Particle-size effects in the formation of bicontinuous Pickering emulsions

Abstract

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway i.e. nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

Nanoparticle Assembly at Liquid–Liquid Interfaces: From the Nanoscale to Mesoscale

“…Two features are essential for the formation and stabilization of bijels: first, the particles must be neutrally wetting with the two liquid phases (contact angle ≈ 90°), so as to avoid the particles imposing a preferred curvature on the liquid–liquid interface during phase separation; second, fluid demixing by spinodal decomposition rather than nucleation is needed to drive a bicontinuous arrangement of the liquid domains, which always requires the fulfilment of several stringent criteria for successful preparation. These difficulties in fabrication are more severe for bijels stabilized by NPs (especially diameter < 50 nm), owing to their comparatively low binding energies to the water/oil interface, especially at the low surface tensions present in bijels, which in turn restricts the smallest accessible domain sizes in these materials …”

Building Reconfigurable Devices Using Complex Liquid–Fluid Interfaces

“…θ can also be derived pictorially by performing a force balance on the three interfacial tensions drawn in Figure , yieldingwhere γ op and γ wp are the oil–particle and water–particle surface tensions, respectively. Typical values of Δ E vary from 1 to 10 kT for nanometer‐sized particles up to 10 6 kT for micrometer‐sized particles . The quadratic dependence of Δ E upon particle size means that as r increases, Δ E increases rapidly and micrometer‐sized particles can be treated as irreversibly adsorbed, even at interfaces with extremely low surface tensions .…”

Bicontinuous structured liquids with sub-micrometre domains using nanoparticle surfactants

“…3b), roughly twice the diffraction limit of the objective used. This represents a reduction in domain size of over an order of magnitude relative to the current state of the art 19 . Significant further reductions in channel diameter could be achieved by using more energetic homogenization methods (for example, a rotor-stator or ultrasonic probe), and we are currently investigating this by hard and soft X-ray and neutron scattering methods.…”

“…Furthermore, a nanoparticle dispersion will stabilize a greater surface area at a given volume fraction, as there will be greater particle cross-sectional area per unit volume 13 . However, ensuring uptake and irreversible binding of nanoparticles to the oil-water interface has proven to be difficult, as has producing the promised reduction in domain size 19 . Applying the binding of .…”