Kühnhammer, Matthias (2022)
Internal structure of aqueous foams stabilised by surfactants or microgels.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00022559
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Foams are abundant in everyday life in the form of cleaning agents or personal care products. In addition, they play an important role in certain industrial processes like flotation in the mining industry or textile manufacturing. Many foam properties like stability or rheology are governed by their structure. However, the internal structure of foams is difficult to determine experimentally. Optical methods are only able to probe the first few layers of bubbles, because of the large difference in refractive indices of gas and (mostly aqueous) liquid phase. This thesis aims to extend the experimental accessibility of the foam structure. For this purpose a sample environment enabling small-angle neutron scattering (SANS) experiments with macroscopic foams is designed and constructed. This sample environment allows measuring foams at different drainage stages, controlling the rate of foam formation, temperature and measurement position. In addition, a sample changer for up to three foam cells is included to utilise the full potential of future high brilliance neutron sources like the European Spallation Source (ESS). In order to extract structural information about the foam from the data, a new model for the description of SANS data from foams is presented. This model is based on an incoherent superposition of reflectivity curves, arising from the foam films, and a small-angle scattering (SAS) contribution arising mainly from the Plateau borders. In addition, a correction factor accounting for the spherical geometry of the foam bubble is introduced. The model is capable of describing the complete scattering curves of a foam stabilised by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. drainage states, and provides information about the thickness distribution of liquid films inside the foam. The validity of the model is tested further by studying foams stabilised by poly(Nisopropylacrylamide) (PNIPAM) microgels (MGs). The macroscopic foam properties in dependence of the cross-linker concentration of the MGs and temperature are investigated as well as the corresponding structuring in single foam films and macroscopic foams. Furthermore, the deformation of MGs inside the foam films is correlated with their elasticity as predicted by the affine network model.

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