in an External Magnetic Field

An ultrafast quench is applied to binary mixtures of superparamagnetic colloidal particles confined at a two-dimensional water-air interface by a sudden increase of an external magnetic field. This quench realizes a virtually instantaneous cooling which is impossible in molecular systems. Using real-space experiments, the relaxation behavior after the quench is explored. Local crystallites with triangular and square symmetry are formed on different time scales, and the correlation peak amplitude of the small particles evolves nonmonotonically in time in agreement with Brownian dynamics computer simulations.

Dielectrophoresis and AC-Induced Assembly in Binary Colloidal Suspensions

Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.

Shear-induced particle migration in binary colloidal suspensions

We present experimental investigations of the spatial and temporal evolution of particle migration in pressure driven flows of Brownian particle suspensions. Binary suspensions of 1.4 and 3.0μm diameter colloidal particles are pumped through a 50×500μm2 rectangular-cross-section capillary tube. Shear rate gradients caused by the resulting parabolic velocity profile drive the particles away from the walls toward the center of the channel, where the shear rate is lowest. The flows are directly imaged using high-speed laser scanning confocal microscopy. Size segregation of the particles is observed. Depending on the conditions, either the large or the small particles enrich the center. We measure the development of the size segregation by tracking the evolution of the cross-stream concentrations of the particles.

Photonic Glass: A Novel Random Material for Light

A new photonic material is presented: “photonic glass”. This is a 3D completely disordered solid arrangement of monodisperse dielectric spheres (see figure). Monodispersity ensures single-sphere optical resonances to be observed. Very thick (from tenths to few millimetres) uniform samples can be grown by attenuating the inter-sphere interaction. Thin (correlated) disordered films can be grown by vertical convective deposition and selective etching of binary colloidal suspensions.

Hypersonic acoustic excitations in binary colloidal crystals: Big versus small hard sphere control

The phononic band structure of two binary colloidal crystals, at hypersonic frequencies, is studied by means of Brillouin light scattering and analyzed in conjunction with corresponding dispersion diagrams of the single colloidal crystals of the constituent particles. Besides the acoustic band of the average medium, the authors' results show the existence of narrow bands originating from resonant multipole modes of the individual particles as well as Bragg-type modes due to the (short-range) periodicity. Strong interaction, leading to the occurrence of hybridization gaps, is observed between the acoustic band and the band of quadrupole modes of the particles that occupy the largest fractional volume of the mixed crystal; the effective radius is either that of the large (in the symmetric NaCl-type crystalline phase) or the small (in the asymmetric NaZn(13)-type crystalline phase) particles. The possibility to reveal a universal behavior of the phononic band structure for different single and binary colloidal crystalline suspensions, by representing in the dispersion diagrams reduced quantities using an appropriate length scale, is discussed.

Partial Clustering in Binary Two-Dimensional Colloidal Suspensions

Strongly interacting binary mixtures of superparamagnetic colloidal particles confined to a two-dimensional water-air interface are examined by theory, computer simulation, and experiment. The mixture exhibits a partial clustering in equilibrium: in the voids of the matrix of unclustered big particles, the small particles form subclusters with a spongelike topology which is accompanied by a characteristic small-wave vector peak in the small-small structure factor. This partial clustering is a general phenomenon occurring for strongly coupled negatively nonadditive mixtures.

Depletion forces and packing models

In binary colloidal suspensions, due to the depletion forces, large spheres tend to pack together so as to make entropy maximum. Based on the complexities of depletion forces and phase behaviors in binary mixtures, two packing models, named nucleation and non-nucleation packing, are proposed in this Letter. Numerical results show that, when the volume fraction is small, the nucleation is possible. As a result, the large spheres tend to pack into a larger and larger cluster. However, when the volume fraction is very large in a way, the hard-spheres will proceed in the non-nucleation packing fashion. The numerical results also show that differences between the two packing models decrease with the decrease of volume fraction.

Gravimetric and electric conductivity studies of settling in binary graphite and kaolin suspensions

This work is devoted to the investigation of the settling process and electric conductivity in binary aqueous colloidal suspensions of graphite and kaolin. The combination of conductometric and gravimetric methods allows to analyze kinetics of both deposit formation and reorganization. Dependences of the characteristic times of mass and conductivity kinetic curves of settling versus pH and surfactant concentration are discussed.

Light-Regulated Electrostatic Interactions in Colloidal Suspensions

The net charge of a colloidal particle was controlled using light and a new photocleavable self-assembled monolayer (SAM). The SAM contained a terminal ammonium group and a centrally located carboxylic acid group that was masked with an ortho-nitrobenzyl functionality. Once exposed to UV light, the 2-nitrobenzyl group was cleaved, therefore transforming the colloidal particle from a net positive (silica-SAM-NH3+) to a net negative (silica-SAM-COO-) charge. By varying the UV exposure time, their zeta potential could be tailored between +26 and -60 mV at neutral pH. To demonstrate a photoinduced gel-to-fluid phase transition, a binary colloidal suspension composed of silica-SAM-NH3+ and negatively charged, rhodamine-labeled silica particles was mixed to form a gel. Exposure to UV light rendered all of the particles negative and therefore converted the system into a colloidal fluid that settles to form a dense sediment.

Direct measurement of relative and collective diffusion in a dilute binary colloidal suspension

Experimental characterization of the dynamics of multicomponent fluids is a problem of general importance to the field of complex fluids. We demonstrate a new experimental approach, termed two-color Fourier imaging correlation spectroscopy, which allows direct measurement of the partial dynamic structure factors, S(11)(k,tau), S(22)(k,tau), and S(12)(k,tau), where 1, 2 label the component species of a binary colloidal suspension. Linear combinations of the partial dynamic structure factors yield the characteristic time-correlation functions of the binary fluid. These are the correlation functions of concentration fluctuations S(CC)(k,tau), number density fluctuations S(NN)(k,tau), and cross-correlations between number density and concentration fluctuations S(NC)(k,tau). Test measurements are performed on a dilute symmetric mixture of fluorescently labeled 0.5 and 1.0 microm polystyrene spheres. From these data, we determine generalized collective and relative diffusion coefficients, and compare them to the predictions for an ideal mixture of noninteracting particles.

Gelcasting process of Al 2O 3/Ni nanocomposites

Al 2O 3/Ni nanocomposites were fabricated by gelcasting an Al 2O 3/NiO mixture, followed by reduction–sintering process. Dispersant quantity was optimized at 1.75 wt.% with measurement of rheological behaviors to determine the best dispersion state of binary oxides colloidal suspension for casting. Hydrogen reduction of NiO in the shaped body was accomplished successfully through pore channels of debinded organic chemical volume. The reduced Al 2O 3/NiO green body comprised Al 2O 3 and Ni phases without a reacted phase. One-dimensional diametric shrinkage reached 21% after pressureless sintering treatment, while fairly high strength of 586 MPa was maintained. Dispersed Ni particles revealed typical ferromagnetic behavior in the Al 2O 3 matrix. Near-net shaping and sintering for the Al 2O 3/Ni nanocomposite were also achieved by present gelcasting, indicating applicability of this process for industrial use.

Electrically guided assembly of planar superlattices in binary colloidal suspensions.

Binary colloidal suspensions are assembled into planar superlattices using ac electric fields. Either triangular or square-packed arrays form, depending on the frequency and relative particle concentrations. The frequency dependence is striking since superlattices develop at low and high frequencies but not at intermediate frequencies. We explain the low frequency behavior (<3 kHz) in terms of induced-dipole repulsion balanced by attraction resulting from electrohydrodynamic (EHD) flow. At high frequencies (20-200 kHz), EHD flow is negligible but aggregation occurs since dipole-dipole interactions become attractive.

Lane formation in colloidal mixtures driven by an external field

The influence of an external field on a binary colloidal mixture performing Brownian dynamics in a solvent is investigated by nonequilibrium computer simulations and simple theory. In our model, one half of the particles are pushed into the field direction while the other half of them are pulled into the opposite direction. For increasing field strength, we show that the system undergoes a nonequilibrium phase transition from a disordered state to a state characterized by lane formation parallel to the field direction. The lanes are formed by the same kind of particles moving collectively with the field. Lane formation accelerates particle transport parallel to the field direction but suppresses massively transport perpendicular to the field. We further show that lane formation also occurs in a time-dependent oscillatory field. If the frequency of the external field exceeds a critical value, however, the system exhibits a transition back to the disordered state. Our results can be experimentally verified in binary colloidal suspensions exposed to external fields under nonequilibrium conditions.

Entropically driven ordering in a binary colloidal suspension near a planar wall.

The local ordering of a binary hard-sphere mixture with a size ratio 1:10 near a planar wall is investigated by means of integral equation theory. We find that when the bulk volume fraction of the smaller particles is greater than 15%, the larger particles (at a bulk volume fraction of 1% and higher) become highly localized on the wall surface, forming a quasi-two-dimensional surface-localized monolayer. Our results are discussed and compared against computer simulation data with an effective one-component Hamiltonian that is based on sphere-sphere and sphere-wall depletion potentials.

SHORT-TIME DYNAMICS OF TWO-COMPONENTCOLLOIDAL SUSPENSIONS

We calculate the short-time translational and rotational self-diffusion coefficients for charge-stabilized binary colloidal suspensions where the density of one component is very dilute.The two-body and leading three-body contributions of hydrodynamic interaction (HI) to the diffusion tensor are taken into account.Due to both electrostatic interactions and HI, the self-diffusion coefficients depend on effective charge numbers,volumn fractions and sizes of two kinds of spheres, and the results can be explained by the effective hard sphere model.The influence of HI on charge-stabilized binary systems is less than that on hard-sphere binary systems.

Charge-Stabilized Liquidlike Ordered Binary Colloidal Suspensions. 1. Ultra-Small-Angle X-ray Scattering Characterization

In this and the following paper we present scattering studies on binary colloidal mixtures made of charge-stabilized polystyrene (PS) and perfluorinated (PFA) particles with diameters σ = 79 and 162 nm, respectively. Both colloidal species were mixed to well-defined compositions. The total concentration was about 9 vol % fraction. By using ultra-small-angle X-ray scattering, we directly obtained the partial scattering intensities of only the PFA particles in liquidlike ordered suspensions. Furthermore, after dividing intensities by the PFA particle form factor PPFA(Q), we got the partial structure factor SPFA-PFA(Q) without any additional treatment. The experimental results are compared with theoretical predictions obtained from the pure repulsive DLVO potentials and the hypernetted chain (HNC) integral equation, as applied to charged colloidal mixtures. It is shown that all measured intensities and extracted structure factors are in good agreement with the theoretical results.

Charge-Stabilized Liquidlike Ordered Binary Colloidal Suspensions. 2. Partial Structure Factors Determined by Small-Angle Neutron Scattering

In this paper we present small-angle neutron scattering (SANS) studies on liquidlike ordered binary colloidal suspensions. Using perfluorinated (σPFA = 162 nm) and polystyrene (σPS = 79 nm) particles, we prepared samples made of the same colloids as in the preceding paper. All system parameters of the mixtures were as before. Via neutron contrast variation we directly obtained all three partial intensities IPFA-PFA(Q), IPFA-PS(Q), and IPS-PS(Q) and partial structure factors SPFA-PFA(Q), SPFA-PS(Q), and SPS-PS(Q). The experimental results are compared with theoretical predictions for binary colloidal mixtures, based on the pure repulsive part of the DLVO potential. Within the hypernetted chain (HNC) closure an accurate agreement is achieved. The particle correlation in the mixtures is clarified by means of the partial distribution functions gij(r) and the one-component effective pair potentials (r) and (r). From this we deduce for the examined suspensions a partial clustering or local demixing by attractive interaction contributions of depletion origin.

Heteroflocculation in Binary Colloidal Suspensions: Monte Carlo Simulations

We have examined heteroflocculation in binary colloidal suspensions by Monte Carlo simulations based on a diffusion‐limited‐cluster‐aggregation (DLCA) model modified with finite attraction energies. The simulations were performed in two dimensions. Under heteroflocculation conditions, i.e., attraction between unlike particles and repulsion between like particles, cluster size undergoes a maximum as the concentration of the second species of particles is increased, similar to the experimental results in both binary suspensions and suspensions with adsorbing polymers. The initial increase in cluster size at low concentrations of the second species of particles is due to the mutual attraction between unlike particles. The decrease in cluster size at higher concentrations of the second species of particles is due to the repulsion between the second species of particles. The distinction between heteroflocculation and particulate depletion flocculation is discussed.

Role of Particle Dissolution in the Stability of Binary Yttria‐Silica Colloidal Suspensions

The stability of yttria‐silica binary aqueous colloidal suspensions is examined as a function of time. It has been observed that initially stable silica particles go through a flocculation period and then restabilize. This phenomenon is attributed to slow dissolution and the specific adsorption of trivalent yttrium cation and its hydroxo complexes on the surface of silica. Furthermore, it is shown that the kinetics of flocculation and restabilization can be controlled by pH adjustment.

Localized density profiles in binary colloidal suspensions

The density profiles of asymmetric binary mixtures of hard spheres are calculated within the local density approximation. The density profiles of the larger spheres are shown to be highly localized, within a narrow strip at intermediate altitudes, under a broad range of physical conditions. The localization results from a subtle competition between entropy and gravity.