Suspensions Of Silica Particles Research Articles

Characterizing stress development and cracking of ceramic particulate coatings during drying

AbstractDuring drying, liquid‐applied particulate coatings develop stress and are consequently prone to stress‐induced defects, such as cracking, curling, and delamination. In this work, the stress development and cracking of coatings, prepared from aqueous silica and zinc oxide particle suspensions, were characterized using cantilever beam deflection with simultaneous imaging of the coating surface. Drying uniformity was improved and lateral or edge‐in drying was discouraged by using thin silicone walls around the perimeter of the cantilever. Coatings prepared from larger monodisperse silica particles (D50 ∼ 0.9 µm) dried uniformly but had a high critical cracking thickness (>150 µm) that prevented simultaneous study of stress development and cracking. Coatings prepared from smaller silica particles (D50 ∼ 0.3 µm) cracked readily at low thicknesses but exhibited edge‐in drying that complicated the stress measurement data. This drying nonuniformity was connected to the potential for these small particles to accumulate at the coating surface during drying. Hence, the selection of particle size and density was critical to drying uniformity when characterizing stress development and cracking. Coatings prepared from suspensions of zinc oxide particles (D50 ∼ 0.4 µm) were well‐suited for these studies, with uniform drying stress peaking at ∼1 MPa. Characteristic features in the stress development data above and below the critical cracking thickness (53 µm) were identified, demonstrating that cantilever beam deflection is a useful tool for studying the effectiveness of crack mitigation methods and the fundamentals of coating fracture during drying.

Effects of salinity on the flow of dense colloidal suspensions.

We experimentally study the effects of salt concentration on the flowing dynamics of dense suspensions of micrometer-sized silica particles in microfluidic drums. In pure water, the particles are fully sedimented under their own weight, but do not touch each other due to their negative surface charges, which results in a "frictionless" dense colloidal suspension. When the pile is inclined above a critical angle θc ∼ 5° a fast avalanche occurs, similar to what is expected for classical athermal granular media. When inclined below this angle, the pile slowly creeps until it reaches flatness. Adding ions in solution screens the repulsive forces between particles, and the flowing properties of the suspension are modified. We observe significant changes in the fast avalanche regime: a time delay appears before the onset of the avalanche and increases with the salt concentration, the whole dynamics becomes slower, and the critical angle θc increases from ∼5° to ∼20°. In contrast, the slow creep regime does not seem to be heavily modified. These behaviors can be explained by considering an increase in both the initial packing fraction of the suspension Φ0, and the effective friction between the particles μp. These observations are confirmed by confocal microscopy measurements to estimate the initial packing fraction of the suspensions, and AFM measurements to quantify the particles surface roughness and the repulsion forces, as a function of the ionic strength of the suspensions.

Advanced Additive Manufacturing of Structurally-Colored Architectures.

Direct ink writing (DIW) stands out as a facile additive manufacturing method, minimizing material waste. Nonetheless, developing homogeneous Bingham inks with high yield stress and swift liquid-to-solid transitions for versatile 3D printing remains a challenge. In this study, high-performance Bingham inks are formulated by destabilizing silica particle suspensions in acrylate-based resin. A colloidal network forms in the shear-free state through interparticle attraction, achieved by disrupting the solvation layer of large resin molecules using polar molecules. The network is highly dense, with evenly distributed linkage strength as monodisperse particles undergo gelation at an ultra-high fraction. Crucially, the strength is calibrated to ensure a sufficiently large yield stress, while still allowing the network to reversibly melt under shear flow. The inks immediately undergo a liquid-to-solid transition upon discharge, while maintaining fluidity without nozzle clogging. The dense colloidal networks develop structural colors due to the short-range order. This enables the rapid and sophisticated drawing of structurally-colored 3D structures, relying solely on rheological properties. Moreover, the printed composite structures exhibit high mechanical stability due to the presence of the colloidal network, which expands the range of potential applications.

Bacterial nanocellulose membrane as novel substrate for biomimetic structural color materials: Application to lysozyme sensing

The development of optical biosensors based on structural colors generated by short-range ordered colloidal particles is attracting growing interest due to their non-iridescent and non-fading features. In this study, a biomimetic approach using biopolymers for the various steps of sensor construction is presented. Bacterial nanocellulose (BNC) has many foreseen applications in biomedical engineering because of its biocompatibility, good mechanical strength, and large modifiable surface area. Herein, a novel approach is taken by using functionalized BNC as a substrate to build a molecularly imprinted photonic sensing layer. BNC was modified with polydopamine (PDA), which improved the adhesion and mechanical properties of the BNC substrate while providing simultaneously a black background for color saturation. A molecularly imprinted polymer (MIP) also made of PDA was used to create the recognition sites for the biomarker lysozyme. A monodisperse colloidal suspension of silica particles was first synthesized and used as core of the MIP shell, and then the photonic structure was assembled on the PDA-BNC membrane. The biosensor showed a detection limit of about 0.8 nmol L−1 of lysozyme in spiked human serum and demonstrated to be selective against cystatin C. These properties, combined with biocompatible, eco-friendly, and low-cost materials, offer a sustainable sensing platform with great potential for healthcare applications.

Effect of a double-chain surfactant on the stabilization of suspensions of silica and titania particles against agglomeration and sedimentation

Dispersions of vesicles of didodecyldimethylammonium bromide (DDAB) were prepared with magnetic stirring of multilamellar liposomes followed by ultrasonication (SS) in order to generate dispersions that contained only unilamellar vesicles, unlike the stirring-only (S) method which also produces many large liposomes. With the SS method, the vesicles had average diameters ranging from 68 to 80 nm, which were much smaller than the diameters generated with the S method. The new vesicle dispersions had nearly infinite viscosities at very low shear stresses at DDAB weight fractions wD from 0.025 to 0.027, even though they were not close-packed, in contrast to the S dispersions. The new SS vesicle dispersions stabilized completely against sedimentation polydisperse nonspherical crystalline titania particles with sizes ranging from ca. 96–156 nm, as well as suspensions of monodisperse spherical amorphous silica particles with diameters of dsed = 454 nm, 691 nm, and 826 nm. These results establish that DDAB vesicle dispersions can stabilize against sedimentation a broad range of particle types and sizes. In addition, at the low values of wD = 0.009 and 0.018, the effective viscosities, ηeff, of the DDAB dispersions, determined from the sedimentation velocities, ranged from 1.35-1.87 cP and 4.34–5.57 cP, respectively. At higher shear stresses, however, these dispersions were highly shear-thinning and still flowable in a capillary under gravity. This behavior is critical for the practical applications of such dispersions for inkjet printing and possibly paints.

Climbing Colloidal Suspension

Mixtures of powder and liquid are ubiquitous in nature as well as industries and exhibit complex flowing and deforming behaviors, including sol to gel transition under shear stress. In order to better understand the characteristic features of this type of mixture, we observed the behavior of a mixture of colloidal silica particles and water as a model system under vibration. The mixture showed different states, from powder-like to viscous fluid-like, with increasing content of water. At certain concentrations of silica particles (around 70 wt. %) and under relatively faster vibration (over 17 Hz), we observed that the colloidal suspension of silica particles and water climbed up the wall of a container against gravity. The main purpose of this paper is to report how we can observe the climbing suspension of colloidal silica. The rheological measurements of the climbing suspension demonstrated that the climbing suspension showed shear-thickening behavior, where force chain networks and normal stress differences are considered to develop. Therefore, we speculate that the transient formation and breaking of force networks and normal stress differences under vibration contribute to the occurrence of the climbing suspension. The tunable nature of colloidal suspensions may help to elucidate the climbing mechanism in the future.

Nanoparticle sizing by focused-beam dynamic ultrasound scattering method

When nanoparticles in Brownian motion in liquid are irradiated with ultrasonic waves in the megahertz frequency range, scattering from the particles occurs, albeit at a very low intensity. The diffusion coefficient and the corresponding particle size can be calculated by analyzing the time correlation function of the ultrasound pulses. Since ultrasonic waves with long wavelengths in comparison with the particle size are unfavorable for detecting such small particles, increasing the energy of the ultrasonic waves is a primary solution. Increasing the energy, conversely, causes an unexpected acoustic flow. Consequently, a method using a strong ultrasound pulse while suppressing this acoustic flow field is required. In this study, we apply (1) a focused transducer with high ultrasonic energy, (2) a high-frequency sensor enhancing scattering performance, and (3) a short pulse repetition time to achieve high-speed and high-precision nanoparticle measurement while confining the sample in a narrow space to eliminate the acoustic flow. This enables direct tracking of nanoparticle motion by observing diffusive motion without perturbation particle dynamics. We specifically investigated a suspension of silica particles with a hydrodynamic radius of 15 nm and also achieved particle size discrimination in mixtures. The time correlation analysis using megahertz ultrasound pulses also takes advantage of ultrasonic waves’ inherent advantages, such as high-speed, high-precision particle size analysis in the submicron range beyond the wavelength of visible light.

Using good vibrations: Melting and controlled shear jamming of dense granular suspensions

Flows of suspensions can be blocked when the suspended particles are densely packed. This makes their formulation and their transport challenging in the industry. In this paper, we study the impact of vibrations on the behavior of dense granular suspensions prepared at a volume fraction above their jamming volume fraction but below the particle assembly random close packing. Vibrations are shown to have a strong effect on their rheological properties and to tune their transition from solidlike to liquidlike behavior. We study suspensions of rough silica particles in a Newtonian fluid. In the absence of vibrations, they have a solidlike behavior: they flow only above a yield stress. Particles are confined by the liquid interface, and the yield stress is of the frictional origin. When vibrations are applied, the yield stress vanishes to give rise to a liquidlike pseudo-Newtonian behavior at a low shear rate. Using shear-reversal experiments, we show that these liquidlike vibrated suspensions of frictional particles behave like nonvibrated suspensions of frictionless particles. As the shear rate is increased, we observe a shear thickening of the vibrated suspensions, eventually leading to shear-jamming. The yield stress behavior is recovered, and vibrations have no more impact. We show that this shear thickening can be tuned by changing the vibration energy injected into the system. We, finally, propose a physical picture based on the competition between contact opening by vibration and contact formation by shear to account for these behaviors. In the framework of the Wyart and Cates [Phys. Rev. Lett. 112, 098302 (2014)] model, vibrations can be seen as introducing a thermal-like repulsive force, yielding a critical stress proportional to the vibration stress introduced by Hanotin et al. [J. Rheol. 59, 253–273 (2015)].

Comparison of effects of homeopathic medicines prepared in glass and plastic vials in macrophage activity in vitro

Background: According to the “silica hypothesis” to explain the action of homeopathic medicines, complex nanoparticles involving the silica extracted from the walls of the glass vial during succussions would be needed to act on target cells. Aims: We aimed to know if suspended particles of silica found in Arsenicum album (AA) could be associated to its biological effects in vitro. Methodology: RAW 264.7 macrophages were co-cultured with yeast (Saccharomyces cerevisiae) to promote phagocytosis. During this process, the cells were exposed to AA in different homeopathic dilutions (30K, 6cH and 200cH) prepared in both, glass or silica-fee plastic vials. All dilutions were prepared from the same 3cH matrix. Since parallel experiments showed random stimulating and inhibiting effects of homeopathic lactose, untreated cells and water were used as controls. The macrophage activity was measured by digital histometry (Metamorph®), reflecting the cell spreading and phagocytosis. A complementary analysis using the acridine orange stain in real time fluorescence microscopy was also performed to evaluate the organization of phago-lysosomes inside the cells. All experiments were performed in triplicate. Microparticles present in the medicines were identified in an EDS (energy dispersive x-ray detector) system (JEOL JMS 6510), according to their chemical elements composition. The morphology of particles was not considered for analysis, since it was considered irrelevant. Results: Experiment one: AA 30K and pure water, when prepared in plastic vials, were able to increase macrophage activity (p≤0.05), but the same result was not seen in cells treated with AA 30K or pure water stocked in glass. Experiment two: AA 200cH prepared in plastic vials also lead to significant increase of macrophage spreading (p≤0.05) in relation to the controls, but no effect was seen in the samples prepared in glass vials. Experiment three: No difference was observed in AA 6cH treated cells in relation to the controls. The morphology of the cells under fluorescence microscopy exhibited increase in lysosome activity in AA 200cH prepared in both plastic and glass vials, in relation to the untreated control. The EDS analysis has shown that only medicines prepared in glass vials contained suspended silica particles. Conclusions: Taken the data together, the unspecific effects on macrophage spreading after treatment with homeopathic AA 30K, 200cH and water prepared in plastic vials, added to the absence of silica particles in these preparations and the increase of lysosomal activity only seen in AA 200cH, independently of the vial nature, corroborate that silica might not be a mandatory condition to justify the biological effects of homeopathic medicines. Additionally, the existence of putative contaminants released from plastic vials during the medicine manipulation deserves further studies.
 Acknowledgements: CAPES, UNIP, ABFH

Separation of Colloidal Particle Using Elastic-Gel-Packed Column

Polymerized spherical gel (46 μm) was packed in a column to permeate silica particle suspension for the separation due to their differences of sizes. The elastic gel in the bottom domain of column deformed more because the pressure of the fluid flow suppressed the packed gel layer, that is, the formed gaps among the gels at the bottom domain had the smaller ‘pore’, resulting in filtering the smaller-size of the particles at the bottom of the column. The larger the size of silica particle, the upper the filtered domain in the column. To elute the filtered silica particle, the gel layer was compacted and restored by on-off of the fluid flow, to open the gaps in the gel layer repeatedly, demonstrating that the elution percentage of silica particle was gradually increased by the repeated on-off permeation of water.

Effect of Particle Specific Surface Area on the Rheology of Non-Brownian Silica Suspensions.

Industrial formulations very often involve particles with a broad range of surface characteristics and size distributions. Particle surface asperities (roughness) and porosity increase particle specific surface area and significantly alter suspension rheology, which can be detrimental to the quality of the end product. We examine the rheological properties of two types of non-Brownian, commercial precipitated silicas, with varying specific surface area, namely PS52 and PS226, suspended in a non-aqueous solvent, glycerol, and compare them against those of glass sphere suspensions (GS2) with a similar size distribution. A non-monotonic effect of the specific surface area () on suspension rheology is observed, whereby PS52 particles in glycerol are found to exhibit strong shear thinning response, whereas such response is suppressed for glass sphere and PS226 particle suspensions. This behaviour is attributed to the competing mechanisms of particle–particle and particle–solvent interactions. In particular, increasing the specific surface area beyond a certain value results in the repulsive interparticle hydration forces (solvation forces) induced by glycerol overcoming particle frictional contacts and suppressing shear thinning; this is evidenced by the response of the highest specific surface area particles PS226. The study demonstrates the potential of using particle specific surface area as a means to tune the rheology of non-Brownian silica particle suspensions.

Optimization of optical transparency of personal care products using the refractive index matching method

Many toothpastes and personal care products consist of suspensions of silica particles in aqueous gels of humectants and other soluble constituents. It is highly desirable to develop new formulations that retain the desired characteristics of the original products but also have high optical transparency. Here, we used the refractive index matching method to manufacture highly transparent toothpastes. Accordingly, we determined the refractive indexes of different liquid gels, each containing a different humectant concentration, and measured the light transmittance and turbidity of formulations containing those gels and the silica. By fine tuning the humectant fraction in the gel, we obtained toothpastes with optimal transparency and also determined the effective refractive indexes of different commercial silica. All silica had similar indexes irrespective of their supplier, particle size distribution, and morphology. We believe that this approach could be extended to other commercial products and therefore contribute to the development of new advanced formulations.

Impact of the Carbon Nanofillers Addition on Rheology and Absorption Ability of Composite Shear Thickening Fluids.

Synthesis and characterization of composite shear thickening fluids (STFs) containing carbon nanofillers are presented. Shear thickening fluids have attracted particular scientific and technological interest due to their unique ability to abruptly increase viscosity in the case of a sudden impact. The fluids have been developed as a potential component of products with high energy absorbing efficiency. This study reports on the rheological behavior, stability, and microstructure of the STFs modified with the following carbon nanofillers: multi-walled carbon nanotubes, reduced graphene oxide, graphene oxide, and carbon black. In the current experiment, the basic STF was made as a suspension of silica particles with a diameter of 500 nm in polypropylene glycol and with a molar mass of 2000 g/mol. The STF was modified with carbon nanofillers in the following proportions: 0.05, 0.15, and 0.25 vol.%. The addition of the carbon nanofillers modified the rheological behavior and impact absorption ability; for the STF containing 0.25 vol.% of carbon nanotubes, an increase of force absorption up to 12% was observed.

Vortex in liquid films from concentrated surfactant solutions containing micelles and colloidal particles

HypothesisNew dynamic phenomena can be observed in evaporating free liquid films from colloidal solutions with bimodal particle size distribution. Such distributions are formed in a natural way in mixed (slightly turbid) solutions of cationic and anionic surfactants, where nanosized micelles coexist with micronsized precipitated particles. ExperimentWithout evaporation of water, the films thin down to thickness < 100 nm. Upon water evaporation from the film, one observes spontaneous film thickening (above 300 nm) and appearance of a dynamic vortex with a spot of thinner film in the center. The vortex wall has a stepwise profile with step-height equal to the effective micelle diameter (ca. 8 nm) and up to 20–30 stratification steps. ResultsFor thicknesses greater than 100 nm, stratification in foam films from micellar solutions has never been observed so far. It evidences for the formation of a thick colloidal crystal of micelles in the evaporating film. The role of the bigger, micronsized particles is to form a filtration cake in the Plateau border, which supports the thick film. The developed quantitative mechanical model shows that the stepwise vortex profile is stabilized by the balance of hydrodynamic and surface tension forces. Vortex is observed not only in films from catanionic surfactant solutions, but also in films from silica and latex particle suspensions, which contain smaller surfactant micelles.

Large-area flexible 2D-colloidal crystals produced directly using roll-to-roll processing

We describe a direct deposition method for the manufacture of 2D-colloidal crystal films on flexible substrates using a roll-to-roll technique (R2R). Suspensions of silica (SiO2) particles in ethanol with different concentrations and particle sizes (diameters between 150 nm and 520 nm) were prepared and directly deposited onto PET, ITO/PET and stainless steel substrates via wire-bar R2R. We propose an empirical equation that allows one to predict an optimal concentration of the colloidal suspension as a function of the processing parameters necessary to produce a colloidal monolayer. We argue that the development of a R2R coating technique for the fabrication of 2D nanostructures can overcome some technological obstacles that currently hinder the development of printed photonics, nanofabrication and colloidal lithography.

Improving tribological properties of oil-based lubricants using hybrid colloidal additives

Colloidal particles and ionic liquids (ILs) are emerging as potential additives for enhancing lubricant performance. We examine a hybrid system containing colloidal suspensions of hairy silica particles (HSPs) in an IL and motor oil. HSPs swollen in IL lower the sliding friction between steel ball-plate tribopairs at room temperature, while the addition of HSP/IL into motor oil at 100 °C dramatically impacts wear and lubrication. This included prevention of significant noise and scuffing from occurring in the tribometer when using motor oil alone. The outstanding performance of the HSP/IL to modulate friction and wear suggests such hybrid lubricants have potential to be the basis for a new class of tuneable friction modifiers to low-viscosity ‘energy efficient’ lubricants.

Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface.

Gas nuclei in water are usually too small to be directly observed. They will grow into bubbles under the negative pressure, which is called cavitation (heterogeneous cavitation). In this study, the gas nuclei in the hydrophilic and hydrophobic silica particle suspension were investigated using the transient cavitation threshold measured by a high-intensity focused ultrasound (HIFU). The transient cavitation bubbles were also observed by a high-speed camera. The results showed that the nuclei only exist on the surface of hydrophobic particles. Furthermore, the aggregation experiments revealed that the aggregates were only formed in the hydrophobic silica suspension by ultrasonic standing waves (USW) at 200kHz. This distinct difference was mainly due to the formation of gas nuclei on hydrophobic silica particles, which grew and coalesced into stable bubbles under the 200kHz USW. The aggregation process in suspension was observed by a CCD camera. Moreover, the cavitation thresholds and acoustic radiation forces were analyzed to explain the mechanism of the acoustic aggregation. This study showed a very promising acoustic method for the selective aggregation of hydrophobic particles, which might be efficiently used in the mineral separation industry.

One-Step Aqueous Spraying Process for the Fabrication of Omniphobic Fabrics Free of Long Perfluoroalkyl Chains.

We report on a simple and versatile method for the preparation in one-step of omniphobic textiles, using only aqueous suspensions of silica particles and polyurethane devoid of long perfluoroalkyl chains (C8) that are now legally-banned because of severe environmental concerns. The omniphobic coatings can be applied on different substrates including fabrics, can resist acidic and basic conditions and a moderate number of washing cycles, and repel liquids such as n-octane, dodecane, hexadecane, ethylene glycol, glycerol, olive oil, and water. Analysis of the wetting properties of coated fabrics indicates that the liquid repellence results from the trapping of air in the re-entrant roughness created by aggregates of silica particles, together with the low surface tension of the polyurethane which bears legally accepted short perfluoroalkyl chains (C4). Our study is a significant step forward toward achieving more environmentally-friendly and robust omniphobic textiles.

Yield stress of mixed suspension of silica particles and lysozymes: The effect of zeta potential and adsorbed amount

To investigate the interactions between colloidal particles in the presence of oppositely charged proteins, the yield stress of a mixed suspension of silica particles and lysozymes was measured as a function of lysozyme dose and pH. Further, the corresponding surface properties of silica particles covered with lysozymes were determined by measuring the adsorbed lysozyme amount and zeta potential. The present results indicated that an increase in the adsorbed lysozyme amount increases the zeta potential of silica particles from negative to positive through an isoelectric point. As expected from the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the maximum value of yield stress is obtained around the isoelectric point and the yield stresses decrease with an increase in the zeta potential magnitude. However, the maximum yield stress depends on the pH, and yield stresses at the same zeta potential magnitude are different depending on the zeta potential sign even if the pH values are similar. That is, the relationship between yield stress and zeta potential is asymmetric with respect to the isoelectric point. These asymmetric results of the yield stress indicate the existence of non-DLVO forces, such as patch-bridging attraction and lateral repulsion between adsorbed lysozymes, affected by the adsorbed amount of lysozyme. Our analysis suggested that the relative adsorbed amount, defined as the ratio of the adsorbed amount to the maximum adsorbed amount, can be used as a parameter to explain the asymmetric feature of the yield stress with respect to the zeta potential. In addition to the zeta potential, the surface occupation of adsorbed substances is critical in controlling particle–particle interactions.

Silica coating with well-defined micro-nano hierarchy for universal and stable surface superhydrophobicity

A universal method is proposed to produce superhydrophobic silica coating on the surfaces of either soft or hard substrates. The well-defined hierarchical structures can be formed by spraying, dipping and dropping the suspension of perfluorosilane-coated silica particles. Further adhesive-layer technique and the corresponding robustness mechanism have been investigated, and the created superhydrophobic surface shows high robustness and performance maintenance after various types of damages, including finger wiping, knife scratching and even 60 of abrasion cycles with sandpaper. Such a surface superhydrophobicity can also fit for the requirements of self-cleaning and oil separation/adsorption.