Rheology and structural properties of protein and particle-stabilized foams - a multi-scale approach

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The project aims at a deeper understanding and a better control of rheology and structural properties of protein and particle-stabilized foams. Therefore, the correlation between structure and dynamics has to be investigated on different length scales. We want to employ a multi-scale approach comprising the macroscopic foam, single foam films/ lamellae as well as the liquid/air interface. This strategy requires different scientific and technical skills not covered by an individual research group. The Willenbacher group (mechanical process engineering) contributes its expertise regarding rheology of foams and interfaces. The Müller-Buschbaum group (physics) will use various scattering techniques to explore the structure of the adsorbed amphiphilic surface layer. The v. Klitzing group (physical chemistry) has profound experience in characterizing interactions in foam films and interfacial absorption phenomena.Yield stress and shear modulus are crucial parameters characterizing rheological properties of foams. Preliminary work on milk protein foams disclosed that these physical parameters are directly related to interfacial elastic properties of the corresponding protein solutions. But under certain physico-chemical conditions drastic deviations from these simple correlations between foam rheology and interfacial elasticity were found presumably due to aggregation and structure formation within foam films interconnecting adjacent surface layers. Based on these findings the proposed project has two goals:1. We want to evaluate whether the simple relationship between yield stress and modulus of foams and interfacial elasticity of corresponding solutions found for milk proteins is also valid for other systems. Different proteins and nanoparticles will be used for foam stabilization. Based on these data a predictive physical model relating foam rheological parameters and interfacial elasticity of corresponding solutions will be developed.2. Under certain physico-chemical conditions (pH, ionic strength and valency) the above mentioned relationship between interfacial and foam rheology fails. Aggregation and structure formation of surface active ingredients at the liquid/ air interface and within foam lamellae will be investigated.Specular and grazing incidence X-ray and neutron reflection will be used to elucidate the lateral and vertical composition of the surface layer (length scale: 1-100 nm). Lateral heterogeneities of the surface layer on a micrometer length scale will be deduced from variations in the thermal diffusivity of tracer particles. Furthermore, aggregation and structure formation will be explored with respect to foam film stability using the Thin Film Pressure Balance technique. Structural investigations during drainage will reveal how and at which characteristic state of drainage structure formation across foam lamellae takes place.