Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk oleogel rheology and interfacial rheology

Abstract

Oleofoams are dispersions of gas bubbles in a continuous oil phase and can be stabilized by crystals of fatty acids or waxes adsorbing at the oil-air interface. Because excess crystals in the continuous phase form an oleogel, an effect of the bulk rheology of the continuous phase is also expected. Here, we evaluate the contributions of bulk and interfacial rheology below and above the melting point of a wax forming an oleogel in sunflower oil. We study the dissolution behaviour of single bubbles using microscopy on a temperature-controlled stage. We compare the behaviour of a bubble embedded in an oleofoam, which owes its stability to both bulk and interfacial rheology, to that of a bubble extracted from the oleofoam and resuspended in oil, for which the interfacial dilatational rheology alone provides stability. We find that below the melting point of the wax, bubbles in the oleofoam are stable whereas bubbles that are only coated with wax crystals dissolve. Both systems dissolve when heated above the melting point of the wax. These findings are rationalized through independent bulk rheological measurements of the oleogel at different temperatures, as well as measurements of the dilatational rheological properties of a wax-coated oil-air interface.

Recent Advances in Understanding and Use of Oleofoams

“…The resulting rheological properties of oleofoams is still debated, but appears to be both due to contribution of the crystals coating the bubbles and to the rheology of the gelled continuous phase in which the bubbles are dispersed (Gunes et al, 2017;Mishra et al, 2020;Saha et al, 2020) (see section Stabilization Mechanisms). Gunes et al determined that the elastic modulus of oleofoams are two orders of magnitude higher than the modulus of a broken (=highly sheared) oleogel at the same concentration of monoglycerides in the oil, but still lower that the initial unbroken network at rest (Gunes et al, 2017).…”

“…In oleofoams, the long-term stability result from both bulk and interfacial contributions (Gunes et al, 2017;Saha et al, 2020). Various destabilizing phenomena tends to destroy such gas dispersions.…”

“…It is therefore important to associate the interfacial and bulk properties only to the destabilization mechanism that they actually hinder. The presence of the crystals layer at the bubble surface has a first stabilizing effect by reducing the surface tension, but they act mostly through the occurrence of a packed elastic layer (Shrestha et al, 2006;Saha et al, 2020). The main goal of such a layer is to prevent coalescence and disproportionation (or ripening).…”

“…The main goal of such a layer is to prevent coalescence and disproportionation (or ripening). By diluting oleofoams with oil, only single bubbles can be observed (Gunes et al, 2017;Saha et al, 2020). The crystals remained attached to the bubbles and the non-spherical shape remained during few minutes to hours depending on the oleofoam systems and process ( Figure 2B) (Gunes et al, 2017;Saha et al, 2020).…”

Acoustic bubble dynamics in a yield-stress fluid

“…Bubble removal should be easier in almost any other yield-stress fluid as they break down under much smaller strains. 59,60 Increasing Z s could also improve bubble removal by raising the critical pressure at which shape oscillations arise.…”