Capillary Suspensions Research Articles

Yield Stress Enhancement of a Ternary Colloidal Suspension via the Addition of Minute Amounts of Sodium Alginate to the Interparticle Capillary Bridges.

Capillary suspensions are ternary solid-liquid-liquid systems produced via the addition of a small amount of secondary fluid to the bulk fluid that contained the dispersed solid particles. The secondary fluid could exert strong capillary forces between the particles and dramatically change the rheological properties of the suspension. So far, research has focused on capillary suspensions that consist of additive-free fluids, whereas capillary suspensions with additives, particularly those of large molecular weight that are highly relevant for industrial purposes, have been relatively less studied. In this study, we performed a systematic analysis of the properties of capillary suspensions that consist of paraffin oil (bulk phase), water (secondary phase), and α-Al2O3 microparticles (particle phase), in which the aqueous secondary phase contained an important eco-friendly polymeric binder, sodium alginate (SA). It was determined that the yield stress of the suspension increased significantly with the increase in the SA content in the aqueous secondary phase, which was attributed to the synergistic effect of the capillary force and hydrogen bonding force that may be related to the increase in the number of capillary bridges. The amounts of SA used to induce a significant change in the yield stress in this study were very small (<0.02% of the total sample volume). The addition of Ca2+ ions to the SA-containing secondary phase further increased the yield stress with possible gelation of the SA chains-in the presence of excess Ca2+ ions, however, the yield stress decreased because of the microscopic phase separation that occurred in the aqueous secondary phase. The microstructures of the sintered porous materials that were produced by using capillary suspensions as precursors were qualitatively well correlated to the rheological behavior of the precursor suspensions, suggesting a new method for the subtle control of the microstructures of porous materials using the addition of minute amounts of polymeric additives.

Microstructures and conformational arrangement in emulsions caused by concentration ratios of pectin-based microgels and oil

Ternary systems of two immiscible liquids and a solid form different microstructures depending on the concentration of each component. Pickering emulsions, capillary suspensions, agglomerates, granulates, emulsion gels or bijels are found in these systems. However, in contrast to solid particles, microgels are deformable, can undergo volume changes, do not have clear contours and might interact with each other in a different manner as solid particles do. We investigated in this study, if previously found conformations and relations for solid particles also apply for pectin-based microgel particles in oil-water-systems. We investigated the microstructures that are formed in oil-in-water-emulsions containing pectin-based microgels and the textures that arise from these microstructures. By varying pectin-based microgel concentration (from 0.5–10 wt%) and oil concentration (from 5 to 20 vol%), we assessed the microscopic conformation of all components in the samples optically, measured the viscosity of the samples and their oil droplet size distributions. With the discussed data, we found concentration regimes for a microgel-stabilised, free-flowing emulsion (from 0.5 to 2.5 wt% microgel concentration) or a micro-structured ternary system (from 5 wt% microgel concentration upwards). Whilst an increase in the oil concentration did not affect the structure of the investigated formulation, an increase in microgel concentration led to the formation of agglomerates. A further increase in this concentration favoured the formation of a continuous particle-oil network throughout the aqueous phase. The change of state due to rearrangement of the components in other structures was visible in the viscosity of the samples. We also showed the influence of the microgel and oil concentration on the oil droplet size in the emulsion state. We obtained mean droplet sizes ranging from around 2 μm–6 μm. The obtained information was summarised in a ternary diagram, where all found microstructure states were qualitatively drawn. With this information, we can show what possibilities pectin-based microgel particles offer in order to adjust the texture of food applications and stabilise oil at the same time.

3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles

A new versatile route is reported to produce tailored porous sintering materials with high mechanical strength. A self-assembly mechanism deposits trace amounts of nanoparticles at contact areas between coarse ceramic particles prior to sintering, resulting in large and uniformly dense sintering bridges. This increases the porosity up to 75 % and simultaneously offers higher mechanical strength in comparison to similar materials even when weaker silica nanoparticles are introduced. This route is ideally suited for robocasting as well as conventional extrusion and, thus, can be used for rapid prototyping and mass production. We report the highest compressive strength-to-weight ratios for lightweight macroporous ceramics, covering an unrivaled density range while offering exceptional flexibility in tailorable microstructure. The strength is dominated by the silica bridges and identical strength is achieved for alumina and aluminosilicate structures. Finally, this route can be used to build hierarchical structures by point-welding particles while preserving inner surface area.

Direct 3D printing of graphene using capillary suspensions.

Conventional 3D printing of graphene requires either a complex formulation of the ink with large quantities of polymers or essential post-processing steps such as freeze drying to allow printability. Here we present a graphene capillary suspension (GCS) containing 16.67 wt% graphene nanoparticles in aqueous suspension with 3.97 wt% carboxymethyl cellulose (CMC) as a stabiliser and a small quantity of the immiscible liquid octanol. This is shown to have the appropriate rheological properties for 3D printing, which is demonstrated through the fabrication of a simple lattice structure by direct writing and air drying at room temperature. The printed structure has a porosity of 81%, is robust for handling with a compression strength of 1.3 MPa and has an electrical conductivity of 250 S m-1. After heat treatment at 350 °C conductivity is 2370 S m-1 but the strength reduces to 0.4 MPa. X-Ray tomography of the internal architecture after printing shows the formation of the capillary suspension eliminates ordering of the 2D materials during extrusion through the printer nozzle. Thus capillary suspensions can be used to direct write graphene 3D structures without the necessity of complicated drying steps or burn-out of large quantities of polymer additions, facilitating shape retention and property control as compared to current 2D material ink formulations used for 3D printing.

Connecting particle clustering and rheology in attractive particle networks.

The structural properties of suspensions and other multiphase systems are vital to overall processability, functionality and acceptance among consumers. Therefore, it is crucial to understand the intrinsic connection between the microstructure of a material and the resulting rheological properties. Here, we demonstrate how the transitions in the microstructural conformations can be quantified and correlated to rheological measurements. We find semi-local parameters from graph theory, the mathematical study of networks, to be useful in linking structure and rheology. Our results, using capillary suspensions as a model system, show that the use of the clustering coefficient, in combination with the coordination number, is able to capture not only the agglomeration of particles, but also measures the formation of groups. These phenomena are tightly connected to the rheological properties. The present sparse networks cannot be described by established techniques such as betweenness centrality.

Semiphenomenological model to predict hardening of solid–liquid–liquid systems by liquid bridges

In this study, we developed two semiphenomenological models to quantitatively predict the rheological properties of capillary suspensions. These models can be used to estimate the critical volume fraction of an additional immiscible fluid above which liquid bridges have formed between all neighboring particles, causing gelation. The models can also be used to estimate the numerical value of the yield stress. This model was derived from the mass balance between the net volume of the liquid bridge and the volume fraction of the secondary fluid assuming monodisperse particles and a cylindrical liquid bridge. The yield stress was constructed based on Rumpf’s equation. The calculation results demonstrated good agreement with the experimental data. We also used the developed model to investigate the effect of particle size on the yield stress. This model qualitatively described the experimental data and reference data, and revealed the mechanism of the particle size dependence of the yield stress. The model applies to capillary suspensions in the pendular state.

How the interaction between styrene-butadiene-rubber (SBR) binder and a secondary fluid affects the rheology, microstructure and adhesive properties of capillary-suspension-type graphite slurries used for Li-ion battery anodes

The distinct structure and flow properties of capillary suspensions, i.e., ternary solid/fluid/fluid systems including two immiscible fluids, make these kinds of slurries promising candidates for the fabrication of Li-ion battery electrodes. Recently, aqueous graphite slurries have been introduced that have beneficial coating properties and yield high-capacity cells with a superior electrochemical performance. Here, we investigate how SBR particles, frequently used as a binder in graphite slurries, affect the structure and flow of the wet paste as well as the adhesion of the dry anode layer. Combining rheological, interfacial and structural investigations revealed that the SBR particles are located at the interface of both liquid phases, and the amount of added SBR and the energy input for dispersing the secondary fluid offer extra degrees of freedom for adjusting the flow properties and microstructure according to processing and product demands. Most importantly, at a given SBR content, the adhesion strength of the capillary-suspension-type graphite slurries to the current collector is substantially higher than for the anode layers made from conventional suspensions. This novel approach promises battery electrodes with extended durability at a low binder content and improved electrical conductivity. Further research on this highly technical highly demanding topic is needed to understand the underlying physics and its impact on electrochemical cell performance.

Microstructure of Rod-Based Capillary Suspensions with Different Rod Aspect Ratios under Quiescent and Shear Flow

A particle-assembled microstructure in ternary capillary suspensions consisting of polyisobutylene (PIB) as the matrix phase, poly(ethylene oxide) (PEO) as the minor fluid phase, and silica (SiO2) particles with different aspect ratios (ARs) is probed by an in situ optical-shear technique, scanning electronic microscopy, and confocal laser scanning microscopy as well as rheometry. It is found that the capillary bridging force brought by the presence of the PEO wetting layer between particles can be influenced by the AR of particles and shear rate, which all play decisive roles in determining the microstructure and rheological properties of suspensions. Moreover, two novel aggregation modes of rods and an interesting spatiotemporal organization phenomenon of particle aggregates (vorticity-aligned particle bands) are found under shear flow.

Lightweight Porous Glass Composite Materials Based on Capillary Suspensions.

In this article, we present a simple, advanced method to produce lightweight tailor-made materials based on capillary suspensions that are made from locally bonded hollow glass spheres with a high total porosity in the range of 70% at apparent densities of 200 kg/m3, having a compressive strength of 0.6 MPa. The amount of added liquid and the particle surface treatment determine the network structure in the pastes and the resulting microstructure of the porous material in a straightforward manner. This structure has a strong impact on the porosity, pore size, and mechanical properties of the final body. The most promising porous materials were made of surface treated hollow glass spheres that create a sample-spanning network in the capillary state, where the added liquid wets the particles worse than the bulk fluid. These samples approach the density of natural balsa wood and they may find application in fields where either weight or structure are important, such as in insulation materials, filters, and membranes, as well as lightweight construction materials for automotive or aerospace engineering.

Shrinkage and dimensional accuracy of porous ceramics derived from capillary suspensions

Shrinkage and dimensional accuracy are of particular importance for industrial material production. High dimensional accuracy directly lowers finishing works and cost. Capillary suspension processing is a novel, easy method to produce highly porous ceramic materials. Their shrinkage and shape accuracy is investigated during processing using a laser microscope. The total shrinkage is reduced by 20% for capillary suspension compared to pure suspension. This results from an increase in linear shape accuracy for top radii as well as height. The linear shape accuracy is increased by 6% in top radius and by 16% in height by using the capillary suspension phenomenon. We also show that this capillary suspension method is applicable for continuous shaping processes, like extrusion. The combination of an easy, robust processing route with known dimensional accuracy and applicability for continuous shaping processes makes this capillary suspension processing route highly desirable for industrial processes.

Wall slip and multi-tier yielding in capillary suspensions

Impact of wall slip on the yield stress measurement is examined for capillary suspensions consisting of cocoa powder as the dispersed phase, vegetable oil as the continuous primary fluid, and water as the secondary fluid using smooth and serrated parallel plates. Using dynamic oscillatory measurements, we investigated the yielding behavior of this ternary solid-fluid-fluid system with varying particle volume fraction, ϕ, from 0.45 to 0.65 and varying water volume fraction, ϕw, from 0.02 to 0.08. Yield stress is defined as the maximum in the elastic stress (G′γ), which is obtained by plotting the product of elastic modulus (G′) and strain amplitude (γ) as a function of applied strain amplitude. With serrated plates, which offer minimal slippage, capillary suspensions with ϕ ≥ 0.45 and a fixed ϕw = 0.06 showed a two-step yielding behavior as indicated by two peaks in the plots of elastic stress as a function of strain amplitude. On the other hand with smooth plates, the capillary suspensions showed strong evidence of wall slip as evident by the presence of three distinct peaks and lowered first yield stresses for all ϕ and ϕw. These results can be interpreted based on the fact that a particle-depleted layer, which is known to be responsible for slip, is present in the vicinity of the smooth surfaces. The slip layer presents itself as an additional “pseudo-microstructure” (characteristic length scale) besides the two microstructures, aqueous bridges and solid particle agglomerates, that may occur in the system. With serrated plates, both the yield stresses (σ1σ2) and storage moduli plateau at lower strain (before the first yield point) and at higher strain (before the second yield point) (G $^{\prime }_{p1}$ , G $^{\prime }_{p2}$ ) were found to increase with ϕ (at a fixed ϕw = 0.06) following power-law dependences. Similarly with increasing ϕw (0.02 – 0.08) at a fixed ϕ = 0.62, the system behaved as a solid-like material in a jammed state with particles strongly held together as manifested by rapidly increasing σ1 and σ2. The usage of smooth surfaces primarily affected σ1 which was reflected by an approximately 70–90% decrement in the measured σ1 for all values of ϕ. By contrast, σ2 and G $^{\prime }_{p2}$ were found to be unaffected as shown by close agreement of values obtained using serrated geometry due to vanishing slip layers at higher strain amplitudes.

The effect of particle wettability on the of rheology particulate suspensions with capillary force

We examined the dependence of the rheology of capillary suspensions on particle wettability using model ternary liquid/liquid/particle mixtures. Experiments were conducted using silica particles suspended in a majority liquid phase, to which a small minority of a second liquid was added. Silane modification was used to realize three different particle wettabilities ranging from particles being fully-wettable by the minority fluid, to being almost completely non-wettable by the minority liquid. In all three cases, the ternary suspensions showed solid-like behavior, with the yield stress and modulus decreasing as particles became less wettable towards the minority fluid. In all three cases, the minority liquid was found to induce particle aggregation into percolating networks which are responsible for the solid-like rheology. Our results suggest two reasons why the solid-like properties of such suspensions diminish as the particles become less wetting by the minority fluid: because particle network relies on multi-particle capillary clusters (rather than pair-wise meniscus binding), and because a certain fraction of particles become too non-wettable to participate in the network.

Structure of capillary suspensions and their versatile applications in the creation of smart materials.

In this article, we review recent research in the field of capillary suspensions and highlight a variety of applications in the field of smart materials. Capillary suspensions are liquid-liquid-solid ternary systems where one liquid is only present in a few percent and induces a strong, capillary-induced particle network. These suspensions have a large potential for exploitation, particularly in the production of porous materials since the paste itself and the properties of the final material can be adapted. We also discuss the rheological properties of the suspension and network structure to highlight the various ways these systems can be tuned.

Fractal approaches to characterize the structure of capillary suspensions using rheology and confocal microscopy.

The rheological properties of a particle suspension can be substantially altered by adding a small amount of a secondary fluid that is immiscible with the bulk phase. The drastic change in the strength of these capillary suspensions arises due to the capillary forces, induced by the added liquid, leading to a percolating particle network. Using rheological scaling models, fractal dimensions are deduced from the yield stress and from oscillatory strain amplitude sweep data as function of the solid volume fraction. Exponents obtained using aluminum-oxide-based capillary suspensions, with a preferentially wetting secondary fluid, indicate an increase in the particle gel's fractal dimension with increasing particle size. This may be explained by a corresponding relative reduction in the capillary force compared to other forces. Confocal images using a glass model system show the microstructure to consist of compact particle flocs interconnected by a sparse backbone. Thus, using the rheological models two different fractal dimensionalities are distinguished - a lower network backbone dimension (D = 1.86-2.05) and an intrafloc dimension (D = 2.57-2.74). The latter is higher due to the higher local solid volume fraction inside of the flocs compared to the sparse backbone. Both of these dimensions are compared with values obtained by analysis of spatial particle positions from 3D confocal microscopy images, where dimensions between 2.43 and 2.63 are computed, lying between the two dimension ranges obtained from rheology. The fractal dimensions determined via this method corroborate the increase in structural compactness with increasing particle size.

Negative normal stress differences N1-N2 in a low concentration capillary suspension.

Here, negative normal stress differences are reported in capillary suspensions, i.e. particle suspensions in a two-fluid system that creates strong capillary attractions, at a solid concentration of 25%, and a volume fraction that has heretofore been considered too low to show such normal stress differences. Such capillary suspensions have strong particle networks and are shear thinning for the entire range of shear rates studied. Capillary suspensions exist in two states: a pendular state when the secondary fluid preferentially wets the particles, and a capillary state when the bulk fluid is preferentially wetting. In the pendular state, the system undergoes a transition from a positive normal stress difference at high shear rates to a negative stress difference at low shear rates. These results are an indication of flexible flocs in the pendular state that are able to rotate to reorientate in the vorticity direction under shear. Analogous experiments were also conducted for the capillary state, where only a negative normal stress difference occurs. The capillary state system forms more network contacts due to droplet breakup at higher shear rates, which enhances the importance of hydrodynamic interactions in the non-colloidal suspension.

Capillary Structured Suspensions from In Situ Hydrophobized Calcium Carbonate Particles Suspended in a Polar Liquid Media.

We demonstrate that capillary suspensions can be formed from hydrophilic calcium carbonate particles suspended in a polar continuous media and connected by capillary bridges formed of minute amounts of an immiscible secondary liquid phase. This was achieved in two different polar continuous phases, water and glycerol, and three different oils, oleic acid, isopropyl myristate, and peppermint oil as a secondary liquid phase. The capillary structuring of the suspension was made possible through local in situ hydrophobization of the calcium carbonate particles dispersed in the polar media by adding very small amounts of oleic acid to the secondary liquid phase. We observed a strong increase in the viscosity of the calcium carbonate suspension by several orders of magnitude upon addition of the secondary oil phase compared with the same suspension without secondary liquid phase or without oleic acid. The stability and the rheological properties of the obtained capillary structured materials were studied in relation to the physical properties of the system such as the particle size, interfacial tension between the primary and secondary liquid phases, as well as the particle contact angle at this liquid-liquid interface. We also determined the minimal concentrations of the secondary liquid phase at fixed particle concentration as well as the minimal particle concentration at fixed secondary phase concentration needed to form a capillary suspension. Capillary suspensions formed by this method can find application in structuring pharmaceutical and food formulations as well as a variety of home and personal care products.

Thermally Responsive Capillary Suspensions.

We demonstrate that stimulus-responsive capillary-structured materials can be formed from hydrophobized calcium carbonate particles suspended in a non-polar phase (silicone oil) and bridged by very small amounts of a hydrogel as the secondary aqueous phase. Inclusion of thermally responsive polymers into the aqueous phase yielded a capillary-structured suspension whose rheology is controlled by a change in temperature and can increase its complex modulus by several orders of magnitude because of the gelation of the capillary bridges between the solid particles. We demonstrate that the rheology of the capillary suspension and its response upon temperature changes can be controlled by the gelling properties as little as 0.1 w/w % of the secondary aqueous phase containing 2 wt % of the gelling carbohydrate. Doping the secondary (aqueous) phase with methyl cellulose, which gels at elevated temperatures, gave capillary-structured materials whose viscosity and structural strength can increase by several orders of magnitude as the temperature is increased past the gelling temperature of the methyl cellulose solution. Increasing the methyl cellulose concentration from 0 to 2 w/w % in the secondary (aqueous) phase increases the complex modulus and the yield stress of the capillary suspension of 10 w/w % hydrophobized calcium carbonate in silicone oil by 2 orders of magnitude at a fixed temperature. By using an aqueous solution of a low melting point agarose as a secondary liquid phase, which melts as the temperature is raised, we produced capillary-structured materials whose viscosity and structural strength can decrease by several orders of magnitude as the temperature is increased past the melting temperature of the agarose solution. The development of thermally responsive capillary suspensions can find potential applications in structuring of smart home and personal care products as well as in temperature-triggered change in rheology and release of flavors in foods and actives in pharmaceutical formulations.

Rheology of particle/water/oil three-phase dispersions: Electrostatic vs. capillary bridge forces

HypothesisParticle/water/oil three-phase capillary suspensions possess the remarkable property to solidify upon the addition of minimal amount of the second (dispersed) liquid. The hardening of these suspensions is due to capillary bridges, which interconnect the particles (pendular state). Electrostatic repulsion across the oily phase, where Debye screening by electrolyte is missing, could also influence the hardness of these suspensions. ExperimentsWe present data for oil-continuous suspensions with aqueous capillary bridges between hydrophilic SiO2 particles at particle volume fractions 35–45%. The hardness is characterized by the yield stress Y for two different oils: mineral (hexadecane) and vegetable (soybean oil). Findings and modellingThe comparison of data for the “mirror” systems of water- and oil-continuous capillary suspensions shows that Y is lower for the oil-continuous ones. The theoretical model of yield stress is upgraded by including a contribution from electrostatic repulsion, which partially counterbalances the capillary-bridge attraction and renders the suspensions softer. The particle charge density determined from data fits is close to that obtained in experiments with monolayers from charged colloid particles at oil/water interfaces. The results could contribute for better understanding, quantitative prediction and control of the mechanical properties of solid/liquid/liquid capillary suspensions.

Dual yielding in capillary suspensions

Rheological measurements were performed to examine the yielding behavior of capillary suspensions prepared by mixing cocoa powder as dispersed phase, vegetable oil as the continuous primary fluid, and water as the secondary fluid. Here, we investigated the yielding behavior of solid-fluid-fluid systems with varying particle volume fraction, ϕ, spanning the regime from a low volume fraction (ϕ = 0.25) to a highly filled regime (ϕ = 0.65) using dynamic oscillatory measurements. While for ϕ ≤ 0.4 with a fixed water volume fraction (ϕ w ) of 0.06 as the secondary fluid, capillary suspensions exhibited a single yield point due to rupturing of aqueous capillary bridges between the particles, while capillary suspensions with ϕ ≥ 0.45 showed a two-step yielding behavior. On plotting elastic stress (G′ γ) as a function of applied strain (γ), two distinct peaks, indicating two yield stresses, were observed. Both the yield stresses and storage modulus at low strains were found to increase with ϕ following a power law dependence. With increasing ϕ w (0 – 0.08) at a fixed ϕ = 0.65, the system shifted to a frustrated, jammed state with particles strongly held together shown by rapidly increasing first and second yield stresses. In particular, the first yield stress was found to increase with ϕ w following a power law dependence, while the second yield stress was found to increase exponentially with ϕ w . Transient steady shear tests were also performed. The single stress overshoot for ϕ ≤ 0.4 with ϕ w = 0.06 reflected one-step yielding behavior. In contrast, for high ϕ (≥ 0.45) values with ϕ w = 0.06, two stress overshoots were observed in agreement with the two-step yielding behavior shown in the dynamic oscillatory measurements. Experiments on the effect of resting time on microstructure recovery demonstrated that aggregates could reform after resting under quiescent conditions.

Radical polymerization of capillary bridges between micron-sized particles in liquid bulk phase as a low temperature route to produce porous solid materials.

We present a generic and versatile low temperature route to produce macro-porous bodies with porosity and pore size distribution that are adjustable in a wide range. Capillary suspensions, where the minor fluid is a monomer, are used as pre-cursors. The monomer is preferentially located between the particles, creating capillary bridges, resulting in a strong, percolating network. Thermally induced polymerization of these bridges at temperatures below 100 °C for less than 5 hours and subsequent removal of the bulk fluid yields macroscopic, self-supporting solid bodies with high porosity. This process is demonstrated using methylmethacrylate and hydroxyethylmethacrlyate with glass particles as a model system. The produced PMMA had a molecular weight of about 500.000 g/mol and dispersity about three. Application specific porous bodies, including PMMA particles connected by PMMA bridges, micron-sized capsules containing phase change material with high inner surface, and porous graphite membranes with high electrical conductivity, are also shown.