Interfacial viscoelasticity and aging effect on droplet formation and breakup
Giménez-Ribes, Gerard ; Sagis, Leonard M.C. ; Habibi, Mehdi - \ 2020
Food Hydrocolloids 103 (2020). - ISSN 0268-005X
Bovine serum albumin - Capillary breakup - Drop formation - Escin - SDS - Surface rheology - Surfactant - Viscoelastic interfaces - β-Lactoglobulin
Interfacial rheology of interfaces stabilized by food ingredients has been studied extensively within the limits of small and slow deformations, within the linear viscoelastic regime (LVR). However, in practice, products such as foams or emulsions undergo fast and large deformations during food processes such as chewing. emulsifying, or foaming. In this study, we used capillary break-up and droplet formation to observe and quantify large and fast deformations of such interfaces, in the presence of surface viscoelasticity. We studied the inertial thinning of the liquid neck during drop formation using high-speed imaging. We tested solutions of several surface active molecules, including two low molecular weight surfactants (SDS and Tween 20), two milk proteins (β-Lactoglobulin, Bovine Serum Albumin) and the saponin Escin. Escin showed the most significant effect on the shape of the neck and neck thinning dynamics. Interfacial shear rheology measurements indicated that the unusual breakup observation of Escin solutions was related to the high viscoelastic surface shear moduli. These high moduli, and the resulting more symmetric and shorter neck shape observed for the proteins and Escin, are associated with the formation of solid-like interfacial structures. However, the presence of a viscoelastic interface did not affect the inertial thinning regime, which indicates that the solid structures were broken down locally at the area of minimum radius (Rmin), and affected only the shape outside this region. The results of this work therefore, show that interfacial measurements within the LVR are not entirely representative for fast and large deformations well beyond linearity.
Interfacial rheology and relaxation behavior of adsorption layers of the triterpenoid saponin Escin
Giménez-Ribes, Gerard ; Habibi, Mehdi ; Sagis, Leonard M.C. - \ 2020
Journal of Colloid and Interface Science 563 (2020). - ISSN 0021-9797 - p. 281 - 290.
Dilatational - Escin - Interfacial rheology - Non-linear Viscoelastic (NLVE) regime - Relaxation - Saponin - Shear - Stretched exponential
Hypothesis: Escin, a monodesmosidic triterpenoid saponin, was shown previously to form viscoelastic interfaces with a very high dilatational and surface shear storage modulus. This is expected to be due to the arrangement of Escin into 2D disordered soft viscoelastic solid interfacial structures, which results in turn in a distribution of relaxation times. Experiments: The responses to dilatational and surface shear deformations of Escin-stabilized air-water interfaces were studied, both in the linear viscoelastic (LVE) and non-linear (NLVE) regime. Step relaxation and amplitude sweeps were performed in dilatation experiments. For surface shear, amplitude sweeps and creep recovery experiments were performed. Findings: Escin stabilized-interfaces displayed a highly non-linear behavior in dilatation as seen in the Lissajous plots. In large oscillatory shear the Lissajous curves had a rhomboidal shape, indicating intracycle yielding and recovery, typical of glassy systems. The relaxation of the interface showed stretched exponential behavior, with stretched exponents typical of disordered solids with dynamic heterogeneity. The use of surface rheological measurements beyond the commonly measured LVE regime clearly has provided new insights into the behavior of these interfaces and their microstructure. These results highlight the need to reconsider other complex interfaces as disordered solids and not as 2D homogenous viscoelastic fluids.