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Emulsion stabilized by proteins – surfactant-covered drops or capsules?

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The deformation of emulsion drops covered with a globular protein layer (β-lactoglobulin, lysozyme) show i) that the adsorption layer restricts the ease of deformation, rather than facilitating it and ii) that shape fluctuations (tank-treading) are caused by the elastic deformation of the (visco-)elastic skin. The obtained data are compared to predictions of the Flumerfeld model inwhich interfacial viscosities and Marangoni stresses are incorporated. We see that the experimental values cannot be reached with the Flumerfelt model if the interfacial rheological model parameters are varied in a realistic range and all other parameters are kept constant. All parameter necessary to compute the interfacial stress boundary condition are characterized by i) interfacial tension or interfacial pressure, ii) interfacial shear rheology, and iii) interfacial dilatational rheology. Alternatively, the data are compared to a model by Barthès-Biesel and Sgaier for the deformation of capsules surrounded by a solid membrane. It is found that for strongly viscoelastic interfacial layer of adsorbed lysozyme and β-lactoglobulin, only the capsule model can give a satisfactory description of the data. In comparison the response of the structurally different protein β-casein (flexible random coil) is briefly discussed. This result suggests that globular protein layer should be seen as “soft elastic membranes“ with interfacial tension properties, i.e. as a hybrid between elastic membranes (no interfacial tension properties) and of soluble emulsifier layers with interfacial tension.

This talk is part of the BSS Formal Seminars series.

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