Hardening of particle/oil/water suspensions due to capillary bridges: Experimental yield stress and theoretical interpretation

Tunable Capillary Suspensions from Aqueous Two-Phase Systems

Adding small amounts of a (partially) immiscible fluid to a suspension can create liquid bridges between particles, leading to interconnected networks known as capillary suspensions. This can be used to structure suspensions and adjust their rheological properties. Typically, these suspensions involve water and oil, where the minority liquid phase wets the particles dispersed in the majority phase. Here, we have demonstrated that oil-free capillary suspensions can also be formed in aqueous two-phase systems (ATPS), where a phase separation occurs between two hydrophilic polymers, dextran and polyethylene glycol (PEG). In this system, silica particles form a self-standing gel when a small amount of the PEG-rich phase is added to the dextran-rich phase. Despite the ultralow interfacial tension in ATPS, a significant increase in storage modulus is achievable. Capillary bridges have been visualized using confocal microscopy. By adjusting the amount of the PEG-rich phase (secondary phase), the network strength and yield stress can be finely tuned, enabling a wide range of rheological responses. Due to the absence of oil and the use of hydrophilic, biocompatible polymers, these capillary suspensions have potential applications in biomedical (where living cells can act as particles), pharmaceutical, and food formulations, as well as in home and personal care products.

Hierarchically porous 3D-printed ceramic scaffolds for bone tissue engineering

Sacrificial templating offers the ability to create interconnected pores within 3D printed filaments and to control pore morphology. Beta-tricalcium phosphate (TCP) bone tissue engineering (BTE) scaffolds were fabricated with multiscale porosity: (i) macropores from direct ink writing (DIW, a material extrusion 3D printing technique), (ii) micropores from oil templating, and (iii) smaller micropores from partial sintering. The hierarchically porous scaffolds possessed a total porosity of 58-70 %, comprising 54-63 % interconnected open pores. The in vitro results demonstrated that scaffolds with macroporosity promoted human osteoblast growth more than scaffolds with only microporosity. The elongated pores from the capillary suspension filament microstructure induced greater cell spreading than the sphere-like pores from the emulsion. Overall, the hierarchically porous scaffold with capillary suspension TCP filaments provided a superior microenvironment for significantly higher cell viability and proliferation than the other scaffolds, including a poly(ε-caprolactone) (PCL) control, a material currently used clinically as porous BTE scaffolds. The cellular response was further enhanced when macropore size was in the range of 570-590 μm. Therefore, the hierarchically porous scaffolds in this study are promising as BTE scaffolds, and the reported process of DIW of oil-templated colloidal pastes is a feasible strategy with potential for further customization.

Tailoring the surface pore morphology of bioceramic scaffolds through colloidal processing for bone tissue engineering

In this study, porous bioceramic scaffolds are developed with two materials, β-tricalcium phosphate (β-TCP) and hydroxyapatite (HA), with order of 10 micron-scale surface pores and approximately 40-60% volume porosity fabricated by soft templating of oil. Suitable oil and surfactant concentrations are determined for the creation of particle-stabilized emulsions with nearly spherical pores, as well as the capillary suspensions with elongated pores. The bioceramic scaffolds surfaces are then assessed for their ability to support osteoblast adhesion and growth, for applications as scaffolds for bone regeneration. The porous scaffolds' surfaces are compared to denser surfaces of the same material, where only submicron porosity arise from partial sintering, to interrogate the impact of surface topography on cell behavior. On the denser surfaces where no large pores are templated, β-TCP supports a larger number of osteoblasts compared to HA. Templated surface porosity significantly impacts the morphology and growth of the osteoblasts. Amongst the pore morphologies, the capillary suspension demonstrates enhanced biological function, whereas the emulsion performs the poorest. The β-TCP capillary suspension scaffold surface appears to provide the most favorable conditions for the osteoblasts.

Oleogel formation based on natural insoluble soybean fiber using capillary force: A novel strategy and application

Insoluble dietary fibers can be used as oleogelators to form oleogels via molecular self-assembly following chemical modification. However, the limitations of traditional chemical modifications and oleogel preparation methods significantly restrict their practical application. This study proposed a novel method to directly form edible oleogels using natural soybean insoluble fiber particles as oil-forming agents and water as a secondary fluid via the capillary suspension force between particles. The results showed that when the particle fraction was 15 % and the secondary fluid content was 0.2, a strong capillary suspension force could be formed to maintain the oil holding capacity of oleogels. The sedimentation coefficient analysis suggested that adding particles and secondary fluids significantly affected the oleogel stability. The polarity of the oils, as well as the ionic strength and pH of the secondary fluids, influenced the rheological properties of oleogels, which correlated with the interfacial tension between the secondary fluids and oils. Moreover, the stable oleogels showed their potential as excellent solid fat substitutes in the preparation of breads (specific volume = 2.029 ± 0.114 cm3/g, weight loss = 12.2 ± 2.6 %, and hardness = 3.321 ± 0.055 N). This study highlighted that insoluble dietary fiber can form oleogels via capillary suspension, which is a relatively rapid and simple strategy. Additionally, it provided a solid foundation for the comprehensive utilization of soybean processing byproducts and the transformation of traditional food-specific oils and fats.

3D printing of Pickering emulsions, Pickering foams and capillary suspensions - A review of stabilization, rheology and applications

Pickering emulsions and foams as well as capillary suspensions are becoming increasingly more popular as inks for 3D printing. However, a lack of understanding of the bulk rheological properties needed for their application in 3D printing is potentially stifling growth in the area, hence the timeliness of this review. Herein, we review the stability and bulk rheology of these materials as well as the applications of their 3D-printed products. By highlighting how the bulk rheology is tuned, and specifically the inks storage modulus, yield stress and critical balance between the two, we present a rheological performance map showing regions where good prints and slumps are observed thus providing clear guidance for future ink formulations. To further advance this field, we also suggest standard experimental protocols for characterizing the bulk rheology of the three types of ink: capillary suspension, Pickering emulsion and Pickering foam for 3D printing by direct ink writing.

Quantitative evaluation of particle-binder interactions in ceramic slurries via differential centrifugal sedimentation

In diverse materials science spanning from fine ceramics to lithium-ion batteries and fuel cells, the particle-binder interactions in slurries play a crucial role in governing the ultimate performance. Despite numerous efforts to date, quantitatively elucidating these hidden interactions has remained a longstanding challenge. Here, we demonstrate a dynamic approach to evaluate adsorptive interactions between ceramic particles and polymeric binders entangled in a slurry utilizing differential centrifugal sedimentation (DCS). Particles settling under a centrifugal force field impart significant viscous resistance on the adsorbed binder, leading to its detachment, influenced by particle size and density. This behaviour directly reflects the particle-binder interactions, and detailed DCS spectrum analysis enables the quantitative assessment of nano-Newton-order adsorption forces. An important finding is the strong correlation of these forces with the mechanical properties of the moulded products. Our results provide insight that forming a flexible network structure with appropriate interactions is essential for desirable formability.

New insights into interaction between oil and solid during hydrothermal treatment of oily sludge

Hydrothermal treatment (HT) can effectively dehydrate and reduce oily sludge (OS) volume, but the resulting hydrothermal oily sludge (HOS) presents greater challenges for washing than the initial oily sludge (IOS). This study examines the effects of HT on OS by analyzing changes in water, oil, and solid. Results indicate that HT considerably decreases the water content in OS while increasing resin and asphaltenes contents. In addition, condensation, side-chain scission, and oxidation reactions occur during the HT process, resulting in coking, agglomeration, and an increase in oxygen-containing groups. This increase, further confirmed by X-ray photoelectron spectroscopy (XPS), enhances the interaction between oil and solids. Calcite, the most prevalent solid-phase component, may form a calcium bridge with the oxygen-containing groups. Moreover, HT reduces the solid particle size, thereby increasing the oil-solid contact area. Interestingly, the process of deasphalting diminishes the interaction between oil and solids, facilitating sludge washing. After washing, the residual oil content in HOS is reduced to less than 0.34%. This study elucidates why HOS is challenging to separate from oil and solids and introduces a novel method that combines dodecylbenzene sulfonic acid (DBSA)-assisted heptane deasphalting with conventional washing techniques. This method shows promise for applications in OS affected by weathering processes.

A hybrid approach to oil structuring - combining wax oleogels and capillary suspensions

There is continuing interest in finding new approaches to gel liquid oil for processed food applications. Here, we combined oleogels and capillary suspensions to generate model oil-continuous networks consisting of a wax oleogel and a water-bridged, glass particle network. The composition map tested comprised 30 vol% polar or non-polar glass beads dispersed in a 70 vol% non-particle phase consisting of water (≤9 vol%) as well as 2 wt% hexatriacontane as oleogelator in canola oil. While the hexatriacontane wax alone gelled the oil, presence of the glass beads (but no water) prevented oleogelation. Self-supporting capillary networks formed with polar particles and 1 vol% water or non-polar glass beads and 3 vol% water in canola oil. The capillary suspensions demonstrated significant differences in rheological behaviour as the polar particles yielded much higher elastic moduli than their non-polar particle counterparts. Polar hybrids were weakened by inclusion of the wax whereas the non-polar particle hybrid network displayed elastic moduli greater than the respective contributions of both capillary and wax gel networks. This hybrid method of oleogelation can be applied to virtually any food particles and uses minimal water and wax.

Effect of liquid volume fraction and shear rate on rheological properties and microstructure formation in ternary particle/oil/water dispersion systems under shear flow: two-dimensional direct numerical simulation

We numerically studied the rheological properties and microstructure formation under shear flow in a ternary particle/oil/water dispersion system. Our numerical simulation method was based on a phase-field model for capturing a free interface, the discrete element method for tracking particle motion, the immersed boundary method for calculating fluid-particle interactions, and a wetting model that assigns an order parameter to the solid surface according to the wettability. The effects of the water-phase volume fraction and shear rate on the microstructure and apparent viscosity were investigated. When the water-phase volume fraction was low, a pendular state was formed, and with an increase in the water-phase volume fraction, the state transitioned into a co-continuous state and a Pickering emulsion. This change in the microstructure state is qualitatively consistent with the results of previous experimental studies. In the pendular state, the viscosity increased with an increase in the water-phase volume fraction. This was due to the development of a network structure connected by liquid bridges, and the increase in the coordination number was quantitatively confirmed. In the case of the pendular state, significant shear thinning was observed, but in the case of the Pickering emulsion, no significant shear thinning was observed. It is concluded that this is due to the difference in the manner in which the microstructure changes with the shear rate. This is the first study to numerically demonstrate the microstructure formation of a ternary dispersion under shear flow and its correlation with the apparent viscosity.

Capillary bridges between unsaturated nano-mineral particles: a molecular dynamics study

Capillary bridges play an important role in the process of cohesion, which is crucial for wet granular media, and engineering of pharmaceuticals and food processing. However, the understanding of capillary bridges at the nanoscale remains unclear because the mechanical performance of nanoscale capillary bridges cannot be fully captured and explained by classical capillary theory. We applied a novel molecular dynamic simulation to investigate the dynamic formation process of nanoscale capillary bridges between quartz asperities. In comparison with classical capillary theory, our results suggested that the application of the toroidal approximation and gorge method will break down at the scale of 1 nm. Below this threshold, a pronounced oscillation in the adhesive force was observed due to inconsistent distribution of water molecules in the capillary bridges. Moreover, we found a non-linear correlation between the adhesive force and the saturation degree. Different from the cohesive stress of sandy soil as a function of saturation degree, we identified an optimal saturation range of 0.5-0.7 instead of 0.2-0.9 for the sandy soil. Our findings enhance the understanding of capillary bridges and provide new insights into the capillary force between particles in the fields of geotechnical engineering, food-process engineering, the pharmaceutical industry and nanotechnology.

Liquid-Suspended and Liquid-Bridged Liquid Metal Microdroplets

Liquid metals (LMs) and alloys are attracting increasing attention owing to their combined advantages of high conductivity and fluidity, and have shown promising results in various emerging applications. Patterning technologies using LMs are being actively researched; among them, direct ink writing is considered a potentially viable approach for efficient LM additive manufacturing. However, true LM additive manufacturing with arbitrary printing geometries remains challenging because of the intrinsically low rheological strength of LMs. Herein, colloidal suspensions of LM droplets amenable to additive manufacturing (or "3D printing") are realized using formulations containing minute amounts of liquid capillary bridges. The resulting LM suspensions exhibit exceptionally high rheological strength with yield stress values well above 10³  Pa, attributed to inter-droplet capillary attraction mediated by the liquid bridges adsorbed on the oxide skin of the LM droplets. Such liquid-bridged LM suspensions, as extrudable ink-type filaments, are based on uncurable continuous-phase liquid media, have a long pot-life and outstanding shear-thinning properties, and shape retention, demonstrating excellent rheological processability suitable for 3D printing. These findings will enable the emergence of a variety of new advanced applications that necessitate LM patterning into highly complicated multidimensional structures.

Structure-Property Relationship in Capillary Foams

The recently discovered capillary foams are aqueous foams stabilized by the synergistic action of colloidal particles and a small amount of oil. Characteristically, their gas bubbles are coated by a particle-stabilized layer of oil and embedded in a gel network of oil-bridged particles. This unique foam architecture offers opportunities for engineering new foam-related materials and processes, but the necessary understanding of its structure-property relations is still in its infancy. Here, we study the effects of particle wettability, particle volume fraction, and oil-to-particle ratio on the structure and selected properties of capillary foams and use our findings to relate measured foamability, foam stability, and rheological key parameters to the observed foam microstructure. We see that particle wettability not only determines the type of gel network formed but also influences the prevalence of oil droplets included within the foam. Our results further show that the stability and rheology of capillary foams are mainly a function of the particle volume fraction whereas the foamability and observed microstructure are sensitive also to the oil-to-particle ratio. These insights expand our fundamental understanding of capillary foams and will greatly facilitate future work on new foam formulations.

Rheology of capillary foams

Aqueous foams are ubiquitous; they appear in products and processes that span the cosmetics, food, and energy industries. The versatile applicability of foams comes as a result of their intrinsic viscous and elastic properties; for example, foams are exploited as drilling fluids in enhanced oil recovery for their high viscosity. Recently, so-called capillary foams were discovered: a class of foams that have excellent stability under static conditions and whose flow properties have so far remained unexplored. The unique architecture of these foams, containing oil-coated bubbles and a gelled network of oil-bridged particles, is expected to affect foam rheology. In this work, we report the first set of rheological data on capillary foams. We study the viscoelastic properties of capillary foams by conducting oscillatory and steady shear tests. We compare our results on the rheological properties of capillary foams to those reported for other aqueous foams. We find that capillary foams, which have low gas volume fractions, exhibit long lasting rheological stability as well as a yielding behavior that is reminiscent of surfactant foams with high gas volume fractions.

Water Bridges in Clay Nanopores: Mechanisms of Formation and Impact on Hydrocarbon Transport

Clays are prevalent in the earth's crust and usually deposited in the presence of water. An unusual finding in clays is that under certain conditions, water molecules can collectively form a bridge across a clay-hosted pore. However, there are relatively few studies focused on the formation mechanism of the water bridge in clay nanopores. In this work, we use molecular dynamics simulations to investigate the formation of the water bridge and its influence on fluid transport in slit-shaped illite nanopores. Two different basal illite surface chemistries are constructed: potassium-hydroxyl (P-H) and hydroxyl-hydroxyl (H-H) structures. Because pore size and water concentration are expected to control the formation of the water bridge, our simulations span a wide range of pore sizes and water concentrations. Generally, positive potassium layers and negative hydroxyl groups in P-H nanopore can induce partial charges which in return produce instant and local electric fields, favoring the formation of the water bridge. In P-H nanopores, the water bridge happens at a relatively low water concentration. However, in H-H nanopores, the water bridge only forms at high water concentrations. Additionally, smaller pore sizes favor the formation of water bridges. However, the presence of an electric field promotes the formation of a water bridge even in larger pore sizes in P-H pores. The results also indicate that in both P-H and H-H nanopores, water adsorption films initially create a smooth surface to promote the hydrocarbon flow. In P-H nanopores, further increases in the water concentration causes a sharp decline in the self-diffusion coefficients of the hydrocarbon and water due to the formation of the water bridge. The presence of electric fields in P-H pores can however weaken the confinement effect of illite and promote the hydrocarbon flow. In contrast, in H-H nanopores, the self-diffusion coefficients decline slowly with the increase of water concentration. This is because no water bridge is formed at low water concentrations in H-H nanopores.

Droplet-Mediated Deterministic Microtransfer Printing: Water as a Temporary Adhesive

Deterministic transfer printing of solid objects has been introduced and demonstrated, where capillary force based on water droplet has been utilized. Droplets on pickup head form capillary bridges with chips to be picked, where the capillarity provides enough force to grip or lift the chip. Release or printing of chips can be achieved by simply drying droplet while the chip is in contact with receiver substrate. That is, water droplet acts as a temporary adhesive, adheres onto solid chip by capillary force, and releases it upon drying. The technique has unique features such as self-alignment during pickup, self-correction on pickup head by short exposure to water mist, and pickup capability of solids having a nonflat, corrugated surface, all of which originated from the fluidic nature of water droplet. The technique has been successfully applied for the fabrication of stretchable micro-light-emitting diode chip arrays. The proposed technique can find wide applications in many fields such as displays, sensors, printed electronics, photovoltaics, etc.

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.

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

HYPOTHESIS

Particle/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.

EXPERIMENTS

We 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 MODELLING

The 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.