Tailoring the structure of colloid-stabilized bicontinuous emulsions via regulated diffusion
Authors:   , de Ruiter Rosa Mathilde 
Published: October 2024

Bicontinuous emulsions are composed of two immiscible liquids that both form an intimately intertwined tunnel network. They are of large interest because they allow mass transport in both phases while providing a large surface area between them. Bicontinuous emulsions can be stabilized through interfacial attachment of colloidal particles. This gives rise a metastable viscoelastic material called bijel in which two interpenetrating, continuous domains of immiscible fluids are separated by a percolating layer of particles.

Chapter 2 describes how bijel fibers are prepared using the method of solvent transfer induced phase separation (STrIPS). A homogeneous bijel precursor mixture, composed of diethyl phthalate (oil), water, 1-propanol and silica nanoparticles, is flown into a toluene phase using microfluidics. When 1-propanol diffuses out of the precursor mixture, phase separation occurs via spinodal decomposition. A bicontinuous liquid arrangement is formed and stabilized through attachment of the silica particles at the oil-water interface. Characterization via confocal laser scanning microscopy (CLSM) reveals the spatial distribution of oil, water and nanoparticles in the bijel fibers. Scanning electron microscopy (SEM) allows visualization of the porous nanoparticle scaffold.

The goal of this thesis is to investigate how the morphology of STrIPS bijels can be tailored through adaptations to the experimental fabrication process. Here, we focus on control over properties that are relevant for the industrial application of bijels: bijel pore size (Chapter 3), connectivity between bijel channel networks and bulk phases of oil and water (Chapter 4) and the scalability of the fabrication process (Chapter 5). The knowledge gains resulting from this work promote the use of bijel in future applications.

In Chapter 3 we demonstrate two ways to control the pore size of STrIPS bijels. First, we find
that decerasing the nanoparticle weight fraction in the bijel precursor mixture decreases the pore sizes of bijels. Second, we tune the pore sizes of the bijels through regulation of the co-solvent diffusion process during STrIPS. To this end, we incorporate an inner toluene stream with variable 1-propanol concentration in the bijel fiber synthesis. Higher 1-propanol concentrations slow down STrIPS, leading to increased pore sizes and introducing asymmetry into the pore size distribution.

How can the bicontinuous pores of bijels be connected to their respective bulk phases? This important question is addressed in Chapter 4, where we selectively connect the bijel’s water channel network with an aqueous bulk phase by co-extruding the bijel precursor mixture with aqueous inner streams of different compositions. The formation of unobstructed channel connections is observed through CLSM imaging. We find that nanoparticle stability and diffusion processes between the precursor mixture and the inner stream play an important role.

The promising application potentials for bijels call for a fabrication method that allows manufacturing of bijels on industrial scales. In Chapter 5, we combine STrIPS with roll-to-roll coating techniques to fabricate bijel films with controlled dimensions at high throughput. Here, it is crucial to control the wetting of the bijel precursor mixture on the solid substrate through the selection of a suitable liquid composition of the precursor mixture.