Pickering Emulsion Transitions in 2,6-Lutidine Plus Water Critical Liquid Mixtures

Pickering stabilization mechanism revealed through direct imaging of particles with tuneable contact angle in a phase-separated binary solvent

Pickering emulsions have attracted increasing attention from multiple fields, including food, cosmetics, healthcare, pharmaceutical, and agriculture. Their stability relies on the presence of colloidal particles instead of surfactant at the droplet interface, providing steric stabilization. Here, we demonstrate the microscopic attachment and detachment of particles with tunable contact angle at the interface underlying the Pickering emulsion stability. We vary the interfacial tension continuously by varying the temperature offset of a phase-separated binary liquid from its critical point, and employ confocal microscopy to directly observe the particles at the interface to determine their coverage and contact angle as a function of the varying interfacial tension. When the interfacial tension decreases upon approaching the binary liquid's critical point, the contact angle and detachment energy (ΔE) drop, and the particles move out of the interface. Microscopic imaging suggests necking and capillary interactions lead to clustering of the particles, before they eventually desorb from the interface. Macroscopic measurements show that concomitantly, coalescence takes place, and the emulsion loses its stability. These results reveal the interplay of interfacial energies, contact angle and surface coverage that underlies the Pickering emulsion stability, opening up ways to manipulate and design the stability through the microscopic behavior of the adsorbed particles.

Effect of the Interfacial Tension on the Stability of Silica Stabilized Pickering Emulsions near the Lower Critical Solution Temperature of 2,6-Lutidine - Water Mixtures

In this paper, the kinetic stability of Pickering emulsions stabilized by spherical silica particles (100 nm in diameter) was examined in the water - 2,6-lutidine mixture. In the close vicinity of the lower critical solution temperature, Pickering emulsions were unstable due to the ultra-low liquid-liquid interfacial tension but increased their stability with increasing the temperature. In this system, the interfacial tension obeys universal scale law and can be tuned by temperature without adding any surface-active agents. Owing to this unique feature, we elucidated the relation between the interfacial tension and the stability of Pickering emulsions.

Curved colloidal crystals of discoids at near-critical liquid-liquid interface

The assembly of disc-shaped particles at curved liquid-liquid interfaces was studied by using confocal microscopy. The interface is formed by a phase-separating critical liquid mixture of 2,6-lutidine and heavy water, where the colloids spontaneously assembled forming a dome. The novelty of this system is three-fold. First, the domes can be constructed and annihilated remotely and reversibly, which allows dynamic control of the colloidal assembly. Second, the effect of curvature can be investigated by analyzing domes of different radii ranging from 5 μm to 125 μm. Third, the slow dynamics due to hydrodynamic interaction among the particles can be utilized to investigate the time-evolution of defect morphology. Unlike the widely studied repulsive colloids, the interparticle potential near the critical point has an attractive component. I contrasted the packing and defects morphology of a solid-like and liquid-like dome differing in particle number density. In the solid-like dome, a chain of 5- and 7-fold coordinated particles was observed. The analysis of trajectories showed that particles were bound in a potential well of a depth of about ten times the thermal energy, which matched well with the calculation of the pair-potential by considering the attractive critical Casimir force among the particles. In the liquid-like dome, 6-fold particles separated by clusters of 5- and 7-coordinated particles were observed, which is suggestive of liquid-solid coexistence. The uniqueness of this system will open up a new research avenue to investigate the effect of varying curvature on the crystallization, defects, and phase diagram of colloidal assemblies.