Mixtures of aqueous and oil foams, both stabilised by colloidal particles

Abstract

Foams and emulsions are crucial colloidal dispersions that are extensively used in a broad range of scientific and industrial applications. Multiphase systems that incorporate both foam and emulsion structures would provide a greater variety of formulation possibilities. Drawing inspiration from the culinary world, we formulated composite soft materials in this thesis, similar to how aqueous egg white and oily butter foams are combined as ingredients in cake frosting.

We created a composite soft material called an emulsion of particle-stabilised foams, which is a combination of two types of well-studied colloidal materials: emulsions and foams. This composite is profoundly different to foamed emulsions that contain immiscible liquid droplets within the liquid phase of foams. In our samples, bubbles exist in both the aqueous and oil phases. Here, aqueous foams are stabilised by hydrophobic silica particles in a mixture of water and propylene glycol; and oil foams are stabilised by partially fluorinated particles in olive oil. Like traditional emulsions, catastrophic and transitional phase inversions occur between oil-in-water (O/W) type and water-in-oil (W/O) type mixed samples. Catastrophic phase inversion is caused by excessive addition of the internal phase, while transitional phase inversion occurs as the contact angle of particles used to stabilise the system alters.

Specifically, we induce catastrophic phase inversion by adjusting the foam proportion, and transitional phase inversion by modulating the concentration of propylene glycol. An intermediate phase-separated state has been observed during the transitional phase inversion process. It contains both W/O and O/W regions. The sample stability highly correlates to its microstructure: the W/O and the phase-separated samples are the most and least stable ones, respectively. In addition to the morphology observed through the microscope, the phase inversions are also evident from the sample flow properties.

Generally, the W/O to O/W inversion is indicated by the sample becoming more and more solid-like. Meanwhile, the rheological signatures of mixed samples are also influenced by the underlying aqueous and oil foam flow properties.

Additionally, we also investigated the mixture of particle-stabilised oil foams and aqueous foams in the form of thin films. Aqueous and oil foams do not tend to combine automatically as these two liquid phases repel each other. However, by spinning the composite foams and then sandwiching them between two coverslips, tortuous air channels across large parts of the thin-film samples appear. The spinning promotes both liquid drainage and bubble mixing, while the coverslips provide external forces and space constraints. In previous research, such as the capillary foam created by adding small quantities of oil into a particle-stabilised aqueous foam, one of the two immiscible liquids commonly functions as a minority component. Unusually, in our samples, with roughly equal quantities, bubbles are found in aqueous regions, oil regions and sometimes spanning both.

In the mid-wet two-dimensional thin-film samples, we noticed a dye absence in some regions of the aqueous phase. Meanwhile, the dye had been observed to aggregate at the air-aqueous and oil–aqueous interfaces. We suspect that this may be consistent with the silica particle distribution. To verify whether this distribution phenomenon also exists in our bulk samples, we froze the emulsion of particle-stabilised foams with liquid nitrogen, then detected and quantified its elemental composition by cryogenic scanning electron microscopy (cryo-SEM) and energy-dispersive X-ray spectroscopy (EDS). The aqueous and oil phases are distinguished by the amount of hydrogen and carbon, respectively. The intersection points of the carbon and hydrogen counts represent the positions of the oil-aqueous interfaces. Independent of the types of samples, the tiny but appreciable quantitative silica peaks corresponding to these interfacial positions indicate that silica particles are critical for emulsion stabilisation.

Furthermore, we observed that our emulsion of particle-stabilised foams, par- ticularly the W/O samples with the smallest average droplet size, remained relatively stable in vials for over a week. We attempted to destroy these samples through centrifuging, using a ball bearing and by drying.

Surprisingly, during the drying process, the mousse-like texture of the W/O samples persisted for at least a week. Based on this stable W/O emulsion of particle-stabilised foams system, we investigated the possibility of using the dispersed phase with foam structure to prevent compositional ripening in W/O emulsions with two aqueous dispersed phases, each containing different proportions of water and propylene glycol. However, the presence of propylene glycol emerged as a key factor in limiting ripening.