Concentrated Silica Suspensions Research Articles

Wavenumber-dependent dynamic light scattering optical coherence tomography measurements of collective and self-diffusion

We demonstrate wavenumber-dependent DLS-OCT measurements of collective and self-diffusion coefficients in concentrated silica suspensions across a broad q-range, utilizing a custom home-built OCT system. Depending on the sample polydispersity, either the collective or self-diffusion is measured. The measured collective-diffusion coefficient shows excellent agreement with hard-sphere theory and serves as an effective tool for accurately determining particle sizes. We employ the decoupling approximation for simultaneously measuring collective and self-diffusion coefficients, even in sufficiently monodisperse suspensions, using a high-speed Thorlabs OCT system. This enables particle size and volume fraction determination without the necessity of wavenumber-dependent measurements. We derive a relationship between the particle number-based polydispersity index and the ratio of self and collective mode amplitudes in the autocorrelation function and utilize it to measure the particle number-based polydispersity index. Notably, the polydispersity determined in this manner demonstrates improved sensitivity to smaller particle sizes compared to the standard intensity-based DLS cumulant analysis performed on dilute samples.

Ultrasound attenuation and phase velocity of moderately concentrated silica suspensions

Ultrasound attenuation coefficient and phase velocity of moderately concentrated suspensions of charged silica particles were measured as a function of frequency. The attenuation coefficients were found to be significantly smaller than the theoretical prediction, and such a difference did not appear in the neutral particle suspensions under the corresponding concentrations. In this study, we have investigated the acoustic spectra of silica particles with different particle sizes, concentrations as well as salt concentrations. It was revealed that the noticeable deviation from the theoretical estimation only appeared in the case of large particle sizes close to the wavelength of ultrasound, and could be circumvented by addition of small amount of salt to suppress the electrostatic interactions of the charged silica particles.

Analysis and optimization of viscosity of concentrated silica suspensions by response surface methodology (RSM): Control of particle modality

ABSTRACTRheology of dense polymer suspensions is of common importance, especially in the ceramics, composites, energetics, drilling fluids, magnetic and pharmaceutical researches. In the present work, the viscosity of concentrated suspensions of polydimethylsiloxane (PDMS) as fluid matrix and silica particles as the dispersed solid phase was modeled by response surface methodology (RSM). The effect of volume fraction of the particles () and modality parameters such as large-to-small particle size (λ) and weight (δ) ratio on the zero shear viscosity of suspensions (µ0) was studied, and a good agreement between the model and experimental data was observed. The results of mathematical modeling showed that the minimum µ0 was obtained at the highest value of (0.768), when δ and λ were equal to 2.75 and 20, respectively. Moreover, the viscosity of the samples with a composition minimized by a theoretical model proposed by Yu and Standish was compared with that of obtained using RSM. At the same solid loading (74.15 vol%), the RSM-optimized zero shear viscosity (∼ 15000 Pa .s) was much lower than that obtained from Yu and Standish method (∼ 32000 Pa .s), which confirms the ability of RSM to optimize the viscosity of highly filled dispersion systems.

Tuning of the rheological properties of concentrated silica suspensions using carbon nanotubes

In this study, the rheological behavior of fumed silica nanoparticles suspended in polyethylene glycol (PEG) and those containing multi-walled carbon nanotubes (MWNTs) was measured at steady and oscillatory shear stress using a stress-controlled rheometer. The obtained results showed that the critical viscosity of the concentrated silica suspension, decreased with the addition of MWNTs. Moreover, the onset of shear thickening for the suspension containing MWNTs appeared at higher shear rate than those without MWNTs. Oscillatory shear measurement shows that the storage modulus and loss modulus of silica suspension were higher than the suspension containing MWNTs. The higher complex viscosity of silica suspensions than the suspension containing MWNTs was also evident over the entire range of the frequency studied. The proposed mechanism of nanotube incorporation in dispersion revealed that the observed shear-thickening behavior of a suspension containing MWNTs could be attributed to the increased interaction force between MWNTs and PEG, as a result of increase in the number of hydrogen bonds. This finding suggests that MWNTs could be used as an efficient tool for tuning of the onset of shear thickening and rheological behavior of concentrated silica suspensions.

Factors controlling the formation and stability of foams used as precursors of porous materials

The remarkable stability of particle-stabilized foams and the opportunity to use them for production of novel porous materials have been attracting the researchers’ attention in the recent years. The major aim of the current study is to clarify the factors, controlling the foamability and stability of foams, formed from concentrated silica suspensions in the presence of the amphoteric surfactant CAPB. The experiments showed that: (1) two regions can be defined with respect to suspension foaminess: Region 1 – good foaming and Region 2 – strongly suppressed foaming. The foam volume decreased linearly with the increase of suspension viscosity, so that Region 2 appears as a result of the excessively high suspension viscosity. (2) Based on foam stability four sub-regions were observed in Region 1: region 1S – stable foams, which can be dried to form stable porous materials; 1UD – foams which are unstable with respect to water drainage and collapse upon drying; 1UC – foams which are stable to drainage, but are unstable to drying, due to crack formation; 1UF – unstable foams which completely fall apart upon drying. (3) Foams in Regions 1S and 1UC had yield stress above 10Pa which prevented liquid drainage, while foams from Region 1UD drained because of their lower yield stress. (4) The particles in the foams assembled in a 3D network in the Plateau channels and the nodes, while surfactant stabilized the foam films between the bubbles. These results define the conditions, appropriate for formation of stable, highly porous silica materials with low mass density, which can be further modified (sintered, impregnated, hydrophobized, etc.) to serve as catalyst supports, porous filters, insulating materials, etc.

1P2-20 Sedimentation Dynamics of Concentrated Silica Suspensions by Means of Dynamic Ultrasound Scattering Method(Poster Session)

1P2-20 Sedimentation Dynamics of Concentrated Silica Suspensions by Means of Dynamic Ultrasound Scattering Method(Poster Session)

Effect of silica sol on the dispersion–gelation process of concentrated silica suspensions for fibre-reinforced ceramic composites

Sol gel process of silica powders dispersed in silica sol has been used to obtain a suitable suspension for the sol infiltration technique of fibre-reinforced ceramics. Efforts have been focused on analysing the effects of polyelectrolyte content, pH, solid load and concentration of gelling agent on the flow behaviour of silica suspensions. The most adequate suspension to manufacture the matrix of composites is a weak flocculated suspension with negligible thixotropic behaviour at pH ≥ 9.5, which is composed of silica microparticles dispersed in silica sol with 41 vol.% of solid load and 1.8 wt.% of Duramax D3005. This suspension easily undergoes a transition to a gel by a slight alteration of stability conditions adding 0.08 M of NH4Cl, which reduces both the electrostatic repulsion and the pH to 8. Moreover, silica sol promotes the densification of ceramics up to 70% after sintering at 900 °C, allowing porous matrix processing of the composites.

Rheological behavior of concentrated silica suspension and its application to soft armor

AbstractThe objective of this study is to develop an advanced stab and/or ballistic proof material composed of shear thickening fluid (STF) and Kevlar composite fabric. In this study, we prepared STF using sphere silica and fumed silica as silica particles and ethylene glycol and polyethylene glycol (PEG 200) as medium fluid, respectively. And the rheological properties of the STF were investigated under different conditions. Also, we impregnated Kevlar fabrics with the STF, and investigated the stab and ballistic resistances of the targets layered by the STF impregnated Kevlar fabrics. From the results, we observed that the STF significantly showed the reversible liquid–solid transition at a certain shear rate, and the STF treatment significantly improved the stab and ballistic resistance of Kevlar fabric. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Yield stress behaviour of concentrated silica suspensions with temperature-responsive polymers

Rheological investigations have determined the influence of temperature and solution pH on particle interactions in concentrated silica suspensions dosed with poly( N-isopropyl acrylamide) (PNIPAM) or polyacrylamide (PAM). At 20 °C, suspensions with PNIPAM displayed DLVO behaviour. Heating suspensions to 50 °C, which is above the lower critical solution temperature (LCST) of PNIPAM, caused the molecular segmental repulsion to switch to attraction with higher adsorbed amounts, promoting attraction between the particles and higher shear yield stresses that deviate from DLVO behaviour. This weak flocculation was partially reversible and lower shear yield stresses resulted after cooling below the LCST back down to 20 °C. PAM caused strong flocculation through bridging-adsorption and electrical double layer screening and much higher shear yield stresses resulted that did not vary much with temperature.

The influence of ingredients of silica suspensions and laser exposure on UV curing behavior of aqueous ceramic suspensions in stereolithography

This paper focused on the influence of ingredients of silica suspensions and laser exposure on curing behavior of the highly concentrated aqueous silica suspensions. As a basic building unit, single cured lines were regarded as research objects to characterize the curing behavior of aqueous ceramic suspensions. The cured width and depth were characteristic parameters of single cured lines and measured with a digital optical microscope. The relationships between two characteristic parameters and ingredients of highly concentrated aqueous ceramic suspensions and laser exposure were investigated. The cured depth and width of single cured lines increased with the ceramic mean diameter and monomer concentrations. The cured width of single cured lines decreased with the solid content, but the cured depth increased with the solid content for silica suspensions. The cured depth and width of the single cured line all decreased with the laser scanning speed. The experimental results show that the ingredients of ceramic suspensions and laser exposure all have great influence on curing behavior of the highly concentrated silica suspensions, which indicates that the formula is an intrinsic factor on the curing behavior of ceramic suspensions and laser exposure is an exterior factor.

Centrifugal separation in tube and disc geometries: experiments and theoretical models

The centrifugal separation of particles or droplets in dispersions is important in a variety of applications, e.g. mineral processing, water and waste water treatment, multistep processing of nanomaterials and biotechnology. Despite numerous studies in the past, separation processes of concentrated dispersions driven by gravity or centrifugal forces are not yet completely understood. Centrifuges can be subdivided into two categories: (1) centrifuges operating with tubes of constant cross section and (2) centrifuges operating with disc or cylinder rotors whose cross-sectional area changes with distance along the axis of rotation. This paper investigates the sedimentation process in centrifuges of these two categories experimentally and theoretically. Based on models for the separation of polydisperse dispersions in centrifugal field numerical calculations were carried out including the conversion of the time dependent concentrations profiles into light transmission profiles. The changes in the dispersion concentration profiles during the sedimentation process were monitored in situ by space- and time-resolved NIR light extinction profiles (STEP-Technology) obtained by multisample analytical centrifugation. Cells having constant and increasing cross sections were used. Results show that the velocity of the boundary between supernatant and dispersion as well as the alteration of the concentration measured radially in the centre of the cells do not depend on the geometry of the cells within the experimental errors. The sediment height in cells with increasing cross section is smaller compared to cells with constant cross section. The simulated sedimentation process is in good agreement with measured data for diluted and concentrated silica suspensions. Based on these results the sedimentation processes in disc or cylinder process centrifuges can be predicted from laboratory tests using analytical centrifugation.

Phase transitions of pyrogenic silica suspensions: A comparison to model laponite

Pyrogenic silica is often used as a thickening agent in paints, pastes, adhesives, or resins. Other applications include, e.g., abrasives in chemical mechanical planarization in the microelectronics industry. In all these applications it is essential to control the state of dispersion. Sometimes, phase transitions from the liquid to the solid state are required while in other cases they have to be completely avoided for the whole shelf life. The nature and influencing parameters of the fluid-solid transition for pyrogenic silica have not been investigated so far. Most investigations deal with the phase transitions of small clay particles such as laponite. Here, we dedicate our interest to the behavior of pyrogenic silica suspensions with varying specific surface area and ionic background concentration. To get an impression of the phase transition behavior we compare our results to model laponite suspensions. We apply dynamic light scattering measurements in the backscattering regime to minimize multiple scattering contributions from concentrated pyrogenic silica suspensions. Further on we exert a decomposition of the measured autocorrelation functions into an ergodic and nonergodic contribution. The analysis of the ergodic spectrum yields two different gelation kinetics for both systems, laponite and pyrogenic silica. For laponite these are in accordance with earlier investigations. The kinetics depend on the ionic background and the solids content of the suspensions. Additionally, we used dynamic extinction spectroscopy to follow the phase transitions of pyrogenic silica on a macroscale.

Shear viscosity of concentrated spherical silica suspension – effect of polymethylsiloxane surface treatment

In determining the formulation of cosmetics dispersed in powder, it is very important to discover the rheological behavior of the suspension. In the case of concentrated particle suspension, it is known that the remarkable behavior of the rheology, which is different from the dilution system, is shown in ways such as shear thickening. In this report, the rheological behaviors of a concentrated spherical silica particle system were investigated in terms of particle–particle and particle–medium interaction. In the case of an aqueous medium, shear thickening occurred under certain conditions. It is noteworthy that the pH of an aqueous medium far from the isoelectric point (pH 4) of silica particles resulted in a decrease in shear viscosity η due to an increase in the double‐layer repulsion. In addition, the rheological behaviors of suspension using the silica particles, which were treated with polydimethylsiloxane and methylhydrogen polysiloxane as the solvent system, were investigated. Shear thickening disappeared in all of the solvent systems, and polydimethylsiloxane system showed lower viscosity than the methylhydrogen polysiloxane system. The surface treatment, which affects the shear viscosity, is changed not only by the particle–particle interaction but also the particle–medium interaction.

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles (ΔpH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH (Δ I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness (≈13 μm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the ΔpH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Δ I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.

Rheological properties of concentrated aqueous silica suspensions: Effects of pH and ions content

The rheological behavior of concentrated aqueous suspensions of nearly monodisperse submicron, spherical silica particles was studied in Couette and vane geometries. End corrections and wall depletion effects were found to be important. The apparent yield stress and shear viscosity were investigated in the light of interactions between the charged silica particles. The effects of pH as well as the addition of electrolyte were examined. The suspensions in de-ionized water without addition of acid, base, or electrolyte gave the largest apparent yield stress and shear viscosity, while the addition of base, acid, and KCl resulted in significant decreases of the apparent yield stress as well as of the viscosity. These effects have been interpreted in light of the DLVO theory and the compression of the double layer around the solid particles (Debye length).

Ion-specific effect on the gelation kinetics of concentrated colloidal silica suspensions

The comparative gelation kinetics of concentrated colloidal silica suspensions at pH 9.8 has been studied in the presence of two different counterions, ammonium as a representative structure-breaker ion and sodium as a representative structure-maker ion. For the same ionic strength of 0.45 M, the experimental results have indicated a pronounced effect of the ion-specificity on the aggregation kinetics and on the microstructure of the gelified networks at the sol–gel transition. This was ascribed to the significant difference of the critical coagulation concentration value in the presence of the two electrolytes.

Rheological behavior and stability of concentrated silica suspensions

In the present article, the rheological behavior and phase stability of concentrated silica suspensions were investigated experimentally by examining the effects of particle size and temperature. The silica particles were stabilized by adsorption of a silane coupling agent, γ-methacryloxypropyl triethoxy silane (MPTES). The MPTES-coated silica particles behaved like hard spheres and exhibited the dispersion stability in tetrahydrofurfuryl alcohol which was used as a refractive-index matching solvent. For a monodisperse suspension, the limiting viscosities at high shear rates were correlated satisfactorily with the Krieger–Dougherty equation until the particle volume fraction φ reached 0.45 above which the limiting high-shear-rate viscosities did not exist. The highly concentrated monodisperse suspensions above φ=0.50 displayed the rapidly shear thinning viscosity at low shear rates and underwent the shear thickening at high shear rates. Specifically, the onset of shear thickening shifted to a higher shear rate either as temperature rose or as the suspension became stable. The dichroism from light passed in the flow-gradient direction probed the most effectively the order–disorder transition such as disappearance of hexagonally ordered layered structure and formation of particle clustering, which caused shear thickening at high shear rates. The abrupt change in dichroism caused by the breakdown of hexagonally layered structure occurred almost 1 decade before the onset of shear thickening. Finally, the bimodal suspensions prepared here possessed the reduced shear viscosity and stress in comparison with the monodisperse suspensions of identical volume fraction. The degree of the viscosity reduction became conspicuous for concentrated suspensions usually above φ=0.40.

Nonlinear rheology of concentrated spherical silica suspensions: 3. Concentration dependence

Nonlinear rheology was examined for concentrated suspensions of spherical silica particles (with radius of 40 nm) in viscous media, 2.27/1 (wt/wt) ethylene glycol/glycerol mixture and pure ethylene glycol. The particles were randomly and isotropically dispersed in the media in the quiescent state, and their effective volume fraction φeff ranged from 0.36 to 0.59. For small strains, the particles exhibited linear relaxation of the Brownian stress σB due to their diffusion. For large step strains γ, the nonlinear relaxation modulus G(t,γ) exhibited strong damping and obeyed the time-strain separability. This damping was related to γ-insensitivity of strain-induced anisotropy in the particle distribution that resulted in decreases of σB/γ. The damping became stronger for larger φeff. This φeff dependence was related to a hard-core volume effect, i.e., strain-induced collision of the particles that is enhanced for larger φeff. Under steady/transient shear flow, the particles exhibited thinning and thickening at low and high γ˙, respectively. The thinning behavior was well described by a BKZ constitutive equation using the G(t,γ) data and attributable to decreases of a Brownian contribution, σB/γ˙. The thickening behavior, not described by this equation, was related to dynamic clustering of the particles and corresponding enhancement of the hydrodynamic stress at high γ˙. In this thickening regime, the viscosity growth η+ after start-up of flow was scaled with a strain γ˙t. Specifically, critical strains γd and γs for the onset of thickening and achievement of the steadily thickened state were independent of γ˙ but decreased with increasing φeff. This φeff dependence was again related to the hard-core volume effect, flow-induced collision of the particles enhanced for larger φeff.

Nonlinear rheology and flow-induced structure in a concentrated spherical silica suspension

In hard-sphere suspensions of solid particles, the stress has the Brownian (thermodynamic) and hydrodynamic components σB and σH, the former reflecting the anisotropy of the particle distribution while the latter being determined by the hydrodynamic interaction between the particles. These two components exhibit nonlinearities under steady shear flow with different mechanisms. The nonlinearity of σB results from the particle distribution insensitive to the shear rate, while σH becomes nonlinear due to flowinduced clustering of the particles. These structural origins of the nonlinearities were confirmed from flow-SANS experiments.

Nonlinear rheology of a concentrated spherical silica suspension: 2. Role of strain in shear-thickening

Nonlinear rheology of a concentrated spherical silica suspension: 2. Role of strain in shear-thickening