Opaque Suspensions Research Articles

Sedimentation and redispersion phenomena in iron-based magnetorheological fluids

In this work, the effect of three different additives (oleic acid, aluminum stearate, and silica nanoparticles) on the aggregation, sedimentation, and redispersibility of concentrated iron-based magnetorheological fluids was investigated. With this aim, the sedimentation behavior was analyzed using an electromagnetic induction method, which is suitable for studying the sedimentation of opaque magnetic suspensions. The redispersibility was studied, in a quantitative way, by means of rheological measurements, both in the presence and in the absence of external magnetic field. For this purpose, samples were subjected to a constant shear stress at different moments (steps) of the settling process. The time evolution of the corresponding shear rate was measured at each step. Interestingly, it was found that although the addition of oleic acid or aluminum stearate does not avoid particle settling, the redispersibility of the suspensions is considerably enhanced. On the contrary, silica nanoparticles behave as a gel-forming agent capable of preventing particle settling under rest conditions. Unfortunately, when iron-silica suspensions are sheared compact sediments are progressively formed, making the redispersion extremely difficult.

Coherent X-rays as a new probe for the investigation of the dynamics of opaque colloidal suspensions

Third generation synchrotron radiation sources open up the possibility of using the coherence properties of the produced X-ray beam. The novel X-ray photon correlation spectroscopy (XPCS) technique probes slow dynamics of disordered material in overcoming the limitations usually occurring using photon correlation spectroscopy with coherent visible light. It is extending the accessible Q-range to short wavelength density fluctuations and is not sensitive to multiple scattering effects; thus allowing the investigation of the dynamics of opaque materials such as magnetic colloidal suspensions.

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

Thermosensitive gold nanoparticles.

Thermosensitive gold nanoparticles were fabricated by conjugating Au with a thiol-terminated poly(N-isopropylacrylamide) or PPA; this polymer stabilizer exhibits a temperature transition while undergoing a hydrophilic to hydrophobic transformation. The introduction of PPA onto gold nanoparticles has sensitized Au nanoparticles with unique temperature dependence. At low temperature (25 degrees C), the solutions containing PPA-functionalized gold nanoparticles are transparent, whereas higher temperatures (30 degrees C) lead to opaque suspensions. The thermosensitive property of PPA-functionalized Au nanoparticles is reversible, and the clear-opaque suspensions can be repeated many times.

Possible optimisation of pastes and the according apparatus in process engineering by MRI flow experiments

In this paper, the spatially resolved determination of velocities in suspensions by means of nuclear magnetic resonance (NMR) imaging (MRI) techniques is described and applied to steady tube flows (with regard to the total flow rate) in different geometries. Three types of suspensions with different solid volume concentrations are examined in order to demonstrate the influence of the material-specific flow-behaviour and of the geometry of the experimental set-up on the observed flow-pattern. MRI offers the possibility to study the flow of multiphase materials with various forms of contrast (e.g. spin density, relaxation and spectroscopic data, diffusive and convective transport) spatially resolved. Thus even optically opaque suspensions can be studied in the interior of the material and the studied device. In this work the local probability distribution of single velocity components is determined. From this probability distribution both the local mean value and the according standard deviation can be derived. The standard deviation can be interpreted as the local dispersion coefficient of the according velocity component.

Microdivers to study sedimentation in polydisperse, concentrated colloidal suspensions

AbstractA small amount of polymer particles with diameters in the 100 μm range can be used as an ensemble of “microdivers” to study sedimentation velocities of polydisperse submicron particles in concentrated opaque suspensions. This tracer technique, based on Berg's (1941) diver method, is simple, measures velocities directly, and does not require underlying theory beyond conservation of momentum. It confirms that the polymer tracers reside at the position in the suspension where the suspension density corresponds to their own density. Aqueous inorganic suspensions of 250–750 nm α‐alumina particles were studied, which is electrostatically stabilized with nitric acid in water, using ∼100 μm polymethylmetacrylate and polystyrene particles as tracers. Experiments were compared with predictions by state‐of‐the‐art continuum models for multicomponent sedimentation to underpin the validity of the tracer technique.

Inversion algorithms for particle sizing with photon migration measurement

AbstractA new linear inverse method is demonstrated for recovery of particle‐size distribution (PSD) and volume fraction of opaque colloidal suspensions from frequency domain photon migration (FDPM) measurement of multiply scattered light. To deal with the ill posedness of PSD retrieving problem, the size distribution is approximated by the B‐spline expansion. Tikhonov regularization, along with both nonnegativity and smoothing constraints, in which the L‐curve method is employed to choose a regularization parameter, are also used to ensure the stable and realistic solution. The algorithm is tested using synthetic FDPM measurements at a variety of size distributions, and results are compared using other inverse methods. The influences of model and measurement error are investigated. To demonstrate our inversion approach, PSD and volume fraction of opaque titanium dioxide suspensions are recovered from actual FDPM measurements. This work demonstrates the ability of FDPM method to characterize colloidal suspensions.

Tailoring nanoparticles of aromatic and dye molecules by excimer laser irradiation

Nanosecond 351nm excimer laser irradiation of microcrystalline powders of aromatic hydrocarbons and phthalocyanine (Pc) dyes in water results in conversion from opaque suspensions to transparent colloidal solutions. Formation of ∼100nm-sized particles are successfully confirmed for oxo(phthalocyaninato)vanadium(IV) (VOPc) by AFM observation. The particles in solution are stable, while some Pc colloid solutions show interesting slow UV absorption spectral changes, which is interpreted in terms of phase transition. It is discussed that the present laser ablation of solution is regarded not only as a new nanoparticle formation method but also as a control way of molecular aggregation in nanoparticles.

Ultrasonics for process monitoring and control

Ultrasonic waves are well suited for use in measurements that characterize multiphase fluids and flows. The waves can interrogate fluids and dense optically opaque suspensions. Transducers can give signals that penetrate vessels. Such transducers, together with supporting instrumentation, can provide on-line real-time measurements for use in process monitoring and control. Staff at the Pacific Northwest National Laboratory have developed a family of devices that measure fluid and slurry density, viscosity, flow rheology, particle size distribution, concentration, and velocity profiles. The transducers have been deployed on model systems to provide data that can be integrated to provide a more complete characterization of a process streams, including to detect time-dependent changing interfaces caused by fouling or phase changes, to track process conditions during mixing, sedimentation, stratification, and slurry transport. A summary of the fundamental physical acoustics for wave-process stream interactions will be provided. Examples that illustrate the capabilities of each type of measurement applied to slurry systems at Hanford site and for wood pulp process streams will be presented. The potential for enhanced process monitoring and control will be illustrated using wood pulp data, where it is required to measure the pulp consistency and degree of refining.

Object delineation within turbid media by backscattering of phase-modulated light

Backscattering of phase-modulated light is shown to reveal the presence of targets embedded in visually opaque suspensions of diffusively scattering particles. Signals at the modulation frequency I( f) and first harmonic I(2 f) give nearly instantaneous measures of the difference in intensities of orthogonal states of circular and linear polarizations, respectively. Examination of different kinds of targets demonstrates the sensitivity of polarization-selective imaging to (a) edges, (b) surface texture, and (c) polarization-dependent absorption of the embedded object.

Hemolysis of mammalian erythrocytes mediated by the myeloperoxidase, lactoperoxidase and saliva peroxidase systems.

The hemolysis of horse, human and pig erythrocytes mediated by myeloperoxidase, lactoperoxidase and saliva peroxidase was studied using turbidimetry and microscopic analysis. The addition of myeloperoxidase, a hydrogen peroxide-generating system (glucose+glucose oxidase) and chloride or bromide caused the opaque suspensions of these mammalian erythrocytes to change into transparent (orange to yellowish green) solutions. Under the microscope, cells became dim circles and faded from the field of vision during the decrease in turbidity. Myeloperoxidase could be replaced by lactoperoxidase or saliva peroxidase in the system containing bromide, but not in that containing chloride. Hemolysis by these peroxidase systems was observed in the range of moderately acidic pH, with the optimum pH values being between 5.0 and 5.5. The present study also showed that turbidimetry and microscopic analysis are useful in assessing hemolysis by the peroxidase-hydrogen peroxide-halide systems.

Book review: Scattering and localization of classical waves in random media

This book contains a collection of papers relating to the propagation of classical waves in multiply scattering random media. It is recommended to anyone who wishes to become familiar with the myriad interesting investigations on this subject that were carried out, in rapid succession, during the period between 1983 and 1990. Although the articles primarily deal with the basic physics of wave propagation, the phenomena discussed in this volume underlie many practical applications. Examples of the latter include the scattering of acoustic waves in disordered solids, the diffusion of thermal waves in random composites, and the transmittance of light through optically opaque suspensions. The articles appearing in this collection deal with many aspects of the subject, and illustrate the richness of both the theoretical and experimental investigations that have been carried out. The book contains 11 chapters, each essentially by a different group of authors. Hence, a number of different points of view and approaches to these problems are covered: several contributions are written as juxtapositions of theory and experiment, others contain rather formal mathematical treatments, others demonstrate the power of phenomenological reasoning and inference. Various chapters contain extensive lists of references, making the book particularly useful to persons seeking to penetrate the literature for the first time. Each of the contributions is of high quality. The editor deserves compliments on his choice of authors and his diligence in guiding the organization and writing of the chapters. The book is an appropriate addition to the personal library of anyone working in a field involving

A novel sedimentation analyser

The authors describe a novel instrumentation system capable of measuring the particle volume fraction and solids flux of optically opaque suspensions undergoing sedimentation. Two independent but complementary measurement techniques-capacitance and pressure-are integrated into the system.

Experimental techniques for monitoring sedimentation in optically opaque suspensions

A review of established and developing techniques for monitoring solid flux, and in some cases particle size, is presented for sedimenting multi-phase systems that are opaque and therefore unsuited to interrogation using conventional optical methods. The techniques are classified according to the physical principle employed for the measurement, relative cost of the equipment, precision, versatility, ease of operation, state of development and their ability to function in non-invasive or invasive modes. Requirements and limitations of these techniques are discussed with respect to their application to colloidal dispersions, emulsions and to mixed colloidal/non-colloidal suspensions. Practical examples of phase-flux determinations are given for devices involving (i) ultrasonics, (ii) capacitance, (iii) inductance and (iv) direct measurement by physical interruption and sampling of a sedimentation process.

A rheological study of packed red blood cell suspensions with an oscillating ball microrheometer.

Rheological properties of concentrated red blood cell suspensions are studied with a magneto acoustic microrheometer in which a ball is suspended in a vertically oriented cylindrical tube. The rheometer uses a conventional falling ball technique to measure steady state viscosity and a vertically oscillating, magnetically driven ball for viscoelastic measurements. The motion of the ball is tracked by ultrasound echo location in which sound waves are transmitted and received by an ultrasound transducer mounted at the base of the tube. The compact size of the rheometer allows rheological studies to be made with microliter quantities of opaque suspensions and permits sudden and accurate changes in temperature. Also, values for the adiabatic compressibility are evaluated from measurements of the speed of sound.

Magnetically driven, acoustically tracked, translating-ball rheometer for small, opaque samples

A new rheometer has been designed to measure the rheological properties at low Reynolds number of microliter quantities of opaque suspensions. The rheometer uses a falling-ball technique to measure steady-state viscosity and a vertically oscillating, magnetically driven ball for viscoelastic measurements. The motion of the ball is tracked by ultrasound echo location, in which sound waves are transmitted and received by an ultrasound transducer mounted at the base of the tube. Concentrated suspensions of red blood cells are used as opaque test samples. The results obtained are in good agreement with those reported in the literature. The data confirm the fact that a concentrated suspension of red blood cells behaves as a shear thinning material and that the energy stored by the suspension during an oscillatory cycle increases with frequency. Testing of the rheometer is also made by using a Newtonian silicone oil. Viscosity measurements obtained with both the falling- and oscillating-ball methods are consistent and are within 2% of the value of 47.3±0.5 cP given by the Cannon-Fenske viscometer. However, it is found that the oscillating-ball technique gives the largest standard deviation, 6%, as opposed to 2% for the falling-ball technique.

Continuum approximations and particle interactions in concentrated suspensions

Falling‐ball rheometry is used to study hydrodynamic forces in concentrated suspensions. The suspensions consist of large, uniform spheres (diameter ds=0.32 cm) neutrally buoyant in a viscous, Newtonian liquid. Falling‐ball experiments are performed by dropping steel balls of various diameters (df) through suspensions held in cylindrical containers of several diameters. In these optically opaque suspensions, the passage of the falling ball is observed with real‐time radiography and is recorded with digitized, high‐speed video. The average terminal velocities of the falling balls are used to calculate an apparent viscosity of each suspension (ηs). Extrapolation of the data to estimate the velocity which would occur in a cylinder of infinite diameter is used to find the apparent viscosity with zero wall effects (ηs∞). Each ηs∞ agrees with independent measurements throughout the solids concentration range of the data (0.0≤φ≤0.45). In the absence of wall effects, ηs∞ is independent of ds /df. That is, all sizes of falling balls (0.75≤df /ds≤12.0) experience the same mean stress field. In addition, it is seen that in dilute and moderately concentrated suspensions (0.0<φ<0.30), wall effects are identical to those in equivalent Newtonian liquids. At higher solids loadings, we observe additional wall effects that can act over much greater distances than in equivalent Newtonian liquids. The magnitude of additional wall effects increase rapidly as φ increases above 0.30.

Some new developments in testing equipment for paints and inks

Optoacoustic spectroscopy is a completely new technique for the examination of solid and liquid samples and a system for this is now commercially available in the UK in the form of EDT's Model OAS 400 Opto‐Acoustic Spectrometer. This attrac‐tive instrument has been developed at EDT Research, 14 Trading Estate Road, London NW10 7LU, with the backing of the National Research Development Corporation from the pioneering work in the field by Dr G. F. Kirkbright of Imperial College, London. The principles of OAS rest on the fact that light absorbed by a solid sample is converted to heat with the sample. If the sample is sealed in a small cell containing a gas e.g. air or nitrogen the heat will be transferred to the gas and cause it to expand. If the incident light beam is now chopped at a steady rate, the gas in the cell will be alternately heated and cooled leading to a series of pressure pulses which can be detected by microphone. Scanning the wave‐length of the light and recording the associated sound amplitude leads to an absorption spectrum analogous to that of more conventional methods of spectrometry. The advantages of OAS lie in the fact that spectra of opaque substances, powders, polymers, suspensions and slurries can be made without difficulties arising from lack of transmission, scattering or reflection.

An improved, totally automated antibiotic bioassay machine.

AbstractA number of improvements have been made in a totally‐automated antibiotic bioassay machine previously described. The new machine accepts unmeasured, untreated, opaque suspensions of fermentation beers three times faster (120 samples per hour) and supplies printed potencies sooner (in just over two hours). Whereas the original machine employed a self‐cleaning filter and used disposable two milliliter beakers, this version involves a batch‐dialysis scheme for effecting sample purification, and provides for automated cleaning of incubation chambers.In operation, a measured, portion of thoroughly‐mixed fermentation beer is automatically diluted and transferred into one side of an incubation chamber, the two halves of which are separated by a dialysis membrane. The other half is filled with inoculated media. During the two hour incubation at 37°, dialyzable antibiotic limits growth of the inoculum in proportion to its concentration. After incubation, the turbidity of the inoculum is simultaneously read by an online computer and plotted on a strip chart recorded. The computer suplies printed potency values and sample identification on site, while the recording provides the operator with an analog record of turbidity. Fiber optics are employed in the turbidmetric readout, and an electric typewrite provides the printout.