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

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.

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.

Linking intermolecular interactions and rheological behaviour in capillary suspensions

HYPOTHESIS

Capillary 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 SIMULATIONS

Through 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.

FINDINGS

Numerical 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.

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.

A pickering emulsion stabilized by chlorella microalgae as an eco-friendly extrusion-based 3D printing ink processable under ambient conditions

Three-dimensional (3D) printing technology is actively utilized in various industrial fields because it facilitates effective and customizable fabrication of complex structures. An important processing route for 3D printing is the extrusion of inks in the form of colloidal suspensions or emulsions, which has recently attracted considerable attention because it allows for selection of a wide range of printing materials and is operable under ambient processing conditions. Herein, we investigate the 3D printability of complex fluids containing chlorella microalgae as an eco-friendly material for 3D printing. Two possible ink types are considered: aqueous chlorella suspensions and emulsions of oil and water mixtures. While the aqueous chlorella suspensions at high particle loading display the 3D-printable rheological properties such as high yield stress and good shape retention, the final structures after extruding and drying the suspensions under ambient conditions show a significant number of macroscopic defects, limiting their practical application. In contrast, the 3D structures produced from the oil-in-water Pickering emulsions stabilized by chlorella microalgae, which are amphiphilic and active at the oil-water interface, show significantly reduced defect formation. Addition of a fast-evaporable oil phase, hexane, is crucial in the mechanisms of enhanced cementation between the individual microalgae via increased inter-particle packing, capillary attraction, and hydrophobic interaction. Furthermore, addition of solid paraffin wax, which is crystalline but well-soluble in the hydrocarbon oil phase under ambient conditions, completely eliminates the undesirable defect formation via enhanced inter-particle binding, while maintaining the overall rheological properties of the emulsion. The optimal formulation of the Pickering emulsion is finally employed to produce a 3D scaffold of satisfactory structural integrity, suggesting that the chlorella-based ink, in the form of an emulsion, has potential as an eco-friendly 3D printing ink processable under ambient conditions.

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.

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.