Capillary Suspensions Research Articles

Hierarchical materials with interconnected pores from capillary suspensions for bone tissue engineering

AbstractThe increasing demand for bone grafts due to the aging population has opened new opportunities for the manufacture of porous ceramics to assist in bone reconstruction. In our study, we investigate a new, promising method to manufacture hierarchically porous structures in a straightforward and tunable way. It consists of combining the novel technology of capillary suspensions, formed by mixing solid particles and two immiscible liquids, one less than 5 vol%, with freeze casting. We have successfully achieved alumina and beta‐tricalcium phosphate (β‐TCP) materials with both <2 µm and 20–50 µm as the smallest and largest pore sizes, respectively. The microstructure exhibits fully open pores and high levels of porosity (>60%). The capillary suspensions’ rheological behavior indicates that silica nano‐suspensions as a secondary fluid creates a stronger internal particle network than sucrose for the alumina system. Conversely, the opposite was observed with the β‐TCP system. These differences were attributed to the change in affinity between the secondary fluids and the solid loading. In our study, both systems have served to deepen the knowledge about the new area of capillary suspensions and proved their use in hierarchical porous scaffolds for bone tissue engineering.

Dynamics of meniscus-bound particle clusters in extensional flow

Capillary suspensions are three-phase mixtures containing a solid particulate phase, a continuous liquid phase, and a second immiscible liquid forming capillary bridges between particles. Capillary suspensions are encountered in a wide array of applications including 3D printing, porous materials, and food formulations, but despite recent progress, the micromechanics of particle clusters in flow is not fully understood. In this work, we study the dynamics of meniscus-bound particle clusters in planar extensional flow using a Stokes trap, which is an automated flow control technique that allows for precise manipulation of freely suspended particles or particle clusters in flow. Focusing on the case of a two-particle doublet, we use a combination of experiments and analytical modeling to understand how particle clusters rearrange, deform, and ultimately break up in extensional flow. The time required for cluster breakup is quantified as a function of capillary number Ca and meniscus volume V. Importantly, a critical capillary number Cacrit for cluster breakup is determined using a combination of experiments and modeling. Cluster relaxation experiments are also performed by deforming particle clusters in flow, followed by flow cessation prior to breakup and observing cluster relaxation dynamics under zero-flow conditions. In all cases, experiments are complemented by an analytical model that accounts for capillary forces, lubrication forces, hydrodynamic drag forces, and hydrodynamic interactions acting on the particles. Results from the analytical models are found to be in good agreement with experiments. Overall, this work provides a new quantitative understanding of the deformation dynamics of capillary clusters in extensional flow.

Enhanced contact flexibility from nanoparticles in capillary suspensions

Hypothesis: Sample-spanning particle networks are used to induce structure and a yield stress, necessary for 3D printing of porous ceramics and paints. In capillary suspensions, a small quantity of immiscible secondary fluid is incorporated into a suspension. By further adding nanoparticles with a range of hydrophobicities, the structure of the bridges and microparticle-microparticle contacts is expected to be modified, resulting in a tunable yield stress and shear moduli. Moreover, the compressibility of these samples, important in many processing and application steps, is expected to be sensitive to these changes.Experiment: The nanoparticle hydrophobicity was altered and their position relative to the microparticles and the bridges was examined using confocal microscopy where the correlation between bridge size and network structure was observed. A step-wise uniaxial compression test on the confocal was conducted to monitor the microparticle movement and structural changes between capillary suspension networks with and without nanoparticles.Findings: Our observation suggests that nanoparticles induce the formation of thin liquid films on the surface of the microparticles, mitigating contact line pinning and promoting internal liquid exchange. Additionally, nanoparticles at microparticle contact regions further diminish Hertzian contact, enhancing the capacity for rearrangement. These effects enhance microparticle movement, narrowing the bridge size distribution.

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.

3D structured capillary cell suspensions aided by aqueous two-phase systems.

We report a facile technique for 3D structuring of living cells by forming capillary cell suspensions based on an aqueous two-phase system (ATPS) of polyethylene glycol (PEG) and dextran (DEX) solutions. We demonstrate the formation of water-in-water (DEX-in-PEG) capillary bridges using concentrated suspensions of yeast cells which show enhanced rheological properties and distinctive 3D patterns. Capillary structured cell suspensions can potentially find applications in novel ways of 3D cell culturing, instant tissue engineering and many biomedical investigations.

Polysiloxane Inks for Multimaterial 3d Printing of High‐Permittivity Dielectric Elastomers

AbstractDielectric elastomer transducers (DET) are promising candidates for electrically‐driven soft robotics. However, the high viscosity and low yield stress of DET formulations prohibit 3D printing, the most common manufacturing method for designer soft actuators. DET inks optimized for direct ink writing (DIW) produce elastomers with high stiffness and mechanical losses, diminishing the utility of DET actuators. To address the antagonistic nature of processing and performance constraints, principles of capillary suspensions are used to engineer DIW DET inks. By blending two immiscible polysiloxane liquids with a filler, a capillary ink suspension is obtained, in which the ink rheology can be tuned independently of the elastomer electromechanical properties. Rheometry is performed to measure and optimize processibility as a function of filler and secondary liquid fraction. Including polar polysiloxanes as the secondary liquid produces a printed elastomer exhibiting a four‐fold permittivity increase over commercial polydimethylsiloxane. The characterization and multimaterial printing into layered DET devices demonstrates that the immiscible capillary suspension improves the processability of the inks and enhances the properties of the elastomers, enabling actuation of the devices at comparatively low voltages. It is anticipated that this formulation approach will allow soft robotics to harness the full potential of DETs.

Manufacture and filtration performance of glass filters made from capillary suspensions

This research highlights the potential of capillary suspensions in tailoring the properties of glass filters and improving filtration performance. Borosilicate glass 3.3 filters fabricated in a wet processing route based on capillary suspensions were developed. Their filter properties and filtration performance as well as control of shrinkage and porosity gradients during sintering were investigated. The use of capillary suspensions significantly reduced green body shrinkage. Long residence times and spatially inhomogeneous heat transfer during sintering favored the formation of porosity gradients. Pore size, porosity, permeability and flexural strength of the filters could be tailored by adjusting capillary suspension composition and sintering conditions. Glass filters with particularly high permeability and small pore sizes were successfully developed. Filtration experiments with Oenococcus oeni and polystyrene spheres (diameter ≈ 1 μm) were conducted, showing that filtration efficiency and flux depend on pore size and open porosity. Crack-free asymmetric glass membranes with improved filtration performance were developed. Compared to dry processed commercial benchmark filters, the capillary suspension-based filters made from the same glass powder had higher pore sizes and permeability. At similar pore size and porosity, filters based on capillary suspensions showed significantly higher filtrate flux than the commercial benchmarks due to a more uniform pore structure.

Oil structuring through capillary suspensions prepared with wheat middlings micronized directly in oil by high-pressure homogenization

Oleogels have been proposed as suitable systems for replacing unhealthy saturated fats in food preparations. This study aimed to structure sunflower oil through capillary suspensions using wheat middlings (WM), a residue of the wheat milling process, as the solid fraction with structuring properties. For the first time, high-pressure homogenization (HPH) was applied directly to WM dispersed in oil to reduce particle size, activate fibers, and release high-value intracellular compounds into sunflower oil. The optimized formulation of WM-in-oil-suspensions was then used to prepare capillary suspensions by adding water as a secondary immiscible fluid. Water addition drastically altered the rheological behavior and strength of oil suspensions due to the establishment of capillary forces and formation of a percolating particle network. The strength of capillary bridges depended on the particle size distribution and surface properties of WM particles and the fraction of added water (ranging between 5 and 80% v/v with respect to oil). The oil suspension at 30% w/w of HPH-treated WM solid fraction (80 MPa for 20 equivalent passes) with the addition of 50% v/v of water exhibited an apparent "transitional" yield stress >300 Pa, corresponding to gel-like behavior. Additionally, HPH treatment released antioxidant compounds contained in the solid matrix into the system. These results highlight that the resulting capillary suspension is a promising material for formulating healthier and more sustainable food products as an alternative to solid fats, with potential applications based on reduced overall caloric content and added health benefits related to dietary fiber.

Structure and rheology of oil-continuous capillary suspensions containing water-swellable cellulose beads and fibres

Cellulose particles are used in a range of materials and applications. Here, we investigated the role of cellulose sorptivity on the rheology and microstructure of oil-continuous capillary suspensions consisting of cellulose beads and fibres at volume fractions of 0.3–0.65 and water at volume fractions up to 0.2. We hypothesised that, in spite of their water-absorbent nature, cellulose particles in oil would be structured into a percolated network by water added as an immiscible secondary phase. The interplay between cellulose bead load and water content was used to generate suspensions ranging from flowable to highly elastic with moduli near 1 MPa. While capillary bridging induced a liquid to semi-solid transition, nearer particle volume fractions of 0.65, we found a transition to a solid, wet-granular material. Liquid-solid transitions occurred even if the water was absorbed by the particles, suggesting that the increase in elasticity was not solely due to the formation of capillary bridges, but also as a result of clustering and the associated increase in effective packing volume fraction. Swapping the beads with fibres increased stiffness by 4 orders whereas swapping half of the beads at the same water fraction increased material stiffness by 3 orders of magnitude. The two conclusions of this study were that, even for water-sorptive particles, capillary and hydrophilic interactions may be used to structure oil-continuous suspensions, and that the use of high aspect ratio particles can increase the rigidity of oil-continuous capillary suspensions to a greater extent than beads at an equivalent volume fraction.

Capillary detachment of a microparticle from a liquid-liquid interface.

The attachment and detachment of microparticles at a liquid-liquid interface are common in many material systems, from Pickering emulsions and colloidal assemblies to capillary suspensions. Properties of these systems rely on how the particles interact with the liquid-liquid interface, including the detachment process. In this study, we simultaneously measure the capillary detachment force of a microparticle from a liquid-liquid interface and visualize the shape of the meniscus by combining colloidal probe microscopy and confocal microscopy. The capillary behavior is studied on both untreated (hydrophilic) and fluorinated (hydrophobic) glass microparticles. The measured force data show good agreement with theoretical calculations based on the extracted geometric parameters from confocal images of the capillary bridge. It is also evident that contact line pinning is an important aspect of detachment for both untreated and fluorinated particles.

Linking intermolecular interactions and rheological behaviour in capillary suspensions

HypothesisCapillary suspensions feature networks of particles connected by liquid bridges, which are obtained by adding a small amount of a second immiscible liquid to a suspension. It is possible to link the network formation as well as the rheological behaviour of capillary suspensions to the intermolecular interactions of their constituents. Experiments and simulationsThrough a combination of experimental and numerical methods, we present a novel approach, based on Hansen solubility parameters computed from Molecular Dynamics (MD) simulations, to rationalize and predict the rheological behaviour of capillary suspensions. We investigated the formation of capillary suspensions for various combinations of bulk and secondary liquids mixed with hydrophilic silica particles. The predictions were confirmed experimentally by rheological analysis, interfacial tension measurements and microscopy (CLSM) imaging. FindingsNumerical and experimental results show that the Hansen solubility parameters theory allows to predict the formation of capillary suspensions, whose strength exponentially decays with decreasing intermolecular interactions between the secondary liquids and the dispersed particles. High immiscibility between the bulk and secondary liquid strengthens the gel up to a critical immiscibility point, above which the strength of the gel remains mostly affected by the affinity between the secondary liquids and the dispersed particles. Furthermore, we find that hydrogen-bonding and polar interactions control the formation of capillary suspensions. This simple approach can guide the selection of adequate solvents and immiscible secondary liquids, allowing an easy formulation of new particulate-based gels.

Back to the future: Fatty acids, the green genie to design smart soft materials

AbstractFatty acids are widely used in industries for various applications as soaps or in their crystalline form. Fatty acid soaps are used as surface‐active agent to stabilize foams and emulsions, for detergency, and surface wetting. Fatty acid molecules in organic solvents are used as low‐molecular‐mass organic gelators. Currently, there is a renewed interest in using fatty acids in industrial applications instead of petrochemical surfactants since fatty acid soaps are green surfactants. Fatty acids exhibit also many advantages, including the design of responsive soft‐materials. Fatty acid soaps are natural pH and thermoresponsive surfactants, which can lead to responsive foams and emulsions. In their crystalline form, fatty acid crystals in both aqueous and non‐aqueous solvents stabilize liquid foams. These crystals are also efficient in the production of liquid marbles. Fatty acids are solvosurfactants with the ability to form microemulsions and capillary suspensions. In this review, we illustrate how fatty acid molecules can be used in the manufacture of multiresponsive soft‐materials ranging from aqueous and non‐aqueous foams, emulsions, nanoemulsions, microemulsions, liquid marbles and capillary suspensions.

Effects of particle roughness on the rheology and structure of capillary suspensions

We show that particle roughness leads to changes in the number, shape and resulting capillary force of liquid bridges in capillary suspensions. We created fluorescently labeled, raspberry-like particles with varying roughness by electrostatically adsorbing silica nanoparticles with sizes between 40 nm and 250 nm on silica microparticles. Rougher particles require more liquid to fill the surface asperities before they form pendular bridges, resulting in smaller and weaker bridges. In a system where the effective bridge volume is adjusted, higher particle roughness leads to less clustered networks, which show a higher yield strain for a matching storage modulus compared to the smooth particle networks. This finding suggests that the particle-particle frictional contacts also affects the strength of capillary suspensions. Using asymptotically nonlinear oscillatory rheology, we corroborate the non-cubical power law scaling of the third harmonic in the shear stress response that results from both Hertzian contacts and friction between particles connected by capillary bridges. We demonstrate that the repulsive Hertzian contact parameter A is sensitive to the liquid bridge strength and that roughness appears to shift the relative scaling of the power law exponents from adhesive-controlled friction to load-controlled friction.

Capillary Interactions, Aggregate Formation, and the Rheology of Particle-Laden Flows: A Lattice Boltzmann Study

The agglomeration of particles caused by the formation of capillary bridges has a decisive impact on the transport properties of a variety of at a first sight very different systems such as capillary suspensions, fluidized beds in chemical reactors, or even sand castles. Here, we study the connection between the microstructure of the agglomerates and the rheology of fluidized suspensions using a coupled lattice Boltzmann and discrete element method approach. We address the influence of the shear rate, the secondary fluid surface tension, and the suspending liquid viscosity. The presence of capillary interactions promotes the formation of either filaments or globular clusters, leading to an increased suspension viscosity. Unexpectedly, filaments have the opposite effect on the viscosity as compared to globular clusters, decreasing the suspension viscosity at larger capillary interaction strengths. In addition, we show that the suspending fluid viscosity also has a non-trivial influence on the effective viscosity of the suspension, a fact usually not taken into account by empirical models.

Giant Functional Properties in Porous Electroceramics through Additive Manufacturing of Capillary Suspensions

Dedicated hierarchical structuring of functional ceramics can be used to shift the limits of functionality. This work presents the manufacturing of highly open porous, hierarchically structured barium titanate ceramics with 3-3 connectivity via direct ink writing of capillary suspension-type inks. The pore size of the printed struts (∼1 μm) is combined with a printed mesostructure (∼100 μm). The self-organized particle network, driven by strong capillary forces in the ternary solid/fluid/fluid ink, results in a high strut porosity, and the distinct flow properties of the ink allow for printing high strut size to pore size ratios, resulting in total porosities >60%. These unique and highly porous additive manufactured log-pile structures with closed bottom and top layers enable tailored dielectric and electromechanical coupling, resulting in an energy harvesting figure of merit FOM33 more than four times higher than any documented data for barium titanate. This clearly demonstrates that combining additive manufacturing of capillary suspensions in combination with appropriate sintering allows for creation of complex architected 3D structures with unprecedented properties. This opens up opportunities in a broad variety of applications, including electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics, or bone tissue engineering with piezoelectrically stimulated cell growth.

Particle contact dynamics as the origin for noninteger power expansion rheology in attractive suspension networks

We show that Hertzian particle contacts are the underlying cause of the as-yet-unexplained noninteger power laws in weakly nonlinear rheology. In the medium amplitude oscillatory shear (MAOS) region, the cubic scaling of the leading order nonlinear shear stress (σ3∼γ0m3, m3=3) is the standard expectation. Expanding on the work by Natalia et al. [J. Rheol. 64, 625–635 (2020)], we report an extensive data set of noncubical, noninteger power law scalings m3 for particle suspensions in two immiscible fluids with a capillary attractive interaction, known as capillary suspensions. Here, we show that distinct power law exponents are found for the storage and loss moduli and these noninteger scalings occur at every secondary fluid concentration for two different contact angles. These compelling results indicate that the noninteger scalings are related to the underlying microstructure of capillary suspensions. We show that the magnitude of the third harmonic elastic stress scaling m3,elastic originates from Hertzian-like contacts in combination with the attractive capillary force. The related third harmonic viscous stress scaling m3,viscous is found to be associated with adhesive-controlled friction. These observations, conducted for a wide range of compositions, can help explain previous reports of noninteger scaling for materials involving particle contacts and offers a new opportunity using the variable power law exponent of MAOS rheology to reveal the physics of particle bonds and friction in the rheological response under low deformation instead of at very high shear rates.

Capillary bridge formation at room temperature in binary liquids with small miscibility

Capillary suspensions, solid-liquid-liquid systems, are conventionally formulated by dispersing the particles in the bulk phase and then adding a small amount of secondary liquid (<5 vol%), leading to increased yield stress. Previous work has shown that partially miscible liquids form capillary bridges between particles upon temperature induced phase separation. In this work, we show that capillary suspensions can be formed by mixing particles with partially saturated 1-alkanol (6–8 carbon atoms)/water solutions, without the need for a temperature induced phase separation. Moreover, we show this effect works with different particle sizes and geometries. In addition, we illustrate that a pure suspension formed with 1-alkanols can create capillary suspensions when stored in humid conditions. This paper also demonstrates how partially miscible oil suspensions form capillary bridges even during the drying process when exposed to high humidity. This can lead to a significant decrease in drying stresses but also a change in structure.

Influence of drying conditions on the stress and weight development of capillary suspensions

AbstractCracking of suspensions during drying is a common problem. While additives, for example, binders and surfactants, can mitigate this problem, some applications, such as printing conductive pastes or sintering green bodies, do not lend themselves to the use of additives. Capillary suspensions provide an alternative formulation without additives. In this work, we use simultaneous stress and weight measurements to investigate the influence of formulation and drying conditions. Capillary suspensions dry more homogeneously and with lower peak stresses, leading to an increased robustness against cracking compared. An increase in dry film porosity is not the key driver for the stress reduction. Instead, the capillary bridges, which create strong particle networks, resist the stress. Increasing the relative humidity enhances this effect, even for pure suspensions. While lower boiling point secondary liquids, for example, water, persist for very long times during drying, higher boiling point liquids offer further potential to tune the drying process.

Rheology of thermo-gelling capillary suspensions

Rheological measurements were performed to examine the behavior of thermally responsive capillary suspensions. These capillary suspensions were prepared by dispersing laponite nanoparticles in a continuous oil phase and an immiscible secondary fluid consisting of an aqueous solution of a thermogelling polymer, Pluronic F127 (PF127). The addition of PF127 (which undergoes sol-gel transition at elevated temperatures) into the secondary phase resulted in a capillary-networked suspension whose rheology can be tailored by increasing the temperature above the gelling temperature of PF127. We investigated the yielding behavior of this system using dynamic oscillatory measurements at different temperatures. These suspensions yield via a two-step process which was observed as two distinct plateaus for storage modulus in a typical plot of storage modulus versus oscillation strain. This yielding behavior is attributed to the rupturing of the bicontinuous network structure at lower strains which is followed by the breakup of particle aggregates at larger strains.

Using an added liquid to suppress drying defects in hard particle coatings

HypothesisLateral accumulation and film defects during drying of hard particle coatings is a common problem, typically solved using polymeric additives and surface active ingredients, which require further processing of the dried film. Capillary suspensions with their tunable physical properties, devoid of polymers, offer new pathways in producing uniform and defect free particulate coatings. ExperimentsWe investigated the effect of small amounts of secondary liquid on the coating’s drying behavior. Stress build-up and weight loss in a temperature and humidity controlled drying chamber were simultaneously measured. Changes in the coating’s reflectance and height profile over time were related with the weight loss and stress curve. FindingsCapillary suspensions dry uniformly without defects. Lateral drying is inhibited by the high yield stress, causing the coating to shrink to an even height. The bridges between particles prevent air invasion and extend the constant drying period. The liquid in the lower layers is transported to the interface via corner flow within surface pores, leading to a partially dry layer near the substrate while the pores above are still saturated. Using capillary suspensions for hard particle coatings results in more uniform, defect free films with better printing characteristics, rendering high additive content obsolete.