Investigation of protein laden droplets in microfluidic applications

This thesis explores the transport and breakup of emulsion droplets in micro-capillaries and capillary constrictions and examines the role of proteins as biological emulsifiers in these systems. The study involves modeling and simulation of liquid/liquid flow through capillary constrictions with varying dynamic contact angles, ranging from highly hydrophilic to highly hydrophobic conditions. Advanced advection schemes with geometric interface reconstruction are employed in the models for high interface advection accuracy in the simulations, alongside a sharp surface tension force model to mitigate spurious currents caused by numerical surface curvature treatment and the implementation of the surface tension as a body force. Stress singularities at the three-phase contact line are addressed using a Navier-slip boundary condition. The simulation results highlight the significant impact of wettability on the contact line propagation and interface deformation, defining distinct displacement regimes within the forced liquid/liquid displacement. These regimes are experimentally validated and evaluated. The research especially investigates the role of proteins as interface active components, particularly used in food technology, where they, due to their amphiphilic nature, stabilize liquid/liquid interfaces by lowering interfacial tension.
Given the increasing use of sensitive animal- and plant-based proteins in the food industry, low-shear, low-stress homogenization methods like the premix-membrane emulsification have gained specific attention. However, challenges such as protein adsorption to membrane surfaces, leading to fouling and pore-blocking, persist. This study addresses how protein adsorption alters the system wettability and affects droplet breakup during emulsification. For this approach, generic configurations (straight micro capillaries with constriction) of membrane structures are utilized.
Through molecular dynamic simulations the research quantifies the impact of protein adsorption on interfacial tension at liquid/liquid and liquid/solid interfaces. The Young-Dupr ́e equation is employed to convert interfacial energies into contact angles, which are validated through experimental studies. The calculated contact angles are used to simulate the droplet propagation through idealized pore structures. Results demonstrate that protein adsorption significantly impacts the wettability, thereby, affecting droplet propagation and interfacial stability. A specific hypothesis is tested regarding disulfide bonds in proteins like the whey protein β- lactoglobulin, which play a crucial role in maintaining secondary and tertiary structures. The study shows that the partial or complete removal of these bonds impacts protein structural rearrangement through intra- and intermolecular interactions, thereby altering their interfacial activity at oil/water interfaces. The numerical investigations contribute to a deeper mechanistic understanding of the structure-function relationship in emulsification processes, particularly concerning the interfacial adsorption behaviour of proteins.