Non-linear impedance analysis of industrial lubricants

This paper examines the application of higher harmonic non-linear electrochemical impedance spectroscopy (NLEIS) for analysis of electrochemical properties of industrial lubricants. NLEIS is a powerful new technique, which provides higher resolution than traditional linear impedance for distinguishing specific mechanisms governing electrode response. High-amplitude NLEIS showed the potential to deliver kinetic parameters not available from small amplitude, fundamental harmonic impedance measurement. Simulation of experimental NLEIS results and evaluation of the 1st (fundamental), 2nd, and 3rd order harmonics impedance data was performed using Mathematica™ software. Results from this simulation study were compared with experimental data leading to a better interpretation and quantification of charge transfer, adsorption and diffusion processes affecting the low-frequency impedance response. This information was utilized in development of more accurate equivalent circuit impedance model for a typical industrial lubricant represented by a combination of bulk solution and interfacial resistances, capacitances, and CPE parameters.     

A radiometric method for the characterization of particulate processes in colloidal suspensions Part 2. Experimental verification of the method

A radiometric method for the characterization of particulate processes is verified using stable hydrosols of silver iodide. Silver iodide hydrosols satisfy the conditions required for the applications of the proposed method. Comparison shows that the values for the change of particle size measured in silver iodide hydrosols by the proposed method are in excellent agreement with the values obtained by other methods on the same systems (electron microscopy, sedimentation analysis, light scattering). This shows that the proposed method is suitable for the characterization of particulate processes in colloidal suspensions.     

Halos mechanism in stabilizing of colloidal suspensions: Nanoparticle weight fraction and pH effects

Nanoparticle halo is a novel method that stabilizes colloidal suspensions with high efficiency compared to conventional methods. But, up to now few articles have been published on the stabilization mechanism of this method. This article investigates the dispersion behavior of colloidal Al₂O₃ aqueous suspensions in the presence of highly charged CeO₂ nanoparticles and the effect of two key parameters; the ceria nanoparticle concentration and pH on the stability of Al₂O₃–CeO₂ bidispersed suspensions. The stability behavior of these suspensions was investigated by sedimentation technique. It was concluded from this work that by increasing ceria nanoparticle concentration the stability of these bidispersed systems reaches an optimum condition, but further increasing of nanoparticles results in decreasing of the colloidal suspension stability. Also, the results showed that the suspensions have optimum dispersion state and high stability at pH 10. The calculated DLVO interparticle interaction potentials verified the experimental results of the pH effect on the stability behavior. The stabilization of the bidispersed suspensions was also evidenced by scanning electron microscopy (SEM) and SEM energy dispersive X-ray analysis (SEM/EDXA) of the sediment layers after 3 weeks.     

Pore size distribution from challenge coreflood testing by colloidal flow

The transport of colloidal and suspension particles and the resultant particle retention occur in a wide range of porous media. The micro scale pore throat size distribution is an important characteristic of porous media, allowing for evaluation of important transport properties. An effective method based on micro scale modelling for the determination of overall pore throat size distribution (PSD) by injection of colloidal particle suspensions into engineered porous media with monitored inlet and breakthrough particle concentrations is developed. The treatment of inlet and outlet colloidal particle concentrations obtained in coreflooding results in a good agreement between the modelling and experimental data. Yet, some deviation was observed between the obtained PSD and that calculated by the Monte Carlo simulation based on the Descartes’ theorem.     

微生物絮凝剂-阳离子型无机微粒体系的混凝脱色研究

从啤酒厂废酵母直接提取微生物絮凝剂,与水合氧化铁微颗粒体系集成应用于混凝脱色,其脱色效果与无机微颗粒体系的电荷密度、微生物絮凝剂剂量有关,提出无机微颗粒与染料集合体的"剩余电荷"的概念解释微生物絮凝剂-阳离子型无机微粒体系的混凝机理.结果显示微生物絮凝剂-水合氧化铁微粒体系混凝脱色在试验范围内均有显著的协同效应,直接黄最高脱色率达到95.5%,分别比单独使用水合氧化铁微粒和微生物絮凝剂提高33.6%和82.6%.     

A radiometric method for the characterization of particulate processes in colloidal suspensions Part 1. Theoretical approach

A theoretical approach to a new radiometric method for the characterization of particulate processes in stable colloidal suspensions is given. Following Rajagopal's studies of Brownian coagulation, the change of particle volume during the Brownian coagulation of sols is correlated with the particle size before the coagulation process. It is proved that the ratio (F_b/F_a)_o of mean particle size before the coagulation process is equal to the ratio (^*F_b/F_a)_tE of imaginary particle size. The ratio of imaginary particle sizes is calculated from heterogeneous exchange fractions attained in sols during the exchange fractions attained in sols during the exchange process (proceeding simultaneously with the coagulation process) using the graphic form of Wagner's solution of differential equation of diffusion. It is shown that this method is suitable for determining the change of the relative mean particle size during the ageing of systems, and thus for the characterization of particulate processes in colloidal suspensions.     

Fouling of ultrafiltration membranes: lateral migration and the particle trajectory model

Lateral migration theory is used both to explain the “flux-paradox” problem for colloidal suspensions and as part of the standard filtration theory for calculating permeation flux as a function of time. Particle trajectory analysis is used to model the physics of colloidal fouling in tangential flow ultrafiltration. A method is also suggested on how to incorporate trajectory analysis into the standard filtration theory. The theoretical results are compared with experiments from the literature and from the authors' laboratory.     

Electrophoretic deposition of multiferroic BiFeO3 sub-micrometric particles from stabilized suspensions

The electrophoretic deposition (EPD) was applied to BiFeO₃ (BFO) powders, one of the most interesting multiferroic compounds characterized by simultaneous magnetic and ferroelectric activity, to form homogeneous films. The preparation and characterization of stable BFO colloidal suspensions in aqueous, organic and mixed solvents were investigated by zeta potential measurements at room temperature in the presence of surfactants. BFO thin films were then deposited on steel substrates from stabilized BFO suspensions, by adjusting the preparative parameters to optimize the film quality. The compositional, morphological and electrical characteristics of the obtained BFO films, together with thickness measurements, were studied using SEM, XRD, AFM, EIS and optical surface profilometer. EPD method applied to BFO stable suspensions produced homogeneous thickness BFO films, free from pinholes and cracks, that were successively sintered and characterized also in terms of photocatalytic response.     

Laser photoacoustic and photothermal spectroscopies as novel characterization methods for microparticles

Photoacoustic and photothermal spectroscopy methods can be effectively applied to the characterization of single microparticles. As the optical beam deflection method, in which a thermal wave generated by non-radiative transition is detected by deflection of a probe beam, is sensitive for a single microparticle, and is a remote-measurement method without using a cell, it is suitable for nondestructive characterization of microparticles. This method was applied to determination of chemical species adsorbed on a single microparticle, and spectral differences between leukemia and normal white blood corpuscles could be obtained. A more violent photothermal conversion phenomenon of a particle, laser breakdown and accompanying plasma and acoustic emission, was applied to individual detection and analysis of ultrafine particles in liquids. Laser-like nonlinear emission from the plasma was observed.     

Macroscopic dynamics of flocculated colloidal suspensions

The macroscopic dynamics of strongly flocculated colloidal suspensions are of both fundamental interest and practical significance across a range of applications from soil mechanics to solid–liquid separation. Over 30 years ago, Buscall and White [Buscall, R., White, L.R., 1987. The consolidation of concentrated suspensions. Part 1. The theory of sedimentation, Journal of the Chemical Society: Faraday Transactions I 83 (3), 873–891] proposed a one-dimensional (1D) theory of sedimentation and consolidation of such suspensions, which spawned suspension characterization tools and process modeling techniques to successfully predict 1D separation behavior. However, many applications such as continuous thickening or cross-flow filtration are multidimensional in practice, involving suspension transport, deformation and separation, and so cannot be described by the 1D BW theory. Multidimensional suspension mechanics has received a little attention over the past 30 years, requiring development of a tensorial rheology of suspensions, capable of describing arbitrary loadings and simultaneous shear and compression. In this paper, we develop a multidimensional theory of the macroscopic mechanics of strongly flocculated colloidal suspensions to describe suspension transport, deformation and separation in terms of the bulk suspension velocity field and local average solids volume fraction alone. This is achieved by consistent extension of the fundamentals of BW theory to multiple dimensions, with constitutive models justified and expressed in terms of experimentally measurable material functions. Constitutive aspects which are outstanding due to lack of empirical evidence are clearly identified, pointing to potential research directions regarding the tensorial rheology of colloidal suspensions. Utility of the theory is demonstrated in an example comparison between 1D and 2D continuous gravity thickener operation.     

Two‐step continuous production of monodisperse colloidal ellipsoids at rates of one gram per day

We report a two‐step process for the continuous production of monodisperse polystyrene colloidal ellipsoids of aspect ratios up to 6.8 at rates that exceed 1.0 g per day, an improvement on previously reported synthetic batch processing rates of nearly a factor of 20. This scale up is accomplished by continuous evaporative processing of a polymer solution into an elastomeric film embedded with colloidal spheres. Subsequently, the film is continuously elongated at a temperature that stretches the embedded spheres into ellipsoids. The method is used to deform initially 1.0 μm diameter spheres into ellipsoids of aspect ratio 1.27 ± 0.15, 3.31 ± 0.44, 3.91 ± 0.72, 4.14 ± 0.47, and 6.77 ± 1.01. The particle production rate reported here opens new possibilities for applications of monodisperse ellipsoids, such as self‐assembly and optical characterization of complex crystalline unit cells, as well as rheological characterization of dilute gels and dense suspensions. © 2017 American Institute of Chemical Engineers AIChE J, 64: 697–707, 2018     

Dynamic light scattering applied to the synthesis of colloidal zeolite

The use of dynamic light scattering as an analysis method within the field of zeolite synthesis has proved to be a powerful and robust tool with which the particle size and the corresponding particle size distribution can be determined. The method has been employed in the evaluation of the crystallization of several types of colloidal zeolite from clear homogeneous solutions. Examples of such zeolites are zeolite N-Y, hydroxysodalite, and TPA-silicalite-1. The fact that the particle size can be determined in as-synthesized zeolite sols as a function of synthesis time enables one to follow, for example, the crystallization process in terms of particle size increase, the process of particle size tailoring as well as to obtain information on the growth mechanism in zeolite synthesis. The colloidal nature of sols following redispersion of zeolite powders and colloidal zeolite organosols has been assessed using dynamic light scattering. The advantages as well as problems associated with the use of dynamic light scattering for characterization of colloidal zeolites are discussed.     

Buckypaper fabrication by liberation of electrophoretically deposited carbon nanotubes

Free-standing films of multi-walled carbon nanotubes (MWCNTs), also known as buckypapers, have been fabricated by a two-step process using electrophoretic deposition (EPD). Films of the multi-walled carbon nanotubes were cast onto stainless steel electrodes from aqueous suspensions by EPD. Using a facile mechanical cleavage technique, the films were liberated from their underlying electrodes to yield the buckypapers. We investigated the films’ thickness, morphology, and surface topology using, respectively, profilometry, scanning electron microscopy, and atomic force microscopy. Mechanical characterization of the buckypapers revealed the average tensile strength and Young’s modulus to be 14.5 MPa and 3.3 GPa, respectively. This fabrication approach provides a cost effective, rapid, and reproducible method to make films of MWCNTs with a range of thicknesses and macroscopic lateral dimensions.     

Phase Behavior of Polyelectrolyte Complexes and Rheological Behavior of Alumina suspensions for Direct Ink Writing

Polyelectrolyte complexes have a bright prospect for fabricating 3D periodic structures by direct ink writing. The phase behavior of complexes containing poly(acrylic acid) and poly(ethylenimine) and rheological behavior of Al₂O₃ colloidal suspensions are characterized. The results reveal that the pH value of solution takes an important role on the phase behavior of polyelectrolyte complexes. When the [COOH]:[NH_x] ratio is higher than or lower than the critical value of 0.6, the pH range of turbid complex solutions narrows down and meanwhile moves to acidic or alkaline region, respectively. The addition of pH regulators prompts polyelectrolyte exchange reaction and soluble complexes are suitable for preparation of cera‐mic suspensions. The polyelectrolyte suspensions with linear viscoelasticity at lower shear stress and good fluidity at higher shear stress are identified for direct ink writing of 3D structures with microsized feature.     

Coagulation–flocculation of colloidal suspensions of kaolinite,bentonite, and alumina by chitosan sulfate

This article reports on the removal of colloidal suspensions of kaolinite, bentonite, and alumina using chitosan sulfate (ChS). ChS was synthesized by partial introduction of sulfate groups in the chitosan (Ch) structure. The polyampholyte (chitosan sulfate) shows variable charge depending on the pH of the solution. ChS was characterized by FTIR, ¹³C‐NMR, elemental analysis, and potentiometric titrations. The ChS coagulation–flocculation capacity for kaolinite, bentonite, and alumina aqueous suspensions was systematically studied. The coagulation–flocculation process was carried out at various pH values and ChS concentrations. The pH range in which the largest ChS removal capacity was observed depended on particle type (4.5–5.5 for kaolinite, 4.5–7.0 for bentonite, and 7.0–8.0 for alumina). The removal of colloidal particles is explained by charge neutralization due to electrostatic interactions between ChS and particles and particle entrapping when the polyelectrolyte precipitates. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and characterization of CdS quantum dots with carboxylic-functionalized poly (vinyl alcohol) for bioconjugation

In the present research it is reported the synthesis and characterization of CdS nanoparticles (NPs) prepared using carboxylic-functionalized poly (vinyl alcohol) (PVA) as the ligand via aqueous route at room temperature and ambient pressure. Different molar concentrations of carboxylic-PVA and PVA were investigated aiming at producing stable colloidal systems. Carboxylic-PVA was conjugated with BSA (bovine serum albumin) and used as capping ligand in the preparation of CdS nanocrystals. UV-visible spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy were used to characterize the kinetics and the relative stability of polymer-capped CdS nanocrystals. The results have clearly indicated that the carboxylic-functionalized PVA was much more effective on nucleating and stabilizing colloidal CdS nanoparticles in aqueous suspensions compared to PVA. In addition, the CdS nanocrystals were obtained in the so-called “quantum-size confinement regime”, with the calculated average size below 4.0 nm and fluorescent activity. Thus, a novel simple route was successfully developed for synthesizing nanohybrids based on quantum dots and water-soluble chemically functionalized polymers with incorporated carboxylic moiety with the possibility of direct bioconjugation.     

Controlling colloidal processing of (K,Na)NbO3-based materials in aqueous medium

K_0.5Na_0.5NbO₃-based materials are serious candidates to replace lead-based piezoceramics since they show excellent electrical and piezoelectric properties. The tape casting technique can be used to obtain highly textured KNN-based ceramics; however, despite industrial and environmental advantages of water-based processing, there are not reports about the control of colloidal processing conditions to obtain optimized K_0.5Na_0.5NbO₃-based slurries in aqueous medium. This paper reports a procedure for controlling colloidal stability and rheological behavior of aqueous (K_0.5Na_0.5)_0.97Li_0.03Nb_0.8Ta_0.2O₃ suspensions. Zeta potential and cationic solubility measurements as a function of pH showed that pH 8.5 is adequate for concentrated suspensions, while flow curves analysis allowed optimizing processing parameters, such as, powder content, amount of deflocculant and binder, and sonication time. Optimized colloidal suspensions were prepared and used to obtain high quality tapes. Processed ceramics from these stacked tapes show equivalent properties to those processed directly from powders, which demonstrates the effectiveness of the colloidal route reported here.     

Atom transfer radical polymerization from silica nanoparticles using the ‘grafting from’ method and structural study via small-angle neutron scattering

Polymer chains were grafted from silica beads (colloidal sol in dimethylacetamide) by atom transfer radical polymerization (ATRP), via the ‘grafting from’ method. The grafting of the initiator onto the silica surface was done in two steps. First, thiol-functionalization of the surface was achieved via silanization with a mercaptopropyl triethoxysilane. Second, we performed an over-grafting of the surface by reacting the thiol with 2-bromoisobutyryl bromide to generate the halogen-functional ATRP initiator. The nanoparticles were kept in solution (in the same solvent) at each stage of the functionalization (even during the purification steps), as this is the only way to avoid irreversible aggregation. Then, the polymerization of styrene was conducted. Control of both the molecular weight and the density of grafted chains can be achieved by this method. Careful characterization such as gel permeation chromatography, ²⁹Si CP/MAS NMR, elemental analysis, infrared spectroscopy and thermo-gravimetric analysis is performed. The state of dispersion of the grafted nanoparticles is followed in details by small angle neutron scattering and results obtained from this technique are presented here as well as the way the SANS data can be treated. Connection is systematically done between the information provided by this technique and the improvement of the synthetic procedure.     

Rheological Behavior and Slip Casting of Al2O3–Ni Aqueous Suspensions

A strong effort has been devoted recently toward processing of metal–ceramic composites with tailored microstructure by colloidal methods. The aim of this work is to optimize the rheological behavior of concentrated Al₂O₃–nickel (Ni) aqueous suspensions and further slip casting in order to obtain dense green composites. Compositions with Ni relative contents ranging from 5 to 75 vol% were prepared from suspensions with high solids loadings (50 vol%) by adjusting the colloidal stability of each component in terms of pH, mobility, dissolution conditions, and influence of polyelectrolytes. The rheological properties were measured under controlled rate and controlled stress conditions at different basic pH conditions and contents of polyelectrolyte. Better rheological conditions of the mixtures were found for pH 10 and 1.0 wt% polyelectrolyte. Minimum viscosity was obtained for suspensions containing 15 vol% of Ni. The analysis of flow curves demonstrates that the suspensions form a structure at very low shear, hindering sedimentation. Homogeneous slip cast bodies with green densities up to 70% of theoretical and up to 75 vol% Ni were sintered in Ar to achieve dense biphasic composites.     

EPD kinetics: A review

The colloidal approach has been studied as an essential step in the tailoring of ceramic nanostructures, but most colloidal processes are limited by the complexity of preparation of highly concentrated and stable suspensions of nanoparticles and their fast ageing. Electrophoretic deposition (EPD) stands out as the most appropriate colloidal process to produce ceramic structures using low solid content sols and suspensions (<1 g/l). This characteristic drastically increases its range of technological applicability to nanoparticle assembly, becoming an alternative to the evaporative coating processes. Modelling of this direct electrically driven assembly process is key for its application to the performance of new materials on length scales of approximately 1–100 nm. In this paper, the key contributions in this field to process control and development of the electrophoretic kinetics equation are summarised.