Marangoni convection during droplet birth and coalescence

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Marangoni effects significantly improve mass transfer and directly influence local hydrodynamics, especially during droplet birth and coalescence. However, a detailed quantitative understanding of local phenomena is missing and therefore neglected in industrial design.The lifetime of a droplet in an extraction column comprises formation, rise or fall and coalescence. These stages are strongly coupled and depend on the fluid dynamic conditions and mass transfer. Mass transfer set in while the droplet is being formed. During the following rise stage, the droplet may collide with other droplets and either coalesce with them or bounce. Both the coalescence probability and the rise time - and, consequently, the droplet residence time in the apparatus - depend on the rise velocity. Moreover, the coalescence behaviour influences the droplet size distribution, which, in turn, governs the interfacial area available for mass transfer. Species transport can evoke interfacial tension gradients, which lead to additional shear stresses at the interface. Thereby, a movement of the interface is induced, creating strong and irregular flow patterns. This phenomenon, called Marangoni effect, leads to extremely strong coupling of momentum and mass transfer.Investigation of these complex phenomena is often performed via studying of fluid dynamics and mass transfer characteristics of single droplets. Correction factors are then used to account for such non-idealities as is Marangoni convection. In the state-of-the-art of apparatus simulation such local phenomena are usually completely ignored.In the research project, new measuring systems and methods have been developed for an in situ, high resolution experimental investigation of the influence of Marangoni convection on mass transport in space and time. However, the first seconds of droplet formation can only be captured with numerical methods. The planned extension period will be used to generate a data pool and to validate newly developed modelling methods enabling creation of new kernels for technical unit simulation. Currently, Marangoni convection is out of consideration in the apparatus design.