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

High-Thermal-Conductivity and High-Fluidity Heat Transfer Emulsion with 89 wt % Suspended Liquid Metal Microdroplets

Colloidal suspensions of thermally conductive particles in a carrier fluid are considered promising heat transfer fluids for various thermal energy transfer applications, such as transportation, plants, electronics, and renewable energy systems. The thermal conductivity (k) of the particle-suspended fluids can be improved substantially by increasing the concentration of conductive particles above a "thermal percolation threshold," which is limited because of the vitrification of the resulting fluid at the high particle loadings. In this study, eutectic Ga-In liquid metal (LM) was employed as a soft high-k filler dispersed as microdroplets at high loadings in paraffin oil (as a carrier fluid) to produce an emulsion-type heat transfer fluid with the combined advantages of high thermal conductivity and high fluidity. Two types of the LM-in-oil emulsions, which were produced via the probe-sonication and rotor-stator homogenization (RSH) methods, demonstrated significant improvements in k, i.e., Δk ∼409 and ∼261%, respectively, at the maximum investigated LM loading of 50 vol % (∼89 wt %), attributed to the enhanced heat transport via high-k LM fillers above the percolation threshold. Despite the high filler loading, the RSH-produced emulsion retained remarkably high fluidity, with a relatively low viscosity increase and no yield stress, demonstrating its potential as a circulatable heat transfer fluid.

Direct-Writable and Thermally One-Step Curable “Water-Stained” Epoxy Composite Inks

In this study, a simple method for preparing direct-writable and thermally one-step curable epoxy composite inks was proposed. Specifically, colloidal inks containing a mixture of ordinary epoxy resin and anhydride-type hardener with the suspended alumina microplates, as exemplary fillers, are “stained” with small amounts of water. This increases the elasticity of the ink via the interparticle capillary attraction and promotes curing of the epoxy matrix in low-temperature ranges, causing the three-dimensional (3D) printed ink to avoid structural disruption during one-step thermal curing without the tedious pre-curing step. The proposed mechanisms for the shape retention of thermally cured water-stained inks were discussed with thorough analyses using shear rheometry, DSC, FTIR, and SEM. Results of the computer-vision numerical analysis of the SEM images reveal that the particles in water-stained inks are oriented more in the vertical direction than those in water-free samples, corroborating the proposed mechanisms. The suggested concept is extremely simple and does not require any additional cost to the one required for the preparation of the common epoxy–filler composites, which is thus expected to be well-exploited in various applications where 3D printing of epoxy-based formulations is necessary.

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.

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.