Suspensions Of Colloidal Particles Research Articles

An Effective Model for Nematic Liquid Crystal Composites with Ferromagnetic Inclusions

Molecules of a nematic liquid crystal respond to an applied magnetic field by reorienting themselves in the direction of the field. Since the dielectric anisotropy of a nematic is small, it takes relatively large fields to elicit a significant liquid crystal response. The interaction may be enhanced in colloidal suspensions of ferromagnetic particles in a liquid crystalline matrix---ferronematics---as proposed by Brochard and de Gennes in 1970. The ability of these particles to align with the field and simultaneously cause reorientation of the nematic molecules greatly increases the magnetic response of the mixture. Essentially the particles provide an easy axis of magnetization that interacts with the liquid crystal via surface anchoring. We derive an expression for the effective energy of ferronematic in the dilute limit, that is, when the number of particles tends to infinity while their total volume fraction tends to zero. The total energy of the mixture is assumed to be the sum of the bulk elastic li...

Rheology of mineral oil-SiO2 nanofluids at high pressure and high temperatures

Nanofluids, engineered colloidal suspensions of nano-sized particles (less than 100 nm) dispersed in a basefluid, have shown potential for use as industrial cooling fluids due to their enhanced heat transfer capabilities. Many industrial applications often involve heat transfer fluids at pressures and temperatures above average atmospheric condition. Understanding the rheological characteristics of nanofluids is necessary for implementing them in these extreme conditions. Even though the effect of temperature on the viscosity of nanofluids at atmospheric pressure has been well studied, viscosity measurements of nanofluids at elevated pressures and temperatures have not yet been investigated. This work investigates the rheological characteristics of mineral oil based nanofluids at high pressure and high temperature (HPHT). The nanofluids used in this work were prepared by mechanically dispersing commercially available SiO2 nanoparticles (∼20 nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO QATAR), which has wide applications in industrial heat exchangers. Mineral oil and nanofluids, with two volume concentrations of 1% and 2%, are studied in this work. The rheological characteristics of the basefluid and nanofluids are measured using an HPHT viscometer. During experimentation, viscosity values of the nanofluids are measured at pressures of 100 kPa and 42 MPa, with temperatures ranging from 25 °C to 140 °C, and at varying shear rates. The results show that the viscosity values of both nanofluids, as well as the basefluid, increased as the pressure increased. In addition, nanofluids exhibit non-Newtonian characteristics at elevated temperatures and pressures.

Influencia del campo magnético estático en la turbidez de la cerveza de alta gravedad

Se aplicó el campo magnético estático a la cerveza de alta gravedad en tres etapas distintas del proceso, a partir de un diseño multifactorial D-óptimo de tres factores (intensidad de corriente, tiempo de residencia y tipo de cerveza). Se determinó la turbidez variable de respuesta por la diferencia de absorbancia por espectrofotometría a 580 nm. Se obtuvo un modelo lineal en los rangos evaluados que relaciona dicha variable con el régimen de tratamiento magnético con efecto significativo sobre la aglutinación de las partículas coloidales en suspensión lo cual favorece la clarificación. La mejor combinación de factores estuvo en el rango de 0,40-0,55 A (100-120 mT) y tiempo de residencia 10 s en la cerveza no diluida y en la diluida, sin afectación de las características sensoriales.

Ferromagnetism in suspensions of magnetic platelets in liquid crystal

More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.

Size and boundary effects on the diffusive behavior of elongated colloidal particles in a strongly confined dense dispersion

In very recent experimental work, diffusive motion of individual particles in a dense columnar phase of colloidal suspension of filamentous virus particles probed by means of fluorescence video microscopy [S. Naderi, E. Pouget, P. Ballesta, P. van der Schoot, M. P. Lettinga, and E. Grelet, Phys. Rev. Lett. 111, 037801 (2013)]. Rare events were observed in which the minority fluorescently labeled particles engage in sudden, jump-like motion along the director. The jump length distribution turned out to be biased towards a half and a full particle length. We suggest these events may be indicative of two types of particle motion, one in which particles overtake other particles in the same column and the other where a column re-equilibrates after a particle leaves a column either to enter into another column or into a void defect on the lattice. Our Brownian dynamics simulations of a quasi one-dimensional system of semi-flexible particles, subject to a Gaussian confinement potentials mimicking the effects of the self-consistent molecular field in the columnar phase, support this idea. We find that the frequency of overtaking depends on the linear fraction of particles and the steepness of the confining potential. The re-equilibration time of a column after a particle is removed from it is much shorter than the self-diffusion timescale. For the case of large system sizes and periodic boundary conditions, overtaking events do not present themselves as full-length jumps. Only if the boundary conditions are reflecting and the system is sufficiently small, full length jumps are observed in particle trajectories. The reason is that only then the amplitude of the background fluctuations is smaller than a particle length. Increasing the bending flexibility of the particles on the one hand enhances the ability of particles to overtake each other but on the other it enhances fluctuations that wash out full jumps in particle trajectories.

Deposition of thin ultrafiltration membranes on commercial SiC microfiltration tubes

Porous SiC based materials present high mechanical, chemical and thermal robustness, and thus have been largely applied to water-filtration technologies. In this study, commercial SiC microfiltration tubes with nominal pore size of 0.04μm were used as carrier for depositing thin aluminum oxide (Al2O3) ultrafiltration membranes. These ultrafiltration membranes were obtained by coating, drying and calcination of a colloidal suspension of boehmite particles. After calcination, the membrane material consisted of nanosized γ-Al2O3 crystallites and had a narrow pore size distribution with average pore size of 5.5nm. Membrane thickness was tuned by repeating the coating of boehmite sol. By doing so, we were able to reduce the defect density on the membrane surface, as evidenced by SEM analysis and by the significant reduction of water permeance after depositing the second γ-Al2O3 layer. After five times coating, a 5.6µm thick γ-Al2O3 layer was obtained. This membrane shows retention of ~75% for polyethylene glycol molecules with Mn of 8 and 35kDa, indicating that, despite their intrinsic surface roughness, commercial SiC microfiltration tubes can be applied as carrier for thin ultrafiltration membranes. This work also indicates that an improvement of the commercial SiC support surface smoothness may greatly enhance permeance and selectivity of γ-Al2O3 ultrafiltration membranes by allowing the deposition of thinner defect-free layers.

Dynamics in thermo-responsive nanogel crystals undergoing melting

We report here the dynamics in thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) nanogel crystals undergoing melting/freezing and verify the applicability of the dynamical criterion for melting/freezing proposed by Löwen et al. [Phys. Rev. Lett. 70, 1557 (1993)]. According to this criterion the ratio of long time diffusion coefficient (D(L)) to short time diffusion coefficients (D(S)) is ~0.1 for colloidal particles in suspension undergoing melting/freezing. Static and dynamic light scattering techniques have been employed to identify the melting/freezing transition of PNIPAM nanogel colloidal crystals of two different volume fractions φ = 0.49 and 0.79 and to measure D(L) and D(S) across the melting. In dense PNIPAM nanogel crystals undergoing melting, the ratio D(L)/D(S) is found to be less than 0.1 for the first time and this deviation is higher in the suspension with higher φ. We also show that the deviation is genuine by measuring D(L)/D(S) on shear melted charged silica colloidal liquid undergoing freezing. The mean square displacement at shorter times, close to the melting, shows subdiffusive behavior. The subdiffusive behavior, arising due to the overlap of the dangling polymer chains between shells of the neighboring particles, is argued to be the reason for the observed deviation.

Dynamics of multiphase systems with complex microstructure. II. Particle-stabilized interfaces

In this paper we use the GENERIC (general equation for nonequilibrium reversible-irreversible coupling) nonequilibrium thermodynamics framework to derive constitutive equations for the surface extra stress tensor of an interface stabilized by a two-dimensional suspension of anisotropic colloidal particles. The dependence of the surface stress tensor on the microstructure of the interface is incorporated through a dependence on a single tensorial structural variable, characterizing the average orientation of the particles. The constitutive equation for the stress tensor is combined with a time-evolution equation describing the changes in the orientation tensor as a result of the applied deformation field. We examine the predictions of the model in in-plane steady shear flow, in-plane oscillatory shear flow, and oscillatory dilatational flow. The model is able to predict the experimentally observed shear thinning behavior in surface shear flow, and also the experimentally observed emergence of even harmonics in the frequency spectrum of the surface stress in oscillatory dilatational flow. Our results show that the highly nonlinear stress-deformation behavior of interfaces with a complex microstructure can be modeled well using simple structural models like the one presented here.

Translational and rotational dynamics of colloidal particles in suspension: Effect of shear

We report a generalization of a nonequilibrium thermodynamic theory for the mesoscopic dynamics of radially symmetric interacting particles to anisotropic pairwise interactions and attain the one- and two-particle Fokker-Planck kinetics equations at a low-density limit that provides the translational-rotational coupling of their motion due to hydrodynamic interactions, from which we derived the balance equations of linear, angular momentum, and energy dissipation due to particle interactions and energy interchange with heat bath. In this low-density approximation, an already-known virial expression for the long-time translational collective diffusion coefficient of an orientational isotropic suspension in terms of the fluid equilibrium microstructure is recovered. An external shear flow induces, in the diffusive regime, vorticity effects into the rotational diffusion property of the colloidal particles. They manifest in the appearance of the particle's rotational viscosity due to vortex flow. The Smoluchowski equation that governs the dynamical relaxation of colloid microstructure due to particle's Brownian motion under stationary flow is provided.

Attenuation Measurement Method to Control Clay Slurries: Application of Dams Water

We propose to study the behavior of diluated suspensions of clay and non colloidal particles by a combination of experiments. We plan to study these diluate particulate systems by changing the masse fraction of clay. We have developed a technique based on the ultrasound, by measuring ultrasonic attenuation waves in clay slurries. This technique makes it possible to control in real time the presence of the clay grains whose dimension is about 10 micron, using an interface conceived under Labview program. The slurries with different weight of clay were controlled. The detection of weak amount of clay’s grains and colloid were provided by the transducer 30Mhz. High sensitivity is obtained by analyzing the attenuation measurements founded from multiple paths through the slurry. The experimental results obtained show that the attenuation of waves due to particles varies linearly with mass fraction and the frequency of transducers. Therefore, such measures of the attenuation at a given frequency give the concentration of clay in the suspension. These frequency charts will be used for the quality control of the dam’s water. General Terms 0.N.E.P: Office National de l’eau Potable, (National office of drinking water). FFT: Fast Fourier Transform.

Enumerating virus-like particles in an optically concentrated suspension by fluorescence correlation spectroscopy.

Fluorescence correlation spectroscopy (FCS) is one of the most sensitive methods for enumerating low concentration nanoparticles in a suspension. However, biological nanoparticles such as viruses often exist at a concentration much lower than the FCS detection limit. While optically generated trapping potentials are shown to effectively enhance the concentration of nanoparticles, feasibility of FCS for enumerating field-enriched nanoparticles requires understanding of the nanoparticle behavior in the external field. This paper reports an experimental study that combines optical trapping and FCS to examine existing theoretical predictions of particle concentration. Colloidal suspensions of polystyrene (PS) nanospheres and HIV-1 virus-like particles are used as model systems. Optical trapping energies and statistical analysis are used to discuss the applicability of FCS for enumerating nanoparticles in a potential well produced by a force field.

Mechanical Stresses Induced by Evaporation in Consolidated Colloidal Suspensions of Hard Particles. Poroelasticity Theory Versus Experiments

Drying of colloidal suspensions often yields intriguing, complex and generally undesirable crack patterns. Despite a consensus on the main underlying physical mechanisms that lead to their formation, quantitative prediction seems at present out of reach. Notably, a prerequisite for this is the determination of the mechanical stress field. Here, our point of view is to provide a sound and well-established continuum mechanics model that can be used safely and easily by a large community and particularly by the fracture mechanics one. We show by comparison with beam deflection experiments, that (i) the use of Biot’s linear poroelasticity theory allows for the prediction of the stresses that lead to the crack formation and that (ii) the setup allows for the identification of some poroelastic constants. This evidenced that the hypothesis of incompressible constituents is reasonable for the nanolatex suspensions considered here, greatly simplifying the constitutive equations and reducing the number of material constants from four to two.

Orientational energy of anisometric particles in liquid-crystalline suspensions

We obtain a general expression for the orientational energy of an individual anisometric particle suspended in uniform nematic liquid crystals when the main axis of the particle rotates with respect to the nematic director. We show that there is a qualitative and quantitative analogy between the internal and external problems for cylindrical volumes of nematic liquid crystals, and on this basis we obtain an estimate of the orientational energy of a particle of cylindrical (rodlike, needlelike, or ellipsoidal) shape. For an ensemble of such particles we propose a modified form of their orientational energy in the nematic matrix. This orientational energy has the usual second-order term, and additional fourth-order term in the scalar product of the nematic director and the vector which characterizes an average direction of the main axes of the particles. As an example we obtain the expression for the free energy density of ferronematics, i.e., colloidal suspensions of needlelike magnetic particles in nematic liquid crystals. Unlike previous models, the free energy density includes the proposed modified form of the particle orientational energy, and also a contribution describing the surface saddle-splay deformations of the liquid crystal matrix.

Integrated Membrane Bioreactor for Water Quality Control in Marine Recirculating Aquaculture Systems

The aquaculture live feed organisms Acartia tonsa (a calanoid copepod, experiment 1) and Brachionus “Cayman” (a rotifer, experiment 2) were cultivated in marine recirculating aquaculture systems (RAS), respectively. The pilot plant was built as a combination of conventional RAS (cRAS) and as a modified RAS which implemented an ultrafiltration membrane bioreactor (MBR) for the removal of fine suspended solids and colloidal particles as part of the treatment system (mRAS). The two treatment schemes were connected to the same biofilter (a moving bed bioreactor). In the first experiment, the membrane was operated with no extraction of concentrate, and hydraulic retention time (HRT) of 6 hours (i.e., water exchange in the cultivation tanks of 4 times per day). In the second experiment, the membrane was operated with daily extraction of concentrate, and HRT of 12 hours. Results show that the MBR option is more efficient in removing particles from the recycle stream than conventional RAS. However, the impact this has on the number of particles in the live feed cultivation tanks is not readily apparent based on particle analysis. The amount of suspended solids added during feeding exceeds the amount removed in the recycle system. This requires a higher recirculation rate and different membrane operating conditions.

Crystallization in a sheared colloidal suspension

We study numerically the crystallization process in a supersaturated suspension of repulsive colloidal particles driven by simple shear flow. The effect of the shear flow on crystallization is two-fold: while it suppresses the initial nucleation, once a large enough critical nucleus has formed its growth is enhanced by the shear flow. Combining both effects implies an optimal strain rate at which the overall crystallization rate has a maximum. To gain insight into the underlying mechanisms, we employ a discrete state model describing the transitions between the local structural configurations around single particles. We observe a time-scale separation between these transitions and the overall progress of the crystallization allowing for an effective Markovian description. By using this model, we demonstrate that the suppression of nucleation is due to the inhibition of a pre-structured liquid.

Dynamical clustering and phase separation in suspensions of self-propelled colloidal particles.

We study experimentally and numerically a (quasi-)two-dimensional colloidal suspension of self-propelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.

Magnetorheology of xanthan-gum-coated soft magnetic carbonyl iron microspheres and their polishing characteristics

Magnetorheological (MR) fluids are colloidal suspensions of soft magnetic particles dispersed in a non-magnetic liquid. Among their applications, MR polishing has attracted considerable attention owing to its smart control of the polishing characteristics for dedicated microelectromechanical system applications. To improve the polishing characteristics of MR fluids, we fabricated carbonyl iron (CI) microspheres coated with xanthan gum (XG) by using a solvent casting method. The morphologies and densities of both pure CI and CI/XG particles were characterized using a scanning electron microscope and a pycnometer, respectively. In addition, the rheological characteristics of the MR fluids under various applied magnetic field strengths were examined using a rotational rheometer. The MR polishing characteristics were conducted using an MR polishing machine to examine the surface roughness and the material removal by MR polishing with added nano-ceria slurry abrasives.

Three dimensional simulation dynamics for the dilute colloidal suspensions of rod-like polymer particles flowing in the bulk and near solid boundaries

Numerical simulations and algorithms are developed to analyze the dynamics of rigid rod-like polymer particles. We developed a theoretical model based on the equations of Jeffrey for the dynamics of rigid polymer particles in fluids and the molecular dynamics by mechanical restitution for the diffusive collisions of the particles at the solid boundaries. The simulations are developed to calculate the dynamic equilibrium probability distribution functions (PDF) distributions for rod-like polymer particles in colloidal suspensions in a fluid under hydrodynamic flow inside pores with solid boundaries. They are carried out for idealized atomically flat and the realistic rough surface boundaries. To accomplish this, we investigate the influence of the surface roughness on the choice of the hydrodynamic boundary conditions. The simulation results for the PDF distributions for the spatial positions and orientations of rod-like polymer particles are calculated, over several orders of magnitude of the rotational Peclet number. They demonstrate the importance and significance of modeling in a three-dimensional spatial frame as compared to the simulation results over a two-dimensional spatial frame. In particular we are able to produce a complete topography for the PDF distributions segmented as a hierarchy in the depletion layer by covering a complete range of orientations in 3D spatial frames. These simulations permit to calculate the nematic order parameter over its tensorial representation for the colloidal suspensions of rod-like polymer particles locally, and throughout the pore space including the depletion layer. Our results for the nematic order parameter are hence innovating and represent a new input for these systems. Open image in new window

Chemotactic Behavior of Catalytic Motors in Microfluidic Channels

Chemotactic Behavior of Catalytic Motors in Microfluidic Channels

Rheological Study of Two-Dimensional Very Anisometric Colloidal Particle Suspensions: From Shear-Induced Orientation to Viscous Dissipation

In the present study, we investigate the evolution with shear of the viscosity of aqueous suspensions of size-selected natural swelling clay minerals for volume fractions extending from isotropic liquids to weak nematic gels. Such suspensions are strongly shear-thinning, a feature that is systematically observed for suspensions of nonspherical particles and that is linked to their orientational properties. We then combined our rheological measurements with small-angle X-ray scattering experiments that, after appropriate treatment, provide the orientational field of the particles. Whatever the clay nature, particle size, and volume fraction, this orientational field was shown to depend only on a nondimensional Péclet number (Pe) defined for one isolated particle as the ratio between hydrodynamic energy and Brownian thermal energy. The measured orientational fields were then directly compared to those obtained for infinitely thin disks through a numerical computation of the Fokker-Plank equation. Even in cases where multiple hydrodynamic interactions dominate, qualitative agreement between both orientational fields is observed, especially at high Péclet number. We have then used an effective approach to assess the viscosity of these suspensions through the definition of an effective volume fraction. Using such an approach, we have been able to transform the relationship between viscosity and volume fraction (ηr = f(φ)) into a relationship that links viscosity with both flow and volume fraction (ηr = f(φ, Pe)).