The behavior of capillary suspensions at diverse length scales: From single capillary bridges to bulk

How bulk liquid viscosity shapes capillary suspensions

HYPOTHESIS

Capillary suspensions offer a new approach to generate novel materials. They are ternary liquid-liquid-solid systems characterized by particles connected by liquid bridges of one fluid suspended in a second immiscible bulk fluid. The viscosity of the bulk liquid can be modulated to customize the structure and rheological properties of capillary suspensions. Experiments and simulations: Using experiments and numerical simulations, we investigated capillary suspensions in the pendular state, using silica particles and water as a bridging liquid. To modulate the viscosity of the bulk fluid, we use different ratios of either dodecane and diisononyl phthalate, or silicone oils with varying chain lengths as bulk liquids. The rheological behavior was characterized using the maximum storage and loss moduli and the yielding behavior. This was related to structural changes of the systems, which was visualized using confocal laser scanning microscopy. In addition, we used Molecular Dynamics (MD) simulations to gain more insights into the behavior of two particles connected by a liquid bridge for various bulk liquids.

FINDINGS

Experiments show that higher bulk liquid viscosity reduces strength, yield stress, and yield strain in capillary suspensions, which is partly attributed to a reduced inter-connectivity of the percolating network. This is caused by the breakup of liquid bridges occurring at shorter distances in the presence of highly viscous bulk liquids, as indicated by numerical simulations.

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.

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.

On the gelation of humins: from transient to covalent networks

Humins are a by-product of many acid-catalyzed biorefinery processes converting polysaccharides into platform chemicals. The valorization of humin residue to increase the profit of biorefinery operations and reduce waste is a field that is growing interest as the production of humins continues to increase. This includes their valorization in materials science. For successful processing of humin-based materials, this study aims to understand the thermal polymerization mechanisms of humins from a rheological perspective. Thermal crosslinking of raw humins leads to an increase in their molecular weight, which in turn leads to the formation of a gel. Humin's gels structure combines physical (thermally reversible) and chemical (thermally irreversible) crosslinks, and temperature plays an essential role in the crosslink density and the gel properties. High temperatures delay the formation of a gel due to the scission of physicochemical interactions, drastically decreasing their viscosity, whereas upon cooling a stronger gel is formed combining the recovered physicochemical bonds and the newly created chemical crosslinks. Thus, a transition from a supramolecular network to a covalently crosslinked network is observed, and properties such as the elasticity or reprocessability of humin gels are influenced by the stage of polymerization.