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

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.

     

Rheometric properties of micron-sized CaCO3 suspensions stabilised by a physical polyol/silica gel for polyurethane foams

This article considers the rheometric properties of mixtures containing a micron-sized mineral filler of calcium carbonate (CaCO₃) in a polymer matrix gelled by adding colloidal silica (CS). These mixtures, consisting of a polymer matrix (polyols, catalysts, surfactant) are used to produce polyurethane foams. The suspending phase (polymer matrix/CS) has a yield stress that has been linked to fractal aggregation of the colloidal filler. Suspensions without any colloidal silica (polymer matrix/CaCO₃), show aggregation of CaCO₃ which is most probably due to the adsorption of catalysts present in the polymer matrix. Beyond a critical CaCO₃ volume fraction, a yield stress is detected indicating a 3D connected structure. In the case of suspensions containing colloidal silica (polymer matrix/CaCO₃/SC), the yield stress is due to a combination of the fractal network formed by the colloidal silica and aggregation of the micron-sized particles of CaCO₃.     

Fractal behavior and scaling law of hydrophobic silica in polyol

This article examines the rheological properties of a system composed of polyol and colloidal silica. Three types of nanosized silicas with hydrophilic and hydrophobic surfaces were studied: A200 with OH surface groups, R974 with CH(3) surface groups, and R805, which is grafted with a C(8)H(17) alkyl chain. Rheometric measurements showed that the dispersions of R805 silicas have a yield stress at low volume fraction, unlike the R974 and A200 silicas. The plastic behavior of the hydrophobic silicas was quantified by a yield stress sigma(0) and an elastic modulus G'. It is observed that these parameters follow scaling laws as a function of the volume fraction of silica introduced, in the form sigma(0) approximately phi(v)(2.9+/-0.2), G' approximately phi(v)(4.1+/-0.3). Static light scattering (SLS) and small angle neutron scattering (SANS) measurements show a fractal arrangement with a fractal dimension D=1.8 ranging from elementary particles of about 32 nm to aggregates measuring about 6 mum. Correlations were established between the theoretical scaling laws and the experimental scaling laws determined by rheometric measurements. The fractal structure observed in this system is explained by the attractive physical interaction of the octyl chains between the silica particles. Contrary to what has been observed in the past by Khan and Zoeller (J. Rheol. 37 (1993) 1225), the lower molecular weight of the polyol studied here, which has a shorter chain length, allows direct bridging of two separate silicates though alkyl chains, giving rise to the formation of a 3D gel network at volume fractions as low as phi(v)=2.2%.